Chapter XXIII
DIRECT AND DEFINITE ACTION OF THE EXTERNAL CONDITIONS OF LIFE.
SLIGHT MODIFICATIONS IN PLANTS FROM THE DEFINITE ACTION OF CHANGED CONDITIONS IN SIZE, COLOUR, CHEMICAL PROPERTIES, AND IN THE STATE OF THE TISSUES--LOCAL DISEASES--CONSPICUOUS MODIFICATIONS FROM CHANGED CLIMATE OR FOOD, ETC.--PLUMAGE OF BIRDS AFFECTED BY PECULIAR NUTRIMENT, AND BY THE INOCULATION OF POISON--LAND-SHELLS--MODIFICATIONS OF ORGANIC BEINGS IN A STATE OF NATURE THROUGH THE DEFINITE ACTION OF EXTERNAL CONDITIONS--COMPARISON OF AMERICAN AND EUROPEAN TREES--GALLS--EFFECTS OF PARASITIC FUNGI--CONSIDERATIONS OPPOSED TO THE BELIEF IN THE POTENT INFLUENCE OF CHANGED EXTERNAL CONDITIONS--PARALLEL SERIES OF VARIETIES--AMOUNT OF VARIATION DOES NOT CORRESPOND WITH THE DEGREE OF CHANGE IN THE CONDITIONS--BUD-VARIATION--MONSTROSITIES PRODUCED BY UNNATURAL TREATMENT--SUMMARY.
If we ask ourselves why this or that character has been modified under domestication, we are, in most cases lost in utter darkness. Many naturalists, especially of the French school, attribute every modification to the "monde ambiant," that is, to changed climate, with all its diversities of heat and cold, dampness and dryness, light and electricity, to the nature of the soil, and to varied kinds and amount of food. By the term definite action, as used in this chapter, I mean an action of such a nature that, when many individuals of the same variety are exposed during several generations to any change in their physical conditions of life, all, or nearly all the individuals, are modified in the same manner. A new sub-variety would thus be produced without the aid of selection.
I do not include under the term of definite action the effects of habit or of the increased use and disuse of various organs. Modifications of this nature, no doubt, are definitely caused by the conditions to which the beings are subjected; but they depend much less on the nature of the conditions than on the laws of growth; hence they are included under a distinct head in the {272} following chapter. We know, however, far too little of the causes and laws of variation to make a sound classification. The direct action of the conditions of life, whether leading to definite or indefinite results, is a totally distinct consideration from the effects of natural selection; for natural selection depends on the survival under various and complex circumstances of the best-fitted individuals, but has no relation whatever to the primary cause of any modification of structure.
I will first give in detail all the facts which I have been able to collect, rendering it probable that climate, food, &c., have acted so definitely and powerfully on the organisation of our domesticated productions, that they have sufficed to form new sub-varieties or races, without the aid of selection by man or of natural selection. I will then give the facts and considerations opposed to this conclusion, and finally we will weigh, as fairly as we can, the evidence on both sides.
When we reflect that distinct races of almost all our domesticated animals exist in each kingdom of Europe, and formerly even in each district of England, we are at first strongly inclined to attribute their origin to the definite action of the physical conditions of each country; and this has been the conclusion of many authors. But we should bear in mind that man annually has to choose which animals shall be preserved for breeding, and which shall be slaughtered. We have also seen that both methodical and unconscious selection were formerly practised, and are now occasionally practised by the most barbarous races, to a much greater extent than might have been anticipated. Hence it is very difficult to judge how far the difference in conditions between, for instance, the several districts in England, could have sufficed without the aid of selection to modify the breeds which have been reared in each. It may be argued that, as numerous wild animals and plants have ranged during many ages throughout Great Britain, and still retain the same character, the difference in conditions between the several districts could not have modified in so marked a manner the various native races of cattle, sheep, pigs, and horses. The same difficulty of distinguishing between selection and the definite effects of the conditions of life, is encountered in a still higher degree when we compare closely allied natural {273} forms, inhabiting two countries, such as North America and Europe, which do not differ greatly in climate, nature of soil, &c., for in this case natural selection will inevitably and rigorously have acted during a long succession of ages.
From the importance of the difficulty just alluded to, it will be advisable to give as large a body of facts as possible, showing that extremely slight differences in treatment, either in different parts of the same country, or during different seasons, certainly cause an appreciable effect, at least on varieties which are already in an unstable condition. Ornamental flowers are good for this purpose, as they are highly variable, and are carefully observed. All floriculturists are unanimous that certain varieties are affected by very slight differences in the nature of the artificial compost in which they are grown, and by the natural soil of the district, and by the season. Thus, a skilful judge, in writing on Carnations and Picotees,[656] asks "where can Admiral Curzon be seen possessing the colour, size, and strength which it has in Derbyshire? Where can Flora's Garland be found equal to those at Slough? Where do high-coloured flowers revel better than at Woolwich and Birmingham? Yet in no two of these districts do the same varieties attain an equal degree of excellence, although each may be receiving the attention of the most skilful cultivators." The same writer then recommends every cultivator to keep five different kinds of soil and manure, "and to endeavour to suit the respective appetites of the plants you are dealing with, for without such attention all hope of general success will be vain." So it is with the Dahlia:[657] the Lady Cooper rarely succeeds near London, but does admirably in other districts; the reverse holds good with other varieties; and again, there are others which succeed equally well in various situations. A skilful gardener[658] states that he procured cuttings of an old and well-known variety (pulchella) of Verbena, which from having been propagated in a different situation presented a slightly different shade of colour; the two varieties were afterwards multiplied by cuttings, being carefully kept distinct; but in the second year they could hardly be distinguished, and in the third year no one could distinguish them.
The nature of the season has an especial influence on certain varieties of the Dahlia: in 1841 two varieties were pre-eminently good, and the next year these same two were pre-eminently bad. A famous amateur[659] asserts that in 1861 many varieties of the Rose came so untrue in character, "that it was hardly possible to recognise them, and the thought was not seldom entertained that the grower had lost his tally." The same amateur[660] states that in 1862 two-thirds of his Auriculas produced central trusses of flowers, and these are remarkable from not keeping true; {274} and he adds that in some seasons certain varieties of this plant all prove good, and the next season all prove bad; whilst exactly the reverse happens with other varieties. In 1845 the editor of the 'Gardener's Chronicle'[661] remarked how singular it was that this year many Calceolarias tended to assume a tubular form. With Heartsease[662] the blotched sorts do not acquire their proper character until hot weather sets in; whilst other varieties lose their beautiful marks as soon as this occurs.
Analogous facts have been observed with leaves: Mr. Beaton asserts[663] that he raised at Shrubland, during six years, twenty thousand seedlings from the Punch Pelargonium, and not one had variegated leaves; but at Surbiton, in Surrey, one-third, or even a greater proportion, of the seedlings from this same variety were more or less variegated. The soil of another district in Surrey has a strong tendency to cause variegation, as appears from information given me by Sir F. Pollock. Verlot[664] states that the variegated strawberry retains its character as long as grown in a dryish soil, but soon loses it when planted in fresh and humid soil. Mr. Salter, who is well known for his success in cultivating variegated plants, informs me that rows of strawberries were planted in his garden in 1859, in the usual way; and at various distances in one row, several plants simultaneously became variegated, and what made the case more extraordinary, all were variegated in precisely the same manner. These plants were removed, but during the three succeeding years other plants in the same row became variegated, and in no instance were the plants in any adjoining row affected.
The chemical qualities, odours, and tissues of plants are often modified by a change which seems to us slight. The Hemlock is said not to yield conicine in Scotland. The root of the Aconitum napellus becomes innocuous in frigid climates. The medicinal properties of the Digitalis are easily affected by culture. The Rhubarb flourishes in England, but does not produce the medicinal substance which makes the plant so valuable in Chinese Tartary. As the Pistacia lentiscus grows abundantly in the South of France, the climate must suit it, but it yields no mastic. The Laurus sassafras in Europe loses the odour proper to it in North America.[665] Many similar facts could be given, and they are remarkable because it might have been thought that definite chemical compounds would have been little liable to change either in quality or quantity.
The wood of the American Locust-tree (Robinia) when grown in England is nearly worthless, as is that of the Oak-tree when grown at the Cape of Good Hope.[666] Hemp and flax, as I hear from Dr. Falconer, flourish and yield plenty of seed on the plains of India, but their fibres are brittle {275} and useless. Hemp, on the other hand, fails to produce in England that resinous matter which is so largely used in India as an intoxicating drug.
The fruit of the Melon is greatly influenced by slight differences in culture and climate. Hence it is generally a better plan, according to Naudin, to improve an old kind than to introduce a new one into any locality. The seed of the Persian Melon produces near Paris fruit inferior to the poorest market kinds, but at Bordeaux yields delicious fruit.[667] Seed is annually brought from Thibet to Kashmir,[668] and produces fruit weighing from four to ten pounds, but plants raised from seed saved in Kashmir next year give fruit weighing only from two to three pounds. It is well known that American varieties of the Apple produce in their native land magnificent and brightly-coloured fruit, but in England of poor quality and a dull colour. In Hungary there are many varieties of the Kidney-bean, remarkable for the beauty of their seeds, but the Rev. M. J. Berkeley[669] found that their beauty could hardly ever be preserved in England, and in some cases the colour was greatly changed. We have seen in the ninth chapter, with respect to wheat, what a remarkable effect transportal from the North to the South of France, and reversely, produced on the weight of the grain.
When man can perceive no change in plants or animals which have been exposed to a new climate or to different treatment, insects can sometimes perceive a marked change. The same species of cactus has been carried to India from Canton, Manilla, Mauritius, and from the hot-houses of Kew, and there is likewise a so-called native kind, formerly introduced from South America; all these plants are alike in appearance, but the cochineal insect flourishes only on the native kind, on which it thrives prodigiously.[670] Humboldt remarks[671] that white men "born in the torrid zone walk barefoot with impunity in the same apartment where a European, recently landed, is exposed to the attacks of the Pulex penetrans." This insect, the too well-known chigoe, must therefore be able to distinguish what the most delicate chemical analysis fails to distinguish, namely, a difference between the blood or tissues of a European and those of a white man born in the country. But the discernment of the chigoe is not so surprising as it at first appears; for {276} according to Liebig[672] the blood of men with different complexions, though inhabiting the same country, emits a different odour.
Diseases peculiar to certain localities, heights, or climates, may be here briefly noticed, as showing the influence of external circumstances on the human body. Diseases confined to certain races of man do not concern us, for the constitution of the race may play the more important part, and this may have been determined by unknown causes. The Plica Polonica stands, in this respect, in a nearly intermediate position; for it rarely affects Germans, who inhabit the neighbourhood of the Vistula, where so many Poles are grievously affected; and on the other hand, it does not affect Russians, who are said to belong to the same original stock with the Poles.[673] The elevation of a district often governs the appearance of diseases; in Mexico the yellow fever does not extend above 924 mètres; and in Peru, people are affected with the verugas only between 600 and 1600 mètres above the sea; many other such cases could be given. A peculiar cutaneous complaint, called the Bouton d'Alep, affects in Aleppo and some neighbouring districts almost every native infant, and some few strangers; and it seems fairly well established that this singular complaint depends on drinking certain waters. In the healthy little island of St. Helena the scarlet-fever is dreaded like the Plague; analogous facts have been observed in Chili and Mexico.[674] Even in the different departments of France it is found that the various infirmities which render the conscript unfit for serving in the army, prevail with remarkable inequality, revealing, as Boudin observes, that many of them are endemic, which otherwise would never have been suspected.[675] Any one who will study the distribution of disease will be struck with surprise at what slight differences in the surrounding circumstances govern the nature and severity of the complaints by which man is at least temporarily affected.
The modifications as yet referred to have been extremely slight, and in most cases have been caused, as far as we can judge, by equally slight changes in the conditions. But can it be safely maintained that such changed conditions, if acting during a long series of generations, would not produce a marked effect? It is commonly believed that the people of the United States differ in appearance from the parent Anglo-Saxon race; and selection cannot have come into action within so short a period. A good observer[676] states that a general absence of fat, {277} a thin and elongated neck, stiff and lank hair, are the chief characteristics. The change in the nature of the hair is supposed to be caused by the dryness of the atmosphere. If immigration into the United States were now stopped, who can say that the character of the whole people would not be greatly modified in the course of two or three thousand years?
The direct and definite action of changed conditions, in contradistinction to the accumulation of indefinite variations, seems to me so important that I will give a large additional body of miscellaneous facts. With plants, a considerable change of climate sometimes produces a conspicuous result. I have given in detail in the ninth chapter the most remarkable case known to me, namely, that in Germany several varieties of maize brought from the hotter parts of America were transformed in the course of only two or three generations. Dr. Falconer informs me that he has seen the English Ribston-pippin apple, a Himalayan oak, Prunus and Pyrus, all assume in the hotter parts of India a fastigate or pyramidal habit; and this fact is the more interesting, as a Chinese tropical species of Pyrus naturally has this habit of growth. Although in these cases the changed manner of growth seems to have been directly caused by the great heat, we know that many fastigate trees have originated in their temperate homes. In the Botanic Gardens of Ceylon the apple-tree[677] "sends out numerous runners under ground, which continually rise into small stems, and form a growth around the parent-tree." The varieties of the cabbage which produce heads in Europe fail to do so in certain tropical countries.[678] The Rhododendron ciliatum produced at Kew flowers so much larger and paler-coloured than those which it bears on its native Himalayan mountain, that Dr. Hooker[679] would hardly have recognised the species by the flowers alone. Many similar facts with respect to the colour and size of flowers could be given.
The experiments of Vilmorin and Buckman on carrots and parsnips prove that abundant nutriment produces a definite and inheritable effect on the so-called roots, with scarcely any change in other parts of the plant. Alum directly influences the colour of the flowers of the Hydrangea.[680] Dryness seems generally to favour the hairyness or villosity of plants. Gärtner found that hybrid Verbascums became extremely woolly when grown in pots. Mr. Masters, on the other hand, states that the Opuntia leucotricha "is well clothed with beautiful white hairs when grown in a damp heat; but in a dry heat exhibits none of this peculiarity."[681] Slight variations of many kinds, not worth specifying in detail, are retained only as {278} long as plants are grown in certain soils, of which Sageret[682] gives from his own experience some instances. Odart, who insists strongly on the permanence of the varieties of the grape, admits[683] that some varieties, when grown under a different climate or treated differently, vary in an extremely slight degree, as in the tint of the fruit and in the period of ripening. Some authors have denied that grafting causes even the slightest difference in the scion; but there is sufficient evidence that the fruit is sometimes slightly affected in size and flavour, the leaves in duration, and the flowers in appearance.[684]
With animals there can be no doubt, from the facts given in the first chapter, that European dogs deteriorate in India, not only in their instincts but in structure; but the changes which they undergo are of such a nature, that they may be partly due to reversion to a primitive form, as in the case of feral animals. In parts of India the turkey becomes reduced in size, "with the pendulous appendage over the beak enormously developed."[685] We have seen how soon the wild duck, when domesticated, loses its true character, from the effects of abundant or changed food, or from taking little exercise. From the direct action of a humid climate and poor pasture the horse rapidly decreases in size in the Falkland Islands. From information which I have received, this seems likewise to be the case to a certain extent with sheep in Australia.
Climate definitely influences the hairy covering of animals; in the West Indies a great change is produced in the fleece of sheep, in about three generations. Dr. Falconer states[686] that the Thibet mastiff and goat, when brought down from the Himalaya to Kashmir, lose their fine wool. At Angora not only goats, but shepherd-dogs and cats, have fine fleecy hair, and Mr. Ainsworth[687] attributes the thickness of the fleece to the severe winters, and its silky lustre to the hot summers. Burnes states positively[688] that the Karakool sheep lose their peculiar black curled fleeces when removed into any other country. Even within the limits of England, I have been assured that with two breeds of sheep the wool was slightly changed by the flocks being pastured in different localities.[689] It has been asserted on good authority[690] that horses kept during several years in the deep coal-mines of Belgium become covered with velvety hair, almost like that on the mole. These cases probably stand in close relation to the natural change of coat in winter and summer. Naked varieties of several domestic animals have occasionally appeared; but there is no reason to {279} believe that this is in any way related to the nature of the climate to which they have been exposed.[691]
It appears at first sight probable that the increased size, the tendency to fatten, the early maturity and altered forms of our improved cattle, sheep, and pigs, have directly resulted from their abundant supply of food. This is the opinion of many competent judges, and probably is to a great extent true. But as far as form is concerned, we must not overlook the equal or more potent influence of lessened use on the limbs and lungs. We see, moreover, as far as size is concerned, that selection is apparently a more powerful agent than a large supply of food, for we can thus only account for the existence, as remarked to me by Mr. Blyth, of the largest and smallest breeds of sheep in the same country, of Cochin-China fowls and Bantams, of small Tumbler and large Runt pigeons, all kept together and supplied with abundant nourishment. Nevertheless there can be little doubt that our domesticated animals have been modified, independently of the increased or lessened use of parts, by the conditions to which they have been subjected, without the aid of selection. For instance, Prof. Rütimeyer[692] shows that the bones of all domesticated quadrupeds can be distinguished from those of wild animals by the state of their surface and general appearance. It is scarcely possible to read Nathusius's excellent 'Vorstudien,'[693] and doubt that, with the highly improved races of the pig, abundant food has produced a conspicuous effect on the general form of the body, on the breadth of the head and face, and even on the teeth. Nathusius rests much on the case of a purely bred Berkshire pig, which when two months old became diseased in its digestive organs, and was preserved for observation until nineteen months old; at this age it had lost several characteristic features of the breed, and had acquired a long, narrow head, of large size relatively to its small body, and elongated legs. But in this case and in some others we ought not to assume that, because certain characters are lost, perhaps through reversion, under one course of treatment, therefore that they had been at first directly produced by an opposite course.
In the case of the rabbit, which has become feral on the island of Porto Santo, we are at first strongly tempted to attribute the whole change--the greatly reduced size, the altered tints of the fur, and the loss of certain characteristic marks--to the definite action of the new conditions to which it has been exposed. But in all such cases we have to consider in addition the tendency to reversion to progenitors more or less remote, and the natural selection of the finest shades of difference.
The nature of the food sometimes either definitely induces certain peculiarities, or stands in some close relation with them. Pallas long ago asserted that the fat-tailed sheep of Siberia degenerated and lost their enormous tails when removed from certain saline pastures; and recently {280} Erman[694] states that this occurs with the Kirgisian sheep when brought to Orenburgh.
It is well known that hemp-seed causes bullfinches and certain other birds to become black. Mr. Wallace has communicated to me some much more remarkable facts of the same nature. The natives of the Amazonian region feed the common green parrot (Chrysotis festiva, Linn.) with the fat of large Siluroid fishes, and the birds thus treated become beautifully variegated with red and yellow feathers. In the Malayan archipelago, the natives of Gilolo alter in an analogous manner the colours of another parrot, namely, the Lorius garrulus, Linn., and thus produce the Lori rajah or King-Lory. These parrots in the Malay Islands and South America, when fed by the natives on natural vegetable food, such as rice and plantains, retain their proper colours. Mr. Wallace has, also, recorded[695] a still more singular fact. "The Indians (of S. America) have a curious art by which they change the colours of the feathers of many birds. They pluck out those from the part they wish to paint, and inoculate the fresh wound with the milky secretion from the skin of a small toad. The feathers grow of a brilliant yellow colour, and on being plucked out, it is said, grow again of the same colour without any fresh operation."
Bechstein[696] does not entertain any doubt that seclusion from light affects, at least temporarily, the colours of cage-birds.
It is well known that the shells of land-mollusca are affected by the abundance of lime in different districts. Isidore Geoffroy St. Hilaire[697] gives the case of Helix lactea, which has recently been carried from Spain to the South of France and to the Rio Plata, and in both these countries now presents a distinct appearance, but whether this has resulted from food or climate is not known. With respect to the common oyster, Mr. F. Buckland informs me that he can generally distinguish the shells from different districts; young oysters brought from Wales and laid down in beds where "natives" are indigenous, in the short space of two months begin to assume the "native" character. M. Costa[698] has recorded a much more remarkable case of the same nature, namely, that young shells taken from the shores of England and placed in the Mediterranean, at once altered their manner of growth and formed prominent diverging rays, like those on the shells of the proper Mediterranean oyster. The same individual shell, showing both forms of growth, was exhibited before a society in Paris. Lastly, it is well known that caterpillars fed on different food sometimes either themselves acquire a different colour or produce moths different in colour.[699]
{281}
It would be travelling beyond my proper limits here to discuss how far organic beings in a state of nature are definitely modified by changed conditions. In my 'Origin of Species' I have given a brief abstract of the facts bearing on this point, and have shown the influence of light on the colours of birds, and of residence near the sea on the lurid tints of insects, and on the succulency of plants. Mr. Herbert Spencer[700] has recently discussed with much ability this whole subject on broad and general grounds. He argues, for instance, that with all animals the external and internal tissues are differently acted on by the surrounding conditions, and they invariably differ in intimate structure. So again the upper and lower surfaces of true leaves, as well as of stems and petioles, when these assume the function and occupy the position of leaves, are differently circumstanced with respect to light, &c., and apparently in consequence differ in structure. But, as Mr. Herbert Spencer admits, it is most difficult in all such cases to distinguish between the effects of the definite action of physical conditions and the accumulation through natural selection of inherited variations which are serviceable to the organism, and which have arisen independently of the definite action of these conditions.
Although we are not here concerned with organic beings in a state of nature, yet I may call attention to one case. Mr. Meehan,[701] in a remarkable paper, compares twenty-nine kinds of American trees, belonging to various orders, with their nearest European allies, all grown in close proximity in the same garden and under as nearly as possible the same conditions. In the American species Mr. Meehan finds, with the rarest exceptions, that the leaves fall earlier in the season, and assume before falling a brighter tint; that they are less deeply toothed or serrated; that the buds are smaller; that the trees are more diffuse in growth and have fewer branchlets; and, lastly, that the seeds are smaller--all in comparison with the corresponding European species. Now, considering that these trees belong to distinct orders, it is out of the question that the peculiarities just specified should have been inherited in the one continent from one progenitor, and in the other from another progenitor; and considering that the trees inhabit widely different stations, these peculiarities can hardly be supposed to be of any special {282} service to the two series in the Old and New Worlds; therefore these peculiarities cannot have been naturally selected. Hence we are led to infer that they have been definitely caused by the long-continued action of the different climate of the two continents on the trees.
Galls.--Another class of facts, not relating to cultivated plants, deserves attention. I allude to the production of galls. Every one knows the curious, bright-red, hairy productions on the wild rose-tree, and the various different galls produced by the oak. Some of the latter resemble fruit, with one face as rosy as the rosiest apple. These bright colours can be of no service either to the gall-forming insect or to the tree, and probably are the direct result of the action of the light, in the same manner as the apples of Nova Scotia or Canada are brighter coloured than English apples. The strongest upholder of the doctrine that organic beings are created beautiful to please mankind would not, I presume, extend this view to galls. According to Osten Sacken's latest revision, no less than fifty-eight kinds of galls are produced on the several species of oak, by Cynips with its sub-genera; and Mr. B. D. Walsh[702] states that he can add many others to the list. One American species of willow, the Salix humilis, bears ten distinct kinds of galls. The leaves which spring from the galls of various English willows differ completely in shape from the natural leaves. The young shoots of junipers and firs, when punctured by certain insects, yield monstrous growths like flowers and cones; and the flowers of some plants become from the same cause wholly changed in appearance. Galls are produced in every quarter of the world; of several sent to me by Mr. Thwaites from Ceylon, some were as symmetrical as a composite flower when in bud, others smooth and spherical like a berry; some protected by long spines, others clothed with yellow wool formed of long cellular hairs, others with regularly tufted hairs. In some galls the internal structure is simple, but in others it is highly complex; thus M. Lucaze-Duthiers[703] has figured in the common ink-gall no less than seven concentric layers, composed of distinct tissue, {283} namely, the epidermic, sub-epidermic, spongy, intermediate, and the hard protective layer formed of curiously thickened woody cells, and, lastly, the central mass abounding with starch-granules on which the larvæ feed.
Galls are produced by insects of various orders, but the greater number by species of Cynips. It is impossible to read M. Lucaze-Duthier's discussion and doubt that the poisonous secretion of the insect causes the growth of the gall, and every one knows how virulent is the poison secreted by wasps and bees, which belong to the same order with Cynips. Galls grow with extraordinary rapidity, and it is said that they attain their full size in a few days;[704] it is certain that they are almost completely developed before the larvæ are hatched. Considering that many gall-insects are extremely small, the drop of secreted poison must be excessively minute; it probably acts on one or two cells alone, which, being abnormally stimulated, rapidly increase by a process of self-division. Galls, as Mr. Walsh[705] remarks, afford good, constant, and definite characters, each kind keeping as true to form as does any independent organic being. This fact becomes still more remarkable when we hear that, for instance, seven out of the ten different kinds of galls produced on Salix humilis are formed by gall-gnats (Cecidomyidæ) which, "though essentially distinct species, yet resemble one another so closely that in almost all cases it is difficult, and in some cases impossible, to distinguish the full-grown insects one from the other."[706] For in accordance with a wide-spread analogy we may safely infer that the poison secreted by insects so closely allied would not differ much in nature; yet this slight difference is sufficient to induce widely different results. In some few cases the same species of gall-gnat produces on distinct species of willows galls which cannot be distinguished; the Cynips fecundatrix, also, has been known to produce on the Turkish oak, to which it is not properly attached, exactly the same kind of gall as on the European oak.[707] These latter facts apparently prove that the nature of the poison is a much more powerful {284} agent in determining the form of the gall than the specific character of the tree which is acted on.
As the poisonous secretion of insects belonging to various orders has the special power of affecting the growth of various plants;--as a slight difference in the nature of the poison suffices to produce widely different results;--and lastly, as we know that the chemical compounds secreted by plants are eminently liable to be modified by changed conditions of life, we may believe it possible that various parts of a plant might be modified through the agency of its own altered secretions. Compare, for instance, the mossy and viscid calyx of a moss-rose, which suddenly appears through bud-variation on a Provence-rose, with the gall of red moss growing from the inoculated leaf of a wild rose, with each filament symmetrically branched like a microscopical spruce-fir, bearing a glandular tip and secreting odoriferous gummy matter.[708] Or compare, on the one hand, the fruit of the peach, with its hairy skin, fleshy covering, hard shell and kernel, and on the other hand one of the more complex galls with its epidermic, spongy, and woody layers, surrounding tissue loaded with starch granules. These normal and abnormal structures manifestly present a certain degree of resemblance. Or, again, reflect on the cases above given of parrots which have had their plumage brightly decorated through some change in their blood, caused by having been fed on certain fishes, or locally inoculated with the poison of a toad. I am far from wishing to maintain that the moss-rose or the hard shell of the peach-stone or the bright colours of birds are actually due to any chemical change in the sap or blood; but these cases of galls and of parrots are excellently adapted to show us how powerfully and singularly external agencies may affect structure. With such facts before us, we need feel no surprise at the appearance of any modification in any organic being.
I may, also, here allude to the remarkable effects which parasitic fungi sometimes produce on plants. Reissek[709] has described a Thesium, affected by an Oecidium, which was greatly modified, and assumed some of the {285} characteristic features of certain allied species, or even genera. Suppose, says Reissek, "the condition originally caused by the fungus to become constant in the course of time, the plant would, if found growing wild, be considered as a distinct species or even as belonging to a new genus." I quote this remark to show how profoundly, yet in how natural a manner, this plant must have been modified by the parasitic fungus.
Facts and Considerations opposed to the belief that the Conditions of Life act in a potent manner in causing definite Modifications of Structure.
I have alluded to the slight differences in species when naturally living in distinct countries under different conditions; and such differences we feel at first inclined, probably to a limited extent with justice, to attribute to the definite action of the surrounding conditions. But it must be borne in mind that there are a far greater number of animals and plants which range widely and have been exposed to great diversities of conditions, yet remain nearly uniform in character. Some authors, as previously remarked, account for the varieties of our culinary and agricultural plants by the definite action of the conditions to which they have been exposed in the different parts of Great Britain; but there are about 200 plants[710] which are found in every single English county; these plants must have been exposed for an immense period to considerable differences of climate and soil, yet do not differ. So, again, some birds, insects, other animals, and plants range over large portions of the world, yet retain the same character.
Notwithstanding the facts previously given on the occurrence of highly peculiar local diseases and on the strange modifications of structure in plants caused by the inoculated poison of insects, and other analogous cases; still there are a multitude of variations--such as the modified skull of the niata ox and bulldog, the long horns of Caffre cattle, the conjoined toes of the solid-hoofed swine, the immense crest and protuberant skull of Polish fowls, the crop of the pouter-pigeon, and a host of other such cases--which we can hardly attribute to the definite action, in the sense before specified, of the external conditions of life. No doubt in every case there must have been some exciting cause; but as we see innumerable individuals exposed to nearly the same conditions, and one alone is affected, we may conclude that the constitution of the individual is of far higher {286} importance than the conditions to which it has been exposed. It seems, indeed, to be a general rule that conspicuous variations occur rarely, and in one individual alone out of many thousands, though all may have been exposed, as far as we can judge, to nearly the same conditions. As the most strongly marked variations graduate insensibly into the most trifling, we are led by the same train of thought to attribute each slight variation much more to innate differences of constitution, however caused, than to the definite action of the surrounding conditions.
We are led to the same conclusion by considering the cases, formerly alluded to, of fowls and pigeons, which have varied and will no doubt go on varying in directly opposite ways, though kept during many generations under nearly the same conditions. Some, for instance, are born with their beaks, wings, tails, legs, &c., a little longer, and others with these same parts a little shorter. By the long-continued selection of such slight individual differences, which occur in birds kept in the same aviary, widely different races could certainly be formed; and long-continued selection, important as is the result, does nothing but preserve the variations which appear to us to arise spontaneously.
In these cases we see that domesticated animals vary in an indefinite number of particulars, though treated as uniformly as is possible. On the other hand, there are instances of animals and plants, which, though exposed to very different conditions, both under nature and domestication, have varied in nearly the same manner. Mr. Layard informs me that he has observed amongst the Caffres of South Africa a dog singularly like an arctic Esquimaux dog. Pigeons in India present nearly the same wide diversities of colour as in Europe; and I have seen chequered and simply barred pigeons, and pigeons with blue and white loins, from Sierra Leone, Madeira, England, and India. New varieties of flowers are continually raised in different parts of Great Britain, but many of these are found by the judges at our exhibitions to be almost identical with old varieties. A vast number of new fruit-trees and culinary vegetables have been produced in North America: these differ from European varieties in the same general manner as the several varieties raised in Europe differ from each other; and no one has ever pretended that the climate of America has given to the many American varieties any general character by which they can be recognised. Nevertheless, from the facts previously advanced on the authority of Mr. Meehan with respect to American and European forest-trees, it would be rash to affirm that varieties raised in the two countries would not in the course of ages assume a distinctive character. Mr. Masters has recorded a striking fact[711] bearing on this subject: he raised numerous plants of Hybiscus Syriacus from seed collected in South Carolina and the Holy Land, where the parent-plants must have been exposed to considerably different conditions; yet the seedlings from both localities broke into two similar strains, one with obtuse leaves and purple or crimson flowers, and the other with elongated leaves and more or less pink flowers.
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We may, also, infer the prepotent influence of the constitution of the organism over the definite action of the conditions of life, from the several cases given in the earlier chapters of parallel series of varieties,--an important subject, hereafter to be more fully discussed. Sub-varieties of the several kinds of wheat, gourds, peaches, and other plants, and to a certain limited extent sub-varieties of the fowl, pigeon, and dog, have been shown either to resemble or to differ from each other in a closely corresponding and parallel manner. In other cases, a variety of one species resembles a distinct species; or the varieties of two distinct species resemble each other. Although these parallel resemblances no doubt often result from reversion to the former characters of a common progenitor; yet in other cases, when new characters first appear, the resemblance must be attributed to the inheritance of a similar constitution, and consequently to a tendency to vary in the same manner. We see something of a similar kind in the same monstrosity appearing and reappearing many times in the same animal, and, as Dr. Maxwell Masters has remarked to me, in the same plant.
We may at least conclude thus far, that the amount of modification which animals and plants have undergone under domestication, does not correspond with the degree to which they have been subjected to changed circumstances. As we know the parentage of domesticated birds far better than of most quadrupeds, we will glance through the list. The pigeon has varied in Europe more than almost any other bird; yet it is a native species, and has not been exposed to any extraordinary change of conditions. The fowl has varied equally, or almost equally, with the pigeon, and is a native of the hot jungles of India. Neither the peacock, a native of the same country, nor the guinea-fowl, an inhabitant of the dry deserts of Africa, has varied at all, or only in colour. The turkey, from Mexico, has varied but little. The duck, on the other hand, a native of Europe, has yielded some well-marked races; and as this is an aquatic bird, it must have been subjected to a far more serious change in its habits than the pigeon or even the fowl, which nevertheless have varied in a much higher degree. The goose, a native of Europe and aquatic like the duck, has varied less than any other domesticated bird, except the peacock.
Bud-variation is, also, important under our present point of view. In some few cases, as when all the eyes or buds on the same tuber of the potato, or all the fruit on the same plum-tree, or all the flowers on the same plant, have suddenly varied in the same manner, it might be argued that the {288} variation had been definitely caused by some change in the conditions to which the plants had been exposed; yet, in other cases, such an admission is extremely difficult. As new characters sometimes appear by bud-variation, which do not occur in the parent-species or in any allied species, we may reject, at least in these cases, the idea that they are due to reversion. Now it is well worth while to reflect maturely on some striking case of bud-variation, for instance that of the peach. This tree has been cultivated by the million in various parts of the world, has been treated differently, grown on its own roots and grafted on various stocks, planted as a standard, against a wall, and under glass; yet each bud of each sub-variety keeps true to its kind. But occasionally, at long intervals of time, a tree in England, or under the widely-different climate of Virginia, produces a single bud, and this yields a branch which ever afterwards bears nectarines. Nectarines differ, as every one knows, from peaches in their smoothness, size, and flavour; and the difference is so great, that some botanists have maintained that they are specifically distinct. So permanent are the characters thus suddenly acquired, that a nectarine produced by bud-variation has propagated itself by seed. To guard against the supposition that there is some fundamental distinction between bud and seminal variation, it is well to bear in mind that nectarines have likewise been produced from the stone of the peach; and, reversely, peaches from the stone of the nectarine. Now is it possible to conceive external conditions more closely alike than those to which the buds on the same tree are exposed? Yet one bud alone, out of the many thousands borne by the same tree, has suddenly without any apparent cause produced a nectarine. But the case is even stronger than this, for the same flower-bud has yielded a fruit, one-half or one-quarter a nectarine, and the other half or three-quarters a peach. Again, seven or eight varieties of the peach have yielded by bud-variation nectarines: the nectarines thus produced, no doubt, differ a little from each other; but still they are nectarines. Of course there must be some cause, internal or external, to excite the peach-bud to change its nature; but I cannot imagine a class of facts better adapted to force on our minds the conviction that what we call the external conditions of life are quite insignificant in {289} relation to any particular variation, in comparison with the organisation or constitution of the being which varies.
It is known from the labours of Geoffroy St. Hilaire, and recently from those of Dareste and others, that eggs of the fowl, if shaken, placed upright, perforated, covered in part with varnish, &c., produce monstrous chickens. Now these monstrosities may be said to be directly caused by such unnatural conditions, but the modifications thus induced are not of a definite nature. An excellent observer, M. Camille Dareste,[712] remarks "that the various species of monstrosities are not determined by specific causes; the external agencies which modify the development of the embryo act solely in causing a perturbation--a perversion in the normal course of development." He compares the result to what we see in illness: a sudden chill, for instance, affects one individual alone out of many, causing either a cold, or sore-throat, rheumatism, or inflammation of the lungs or pleura. Contagious matter acts in an analogous manner.[713] We may take a still more specific instance: seven pigeons were struck by rattle-snakes;[714] some suffered from convulsions; some had their blood coagulated, in others it was perfectly fluid; some showed ecchymosed spots on the heart, others on the intestines, &c.; others again showed no visible lesion in any organ. It is well known that excess in drinking causes different diseases in different men; but men living under a cold and tropical climate are differently affected:[715] and in this case we see the definite influence of opposite conditions. The foregoing facts apparently give us as good an idea as we are likely for a long time to obtain, how in many cases external conditions act directly, though not definitely, in causing modifications of structure.
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Summary.--There can be no doubt, from the facts given in the early part of this chapter, that extremely slight changes in {290} the conditions of life sometimes act in a definite manner on our already variable domesticated productions; and, as the action of changed conditions in causing general or indefinite variability is accumulative, so it may be with their definite action. Hence it is possible that great and definite modifications of structure may result from altered conditions acting during a long series of generations. In some few instances a marked effect has been produced quickly on all, or nearly all, the individuals which have been exposed to some considerable change of climate, food, or other circumstance. This has occurred, and is now occurring, with European men in the United States, with European dogs in India, with horses in the Falkland Islands, apparently with various animals at Angora, with foreign oysters in the Mediterranean, and with maize grown in Europe from tropical seed. We have seen that the chemical compounds secreted by plants and the state of their tissues are readily affected by changed conditions. In some cases a relation apparently exists between certain characters and certain conditions, so that if the latter be changed the character is lost--as with cultivated flowers, with some few culinary plants, with the fruit of the melon, with fat-tailed sheep, and other sheep having peculiar fleeces.
The production of galls, and the change of plumage in parrots when fed on peculiar food or when inoculated by the poison of a toad, prove to us what great and mysterious changes in structure and colour may be the definite result of chemical changes in the nutrient fluids or tissues.
We have also reason to believe that organic beings in a state of nature may be modified in various definite ways by the conditions to which they have been long exposed, as in the case of American trees in comparison with their representatives in Europe. But in all such cases it is most difficult to distinguish between the definite results of changed conditions, and the accumulation through natural selection of serviceable variations which have arisen independently of the nature of the conditions. If, for instance, a plant had to be modified so as to become fitted to inhabit a humid instead of an arid station, we have no reason to believe that variations of the right kind would occur more frequently if the parent-plant inhabited a station a little more {291} humid than usual. Whether the station was unusually dry or humid, variations adapting the plant in a slight degree for directly opposite habits of life would occasionally arise, as we have reason to believe from what we know in other cases.
In most, perhaps in all cases, the organisation or constitution of the being which is acted on, is a much more important element than the nature of the changed conditions, in determining the nature of the variation. We have evidence of this in the appearance of nearly similar modifications under different conditions, and of different modifications under apparently nearly the same conditions. We have still better evidence of this in closely parallel varieties being frequently produced from distinct races, or even distinct species, and in the frequent recurrence of the same monstrosity in the same species. We have also seen that the degree to which domesticated birds have varied, does not stand in any close relation with the amount of change to which they have been subjected.
To recur once again to bud-variations. When we reflect on the millions of buds which many trees have produced, before some one bud has varied, we are lost in wonder what the precise cause of each variation can be. Let us recall the case given by Andrew Knight of the forty-year-old tree of the yellow magnum bonum plum, an old variety which has been propagated by grafts on various stocks for a very long period throughout Europe and North America, and on which a single bud suddenly produced the red magnum bonum. We should also bear in mind that distinct varieties, and even distinct species,--as in the case of peaches, nectarines, and apricots,--of certain roses and camellias,--although separated by a vast number of generations from any progenitor in common, and although cultivated under diversified conditions, have yielded by bud-variation closely analogous varieties. When we reflect on these facts we become deeply impressed with the conviction that in such cases the nature of the variation depends but little on the conditions to which the plant has been exposed, and not in any especial manner on its individual character, but much more on the general nature or constitution, inherited from some remote progenitor, of the whole group of allied beings to which the plant belongs. We are thus driven to conclude that in most {292} cases the conditions of life play a subordinate part in causing any particular modification; like that which a spark plays, when a mass of combustibles bursts into flame--the nature of the flame depending on the combustible matter, and not on the spark.
No doubt each slight variation must have its efficient cause; but it is as hopeless an attempt to discover the cause of each as to say why a chill or a poison affects one man differently from another. Even with modifications resulting from the definite action of the conditions of life, when all or nearly all the individuals, which have been similarly exposed, are similarly affected, we can rarely see the precise relation between cause and effect. In the next chapter it will be shown that the increased use or disuse of various organs, produces an inherited effect. It will further be seen that certain variations are bound together by correlation and other laws. Beyond this we cannot at present explain either the causes or manner of action of Variation.
Finally, as indefinite and almost illimitable variability is the usual result of domestication and cultivation, with the same part or organ varying in different individuals in different or even in directly opposite ways; and as the same variation, if strongly pronounced, usually recurs only after long intervals of time, any particular variation would generally be lost by crossing, reversion, and the accidental destruction of the varying individuals, unless carefully preserved by man. Hence, although it must be admitted that new conditions of life do sometimes definitely affect organic beings, it may be doubted whether well-marked races have often been produced by the direct action of changed conditions without the aid of selection either by man or nature.
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XXIV. LAWS OF VARIATION--USE AND DISUSE, ETC
NISUS FORMATIVUS, OR THE CO-ORDINATING POWER OF THE ORGANISATION--ON THE EFFECTS OF THE INCREASED USE AND DISUSE OF ORGANS--CHANGED HABITS OF LIFE--ACCLIMATISATION WITH ANIMALS AND PLANTS--VARIOUS METHODS BY WHICH THIS CAN BE EFFECTED--ARRESTS OF DEVELOPMENT--RUDIMENTARY ORGANS.
In this and the two following chapters I shall discuss, as well as the difficulty of the subject permits, the several laws which govern Variability. These may be grouped under the effects of use and disuse, including changed habits and acclimatisation--arrests of development--correlated variation--the cohesion of homologous parts--the variability of multiple parts--compensation of growth--the position of buds with respect to the axis of the plant--and lastly, analogous variation. These several subjects so graduate into each other that their distinction is often arbitrary.
It may be convenient first briefly to discuss that co-ordinating and reparative power which is common, in a higher or lower degree, to all organic beings, and which was formerly designated by physiologists as the nisus formativus.
Blumenbach and others[716] have insisted that the principle which permits a Hydra, when cut into fragments, to develop itself into two or more perfect animals, is the same with that which causes a wound in the higher animals to heal by a cicatrice. Such cases as that of the Hydra are evidently analogous with the spontaneous division or fissiparous generation of the lowest animals, and likewise with the budding of plants. Between these extreme cases and that of a mere cicatrice we have every gradation. Spallanzani,[717] by cutting off the legs and tail of a Salamander, got in the course of three months six crops of these members; so that 687 perfect bones were reproduced by one animal during one season. At whatever {294} point the limb was cut off, the deficient part, and no more, was exactly reproduced. Even with man, as we have seen in the twelfth chapter, when treating of polydactylism, the entire limb whilst in an embryonic state, and supernumerary digits, are occasionally, though imperfectly, reproduced after amputation. When a diseased bone has been removed, a new one sometimes "gradually assumes the regular form, and all the attachments of muscles, ligaments, &c., become as complete as before."[718]
This power of regrowth does not, however, always act perfectly: the reproduced tail of a lizard differs in the forms of the scales from the normal tail: with certain Orthopterous insects the large hind legs are reproduced of smaller size:[719] the white cicatrice which in the higher animals unites the edges of a deep wound is not formed of perfect skin, for elastic tissue is not produced till long afterwards.[720] "The activity of the nisus formativus," says Blumenbach, "is in an inverse ratio to the age of the organised body." To this may be added that its power is greater in animals the lower they are in the scale of organisation; and animals low in the scale correspond with the embryos of higher animals belonging to the same class. Newport's observations[721] afford a good illustration of this fact, for he found that "myriapods, whose highest development scarcely carries them beyond the larvæ of perfect insects, can regenerate limbs and antennæ up to the time of their last moult;" and so can the larvæ of true insects, but not the mature insect. Salamanders correspond in development with the tadpoles or larvæ of the tailless Batrachians, and both possess to a large extent the power of regrowth; but not so the mature tailless Batrachians.
Absorption often plays an important part in the repairs of injuries. When a bone is broken, and does not unite, the ends are absorbed and rounded, so that a false joint is formed; or if the ends unite, but overlap, the projecting parts are removed.[722] But absorption comes into action, as Virchow remarks, during the normal growth of bones; parts which are solid during youth become hollowed out for the medullary tissue as the bone increases in size. In trying to understand the many well-adapted cases of regrowth when aided by absorption, we should remember that most parts of the organisation, even whilst retaining the same form, undergo constant renewal; so that a part which was not renewed would naturally be liable to complete absorption.
Some cases, usually classed under the so-called nisus formativus, at first appear to come under a distinct head; for not only are old structures reproduced, but structures which appear new are formed. Thus, after inflammation "false membranes," furnished with blood-vessels, lymphatics, and nerves, are developed; or a foetus escapes from the Fallopian tubes, and falls into the abdomen, "nature pours out a quantity of plastic lymph, which forms itself into organised membrane, richly supplied with blood-vessels," and the foetus is nourished for a time. In certain cases of {295} hydrocephalus the open and dangerous spaces in the skull are filled up with new bones, which interlock by perfect serrated sutures.[723] But most physiologists, especially on the Continent, have now given up the belief in plastic lymph or blastema, and Virchow[724] maintains that every structure, new or old, is formed by the proliferation of pre-existing cells. On this view false membranes, like cancerous or other tumours, are merely abnormal developments of normal growths; and we can thus understand how it is that they resemble adjoining structures; for instance, that "false membrane in the serous cavities acquires a covering of epithelium exactly like that which covers the original serous membrane; adhesions of the iris may become black apparently from the production of pigment-cells like those of the uvea."[725]
No doubt the power of reparation, though not always quite perfect, is an admirable provision, ready for various emergencies, even for those which occur only at long intervals of time.[726] Yet this power is not more wonderful than the growth and development of every single creature, more especially of those which are propagated by fissiparous generation. This subject has been here noticed, because we may infer that, when any part or organ is either greatly increased in size or wholly suppressed through variation and continued selection, the co-ordinating power of the organisation will continually tend to bring all the parts again into harmony with each other.
On the Effects of the Increased Use and Disuse of Organs.
It is notorious, and we shall immediately adduce proofs, that increased use or action strengthens muscles, glands, sense-organs, &c.; and that disuse, on the other hand, weakens them. I have not met with any clear explanation of this fact in works on Physiology. Mr. Herbert Spencer[727] maintains that when muscles are much used, or when intermittent pressure is applied to the epidermis, an excess of nutritive matter exudes from the vessels, and that this gives additional development to the adjoining parts. That an increased flow of blood towards an organ leads to its greater development is probable, if not certain. Mr. Paget[728] thus accounts for the long, thick, and dark-coloured hair which occasionally grows, even in young children, near old-standing inflamed surfaces or fractured bones. When Hunter {296} inserted the spur of a cock into the comb, which is well supplied with blood-vessels, it grew in one case in a spiral direction to a length of six inches, and in another case forward, like a horn, so that the bird could not touch the ground with its beak. But whether Mr. Herbert Spencer's view of the exudation of nutritive matter due to increased movement and pressure, will fully account for the augmented size of bones, ligaments, and especially of internal glands and nerves, seems doubtful. According to the interesting observations of M. Sedillot,[729] when a portion of one bone of the leg or fore-arm of an animal is removed and is not replaced by growth, the associated bone enlarges till it attains a bulk equal to that of the two bones, of which it has to perform the functions. This is best exhibited in dogs in which the tibia has been removed; the companion bone, which is naturally almost filiform and not one-fifth the size of the other, soon acquires a size equal to or greater than the tibia. Now, it is at first difficult to believe that increased weight acting on a straight bone could, by alternately increased and diminished pressure, cause nutritive matter to exude from the vessels which permeate the periosteum. Nevertheless, the observations adduced by Mr. Spencer,[730] on the strengthening of the bowed bones of rickety children, along their concave sides, leads to the belief that this is possible.
Mr. H. Spencer has also shown that the ascent of the sap in trees is aided by the rocking movement caused by the wind; and the sap strengthens the trunk "in proportion to the stress to be borne; since the more severe and the more repeated the strains, the greater must be the exudation from the vessels into the surrounding tissue, and the greater the thickening of this tissue by secondary deposits."[731] But woody trunks may be formed of hard tissue without their having been subjected to any movement, as we see with ivy closely attached to old walls. In all these cases, it is very difficult to disentangle the effects of long-continued selection from those consequent on the increased action or movement of the part. Mr. H. Spencer[732] acknowledges this difficulty, and gives as an instance the spines {297} or thorns of trees, and the shells of nuts. Here we have extremely hard woody tissue without the possibility of any movement to cause exudation, and without, as far as we can see, any other directly exciting cause; and as the hardness of these parts is of manifest service to the plant, we may look at the result as probably due to the selection of so-called spontaneous variations. Every one knows that hard work thickens the epidermis on the hands; and when we hear that with infants long before their birth the epidermis is thicker on the palms and soles of the feet than on any other part of the body, as was observed with admiration by Albinus,[733] we are naturally inclined to attribute this to the inherited effects of long-continued use or pressure. We are tempted to extend the same view even to the hoofs of quadrupeds; but who will pretend to determine how far natural selection may have aided in the formation of structures of such obvious importance to the animal?
That use strengthens the muscles may be seen in the limbs of artisans who follow different trades; and when a muscle is strengthened, the tendons, and the crests of bone to which they are attached, become enlarged; and this must likewise be the case with the blood-vessels and nerves. On the other hand, when a limb is not used, as by Eastern fanatics, or when the nerve supplying it with nervous power is effectually destroyed, the muscles wither. So again, when the eye is destroyed the optic nerve becomes atrophied, sometimes even in the course of a few months.[734] The Proteus is furnished with branchiæ as well as with lungs: and Schreibers[735] found that when the animal was compelled to live in deep water the branchiæ were developed to thrice their ordinary size, and the lungs were partially atrophied. When, on the other hand, the animal was compelled to live in shallow water, the lungs became larger and more vascular, whilst the branchiæ disappeared in a more or less complete degree. Such modifications as these are, however, of comparatively little value for us, as we do not actually know that they tend to be inherited.
In many cases there is reason to believe that the lessened use of various organs has affected the corresponding parts in the offspring. But there is no good evidence that this ever follows in the course of a single generation. It appears, as in the case of general or indefinite variability, that several generations must be subjected to changed habits for any appreciable result. Our domestic fowls, ducks, and geese have almost lost, not {298} only in the individual but in the race, their power of flight; for we do not see a chicken, when frightened, take flight like a young pheasant. Hence I was led carefully to compare the limb-bones of fowls, ducks, pigeons, and rabbits, with the same bones in the wild parent-species. As the measurements and weights were fully given in the earlier chapters, I need here only recapitulate the results. With domestic pigeons, the length of the sternum, the prominence of its crest, the length of the scapulæ and furcula, the length of the wings as measured from tip to tip of the radius, are all reduced relatively to the same parts in the wild pigeon. The wing and tail feathers, however, are increased in length, but this may have as little connection with the use of the wings or tail, as the lengthened hair on a dog with the amount of exercise which the breed has habitually taken. The feet of pigeons, except in the long-beaked races, are reduced in size. With fowls the crest of the sternum is less prominent, and is often distorted or monstrous; the wing-bones have become lighter relatively to the leg-bones, and are apparently a little shorter in comparison with those of the parent-form, the Gallus bankiva. With ducks, the crest of the sternum is affected in the same manner as in the foregoing cases: the furcula, coracoids, and scapulæ are all reduced in weight relatively to the whole skeleton: the bones of the wings are shorter and lighter, and the bones of the legs longer and heavier, relatively to each other, and relatively to the whole skeleton, in comparison with the same bones in the wild-duck. The decreased weight and size of the bones, in the foregoing cases, is probably the indirect result of the reaction of the weakened muscles on the bones. I failed to compare the feathers of the wings of the tame and wild duck; but Gloger[736] asserts that in the wild duck the tips of the wing-feathers reach almost to the end of the tail, whilst in the domestic duck they often hardly reach to its base. He remarks, also, on the greater thickness of the legs, and says that the swimming membrane between the toes is reduced; but I was not able to detect this latter difference.
With the domesticated rabbit the body, together with the whole skeleton, is generally larger and heavier than in the wild animal, and the leg-bones are heavier in due proportion; but whatever standard of comparison be taken, neither the leg-bones nor the scapulæ have increased in length proportionally with the increased dimensions of the rest of the skeleton. The skull has become in a marked manner narrower, and, from the measurements of its capacity formerly given, we may conclude, that this narrowness results from the decreased size of the brain, consequent on the mentally inactive life led by these closely-confined animals.
We have seen in the eighth chapter that silk-moths, which have been kept during many centuries closely confined, emerge from their cocoons with their wings distorted, incapable of flight, often greatly reduced in size, or even, according to Quatrefages, quite rudimentary. This condition of the wings may be largely owing to the same kind of monstrosity which often affects wild Lepidoptera when artificially reared from the cocoon; or it may {299} be in part due to an inherent tendency, which is common to the females of many Bombycidæ, to have their wings in a more or less rudimentary state; but part of the effect may probably be attributed to long-continued disuse.
From the foregoing facts there can be no doubt that certain parts of the skeleton in our anciently domesticated animals, have been modified in length and weight by the effects of decreased or increased use; but they have not been modified, as shown in the earlier chapters, in shape or structure. We must, however, be cautious in extending this latter conclusion to animals living a free life; for these will occasionally be exposed during successive generations to the severest competition. With wild animals it would be an advantage in the struggle for life that every superfluous and useless detail of structure should be removed or absorbed; and thus the reduced bones might ultimately become changed in structure. With highly-fed domesticated animals, on the other hand, there is no economy of growth; nor any tendency to the elimination of trifling and superfluous details of structure.
Turning now to more general observations, Nathusius has shown that, with the improved races of the pig, the shortened legs and snout, the form of the articular condyles of the occiput, and the position of the jaws with the upper canine teeth projecting in a most anomalous manner in front of the lower canines, may be attributed to these parts not having been fully exercised. For the highly-cultivated races do not travel in search of food, nor root up the ground with their ringed muzzles. These modifications of structure, which are all strictly inherited, characterise several improved breeds, so that they cannot have been derived from any single domestic or wild stock.[737] With respect to cattle, Professor Tanner has remarked that the lungs and liver in the improved breeds "are found to be considerably reduced in size when compared with those possessed by animals having perfect liberty;"[738] and the reduction of these organs affects the general shape of the body. The cause of the reduced lungs in highly-bred animals which take little exercise is {300} obvious; and perhaps the liver may be affected by the nutritious and artificial food on which they largely subsist.
It is well known that, when an artery is tied, the anastomosing branches, from being forced to transmit more blood, increase in diameter; and this increase cannot be accounted for by mere extension, as their coats gain in strength. Mr. Herbert Spencer[739] has argued that with plants the flow of sap from the point of supply to the growing part first elongates the cells in this line; and that the cells then become confluent, thus forming the ducts; so that, on this view, the vessels in plants are formed by the mutual reaction of the flowing sap and cellular tissue. Dr. W. Turner has remarked,[740] with respect to the branches of arteries, and likewise to a certain extent with nerves, that the great principle of compensation frequently comes into play; for "when two nerves pass to adjacent cutaneous areas, an inverse relation as regards size may subsist between them; a deficiency in one may be supplied by an increase in the other, and thus the area of the former may be trespassed on by the latter nerve." But how far in these cases the difference in size in the nerves and arteries is due to original variation, and how far to increased use or action, is not clear.
In reference to glands, Mr. Paget observes that "when one kidney is destroyed the other often becomes much larger, and does double work."[741] If we compare the size of the udders and their power of secretion in cows which have been long domesticated, and in certain goats in which the udders nearly touch the ground, with the size and power of secretion of these organs in wild or half-domesticated animals, the difference is great. A good cow with us daily yields more than five gallons, or forty pints of milk, whilst a first-rate animal, kept, for instance, by the Damaras of South Africa,[742] "rarely gives more than two or three pints of milk daily, and, should her calf be taken from her, she absolutely refuses to give any." We may attribute the excellence of our cows, and of certain goats, partly to the continued selection of the best milking animals, and partly to the inherited effects of the increased action, through man's art, of the secreting glands.
It is notorious, as was remarked in the twelfth chapter, that short-sight is inherited; and if we compare watchmakers or engravers with, for instance, sailors, we can hardly doubt that vision continually directed towards a near object permanently affects the structure of the eye.
Veterinarians are unanimous that horses become affected with spavins, splints, ringbones, &c., from being shod, and from travelling on hard roads, and they are almost equally unanimous that these injuries are transmitted. Formerly horses were not shod in North Carolina, and it has been asserted that they did not then suffer from these diseases of the legs and feet.[743]
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Our domesticated quadrupeds are all descended, as far as is known, from species having erect ears; yet few kinds can be named, of which at least one race has not drooping ears. Cats in China, horses in parts of Russia, sheep in Italy and elsewhere, the guinea-pig in Germany, goats and cattle in India, rabbits, pigs, and dogs in all long-civilised countries, have dependent ears. With wild animals, which constantly use their ears like funnels to catch every passing sound, and especially to ascertain the direction whence it comes, there is not, as Mr. Blyth has remarked, any species with drooping ears except the elephant. Hence the incapacity to erect the ears is certainly in some manner the result of domestication; and this incapacity has been attributed by various authors[744] to disuse, for animals protected by man are not compelled habitually to use their ears. Col. Hamilton Smith[745] states that in ancient effigies of the dog, "with the exception of one Egyptian instance, no sculpture of the earlier Grecian era produces representations of hounds with completely drooping ears; those with them half pendulous are missing in the most ancient; and this character increases, by degrees, in the works of the Roman period." Godron also has remarked that "the pigs of the ancient Egyptians had not their ears enlarged and pendent."[746] But it is remarkable that the drooping of the ears, though probably the effect of disuse, is not accompanied by any decrease in size; on the contrary, when we remember that animals so different as fancy rabbits, certain Indian breeds of the goat, our petted spaniels, bloodhounds, and other dogs, have enormously elongated ears, it would appear as if disuse actually caused an increase in length. With rabbits, the drooping of the much elongated ears has affected even the structure of the skull.
The tail of no wild animal, as remarked to me by Mr. Blyth, is curled; whereas pigs and some races of dogs have their tails much curled. This deformity, therefore, appears to be the result of domestication, but whether in any way connected with the lessened use of the tail is doubtful.
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The epidermis on our hands is easily thickened, as every one knows, by hard work. In a district of Ceylon the sheep have "horny callosities that defend their knees, and which arise from their habit of kneeling down to crop the short herbage, and this distinguishes the Jaffna flocks from those of other portions of the island;" but it is not stated whether this peculiarity is inherited.[747]
The mucous membrane which lines the stomach is continuous with the external skin of the body; therefore it is not surprising that its texture should be affected by the nature of the food consumed, but other and more interesting changes likewise follow. Hunter long ago observed that the muscular coat of the stomach of a gull (Larus tridactylus) which had been fed for a year chiefly on grain was thickened; and, according to Dr. Edmondston, a similar change periodically occurs in the Shetland Islands in the stomach of the Larus argentatus, which in the spring frequents the corn-fields and feeds on the seed. The same careful observer has noticed a great change in the stomach of a raven which had been long fed on vegetable food. In the case of an owl (Strix grallaria) similarly treated, Menetries states that the form of the stomach was changed, the inner coat became leathery, and the liver increased in size. Whether these modifications in the digestive organs would in the course of generations become inherited is not known.[748]
The increased or diminished length of the intestines, which apparently results from changed diet, is a more remarkable case, because it is characteristic of certain animals in their domesticated condition, and therefore must be inherited. The complex absorbent system, the blood-vessels, nerves, and muscles, are necessarily all modified together with the intestines. According to Daubenton, the intestines of the domestic cat are one-third longer than those of the wild cat of Europe; and although this species is not the parent-stock of the domestic animal, yet, as Isidore Geoffroy has remarked, the several species {303} of cats are so closely allied that the comparison is probably a fair one. The increased length appears to be due to the domestic cat being less strictly carnivorous in its diet than any wild feline species; I have seen a French kitten eating vegetables as readily as meat. According to Cuvier, the intestines of the domesticated pig exceed greatly in proportionate length those of the wild boar. In the tame and wild rabbit the change is of an opposite nature, and probably results from the nutritious food given to the tame rabbit.[749]
* * * * *
Changed Habits of Life, independently of the Use or Disuse of particular Organs.--This subject, as far as the mental powers of animals are concerned, so blends into instinct, on which I shall treat in a future work, that I will here only remind the reader of the many cases which occur under domestication, and which are familiar to every one--for instance the tameness of our animals--the pointing or retrieving of dogs--their not attacking the smaller animals kept by man--and so forth. How much of these changes ought to be attributed to inherited habit, and how much to the selection of individuals which have varied in the desired manner, irrespectively of the special circumstances under which they have been kept, can seldom be told. We have already seen that animals may be habituated to a changed diet; but a few additional instances may here be given.
In the Polynesian Islands and in China the dog is fed exclusively on vegetable matter, and the taste for this kind of food is to a certain extent inherited.[750] Our sporting dogs will not touch the bones of game birds, whilst other dogs devour them with greediness. In some parts of the world sheep have been largely fed on fish. The domestic hog is fond of barley, the wild boar is said to disdain it; and the disdain is partially inherited, for some young wild pigs bred in captivity showed an aversion for this grain, whilst others of the same brood relished it.[751] One of my relations bred some young pigs from {304} a Chinese sow by a wild Alpine boar; they lived free in the park, and were so tame that they came to the house to be fed; but they would not touch swill, which was devoured by the other pigs. An animal when once accustomed to an unnatural diet, which can generally be effected only during youth, dislikes its proper food, as Spallanzani found to be the case with a pigeon which had been long fed on meat. Individuals of the same species take to new food with different degrees of readiness; one horse, it is stated, soon learned to eat meat, whilst another would have perished from hunger rather than have partaken of it.[752]
The caterpillars of the Bombyx hesperus feed in a state of nature on the leaves of the Café diable, but, after having been reared on the Ailanthus, they would not touch the Café diable, and actually died of hunger.[753]
It has been found possible to accustom marine fish to live in fresh water; but as such changes in fish, and other marine animals, have been chiefly observed in a state of nature, they do not properly belong to our present subject. The period of gestation and of maturity, as shown in the earlier chapters,--the season and the frequency of the act of breeding,--have all been greatly modified under domestication. With the Egyptian goose the rate of change in the season has been recorded.[754] The wild drake pairs with one female, the domestic drake is polygamous. Certain breeds of fowls have lost the habit of incubation. The paces of the horse, and the manner of flight in certain breeds of the pigeon, have been modified, and are inherited. The voice differs much in certain fowls and pigeons. Some breeds are clamorous and others silent, as in the Call and common duck, or in the Spitz and pointer dog. Every one knows how dogs differ from each other in their manner of hunting, and in their ardour after different kinds of game or vermin.
With plants the period of vegetation is easily changed and is inherited, as in the case of summer and winter wheat, barley, {305} and vetches; but to this subject we shall immediately return under acclimatisation. Annual plants sometimes become perennial under a new climate, as I hear from Dr. Hooker is the case with the stock and mignonette in Tasmania. On the other hand, perennials sometimes become annuals, as with the Ricinus in England, and as, according to Captain Mangles, with many varieties of the heartsease. Von Berg[755] raised from seed of Verbascum phoenicium, which is usually a biennial, both annual and perennial varieties. Some deciduous bushes become evergreen in hot countries.[756] Rice requires much water, but there is one variety in India which can be grown without irrigation.[757] Certain varieties of the oat and of our other cereals are best fitted for certain soils.[758] Endless similar facts could be given in the animal and vegetable kingdoms. They are noticed here because they illustrate analogous differences in closely allied natural species, and because such changed habits of life, whether due to use and disuse, or to the direct action of external conditions, or to so-called spontaneous variation, would be apt to lead to modifications of structure.
* * * * *
Acclimatisation.--From the previous remarks we are naturally led to the much disputed subject of acclimatisation. There are two distinct questions: Do varieties descended from the same species differ in their power of living under different climates? And secondly, if they so differ, how have they become thus adapted? We have seen that European dogs do not succeed well in India, and it is asserted,[759] that no one has succeeded in there keeping the Newfoundland long alive; but then it may be argued, probably with truth, that these northern breeds are specifically distinct from the native dogs which flourish in India. The same remark may be made with respect to different breeds of sheep, of which, according to Youatt,[760] not one brought "from a torrid climate lasts out the second year," in the Zoological Gardens. But sheep are capable of some degree of acclimatisation, for Merino sheep bred at the Cape of Good Hope have been found {306} far better adapted for India than those imported from England.[761] It is almost certain that the breeds of the fowl are descended from the same species; but the Spanish breed, which there is good reason to believe originated near the Mediterranean,[762] though so fine and vigorous in England, suffers more from frost than any other breed. The Arrindy silk-moth introduced from Bengal, and the Ailanthus moth from the temperate province of Shan Tung, in China, belong to the same species, as we may infer from their identity in the caterpillar, cocoon, and mature states;[763] yet they differ much in constitution: the Indian form "will flourish only in warm latitudes," the other is quite hardy and withstands cold and rain.
Plants are more strictly adapted to climate than are animals. The latter when domesticated withstand such great diversities of climate, that we find nearly the same species in tropical and temperate countries; whilst the cultivated plants are widely dissimilar. Hence a larger field is open for inquiry in regard to the acclimatisation of plants than of animals. It is no exaggeration to say that with almost every plant which has long been cultivated varieties exist, which are endowed with constitutions fitted for very different climates; I will select only a few of the more striking cases, as it would be tedious to give all. In North America numerous fruit-trees have been raised, and in horticultural publications,--for instance, in Downing,--lists are given of the varieties which are best able to withstand the severe climate of the northern States and Canada. Many American varieties of the pear, plum, and peach are excellent in their own country, but until recently hardly one was known that succeeded in England; and with apples,[764] not one succeeds. Though the American varieties can withstand a severer winter than ours, the summer here is not hot enough. Fruit-trees have originated in Europe as in America with different constitutions, but they are not here much noticed, as the same nurserymen do not supply a wide area. The Forelle pear flowers early, and when the flowers have just set, and this is the critical period, they have been observed, both in France and England, to withstand with complete impunity a frost of 18° and even 14° Fahr., which killed the flowers, whether fully expanded or in bud, of all other kinds of pears.[765] This power in the flower of resisting cold and afterwards producing fruit does not invariably depend, as we know on good authority,[766] on general constitutional vigour.
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In proceeding northward, the number of varieties which are enabled to resist the climate rapidly decreases, as may be seen in the list of the varieties of the cherry, apple, and pear, which can be cultivated in the neighbourhood of Stockholm.[767] Near Moscow, Prince Troubetzkoy planted for experiment in the open ground several varieties of the pear, but one alone, the Poire sans Pepins, withstood the cold of winter.[768] We thus see that our fruit-trees, like distinct species of the same genus, certainly differ from each other in their constitutional adaptation to different climates.
With the varieties of many plants, the adaptation to climate is often very close. Thus it has been proved by repeated trials "that few if any of the English varieties of wheat are adapted for cultivation in Scotland;"[769] but the failure in this case is at first only in the quantity, though ultimately in the quality, of the grain produced. The Rev. J. M. Berkeley sowed wheat-seed from India, and got "the most meagre ears," on land which would certainly have yielded a good crop from English wheat.[770] In these cases varieties have been carried from a warmer to a cooler climate; in the reverse case, as "when wheat was imported directly from France into the West Indian Islands, it produced either wholly barren spikes or furnished with only two or three miserable seeds, while West Indian seed by its side yielded an enormous harvest."[771] Here is another case of close adaptation to a slightly cooler climate; a kind of wheat which in England may be used indifferently either as a winter or summer variety, when sown under the warmer climate of Grignan, in France, behaved exactly as if it had been a true winter wheat.[772]
Botanists believe that all the varieties of maize belong to the same species; and we have seen that in North America, in proceeding northward, the varieties cultivated in each zone produce their flowers and ripen their seed within shorter and shorter periods. So that the tall, slowly maturing southern varieties do not succeed in New England, and the New English varieties do not succeed in Canada. I have not met with any statement that the southern varieties are actually injured or killed by a degree of cold which the northern varieties withstand with impunity, though this is probable; but the production of early flowering and early seeding varieties deserves to be considered as one form of acclimatisation. Hence it has been found possible, according to Kalm, to cultivate maize further and further northwards in America. In Europe, also, as we learn from the evidence given by Alph. De Candolle, the culture of maize has extended since the end of the last century thirty leagues north of its former boundary.[773] On the authority of the great Linnæus,[774] I may quote an {308} analogous case, namely, that in Sweden tobacco raised from home-grown seed ripens its seed a month sooner and is less liable to miscarry than plants raised from foreign seed.
With the Vine, differently from the maize, the line of practical culture has retreated a little southward since the middle ages;[775] but this seems due to commerce, including that of wine, being now freer or more easy. Nevertheless the fact of the vine not having spread northward shows that acclimatisation has made no progress during several centuries. There is, however, a marked difference in the constitution of the several varieties,--some being hardy, whilst others, like the muscat of Alexandria, require a very high temperature to come to perfection. According to Labat,[776] vines taken from France to the West Indies succeed with extreme difficulty, whilst those imported from Madeira, or the Canary Islands, thrive admirably.
Gallesio gives a curious account of the naturalisation of the Orange in Italy. Daring many centuries the sweet orange was propagated exclusively by grafts, and so often suffered from frosts that it required protection. After the severe frost of 1709, and more especially after that of 1763, so many trees were destroyed that seedlings from the sweet orange were raised, and, to the surprise of the inhabitants, their fruit was found to be sweet. The trees thus raised were larger, more productive, and hardier than the former kinds; and seedlings are now continually raised. Hence Gallesio concludes that much more was effected for the naturalisation of the orange in Italy by the accidental production of new kinds during a period of about sixty years, than had been effected by grafting old varieties during many ages.[777] I may add that Risso[778] describes some Portuguese varieties of the orange as extremely sensitive to cold, and as much tenderer than certain other varieties.
The peach was known to Theophrastus, 322 B.C.[779] According to the authorities quoted by Dr. F. Rolle,[780] it was tender when first introduced into Greece, and even in the island of Rhodes only occasionally bore fruit. If this be correct, the peach, in spreading during the last two thousand years over the middle parts of Europe, must have become much hardier. At the present day different varieties differ much in hardiness: some French varieties will not succeed in England; and near Paris, the Pavie de Bonneuil does not ripen its fruit till very late, even when grown on a wall; "it is, therefore, only fit for a very hot southern climate."[781]
I will briefly give a few other cases. A variety of Magnolia grandiflora, raised by M. Roy, withstands cold several degrees lower than that which any other variety can resist. With camellias there is much difference in hardiness. One particular variety of Noisette rose withstood the severe frost of 1860 "untouched and hale amidst a universal destruction of other {309} Noisettes." In New York the "Irish yew is quite hardy, but the common yew is liable to be cut down." I may add that there are varieties of the sweet potato (Convolvulus batatas) which are suited for warmer, as well as for colder, climates.[782]
The plants as yet mentioned have been found capable of resisting an unusual degree of cold or heat, when fully grown. The following cases refer to plants whilst young. In a large bed of young Araucarias of the same age, growing close together and equally exposed, it was observed,[783] after the unusually severe winter of 1860-61, that, "in the midst of the dying, numerous individuals remained on which the frost had absolutely made no kind of impression." Dr. Lindley, after alluding to this and other similar cases, remarks, "Among the lessons which the late formidable winter has taught us, is that, even in their power of resisting cold, individuals of the same species of plants are remarkably different." Near Salisbury, there was a sharp frost on the night of May 24th, 1836, and all the French beans (Phaseolus vulgaris) in a bed were killed except about one in thirty, which completely escaped.[784] On the same day of the month, but in the year 1864, there was a severe frost in Kent, and two rows of scarlet-runners (P. multiflorus) in my garden, containing 390 plants of the same age and equally exposed, were all blackened and killed except about a dozen plants. In an adjoining row of "Fulmer's dwarf bean" (P. vulgaris), one single plant escaped. A still more severe frost occurred four days afterwards, and of the dozen plants which had previously escaped only three survived; these were not taller or more vigorous than the other young plants, but they escaped completely, with not even the tips of their leaves browned. It was impossible to behold these three plants, with their blackened, withered, and dead brethren all round them, and not see at a glance that they differed widely in constitutional power of resisting frost.
This work is not the proper place to show that wild plants {310} of the same species, naturally growing at different altitudes or under different latitudes, become to a certain extent acclimatised, as is proved by the different behaviour of their seedlings when raised in England. In my 'Origin of Species' I have alluded to some cases, and I could add others. One instance must suffice: Mr. Grigor, of Forres,[785] states that seedlings of the Scotch fir (Pinus sylvestris), raised from seed from the Continent and from the forests of Scotland, differ much. "The difference is perceptible in one-year-old, and more so in two-year-old seedlings; but the effects of the winter on the second year's growth almost uniformly makes those from the Continent quite brown, and so damaged, that by the month of March they are quite unsaleable, while the plants from the native Scotch pine, under the same treatment, and standing alongside, although considerably shorter, are rather stouter and quite green, so that the beds of the one can be known from the other when seen from the distance of a mile." Closely similar facts have been observed with seedling larches.
Hardy varieties would alone be valued or noticed in Europe; whilst tender varieties, requiring more warmth, would generally be neglected; but such occasionally arise. Thus Loudon[786] describes a Cornish variety of the elm which is almost an evergreen, and of which the shoots are often killed by the autumnal frosts, so that its timber is of little value. Horticulturists know that some varieties are much more tender than others: thus all the varieties of the broccoli are more tender than cabbages; but there is much difference in this respect in the sub-varieties of the broccoli; the pink and purple kinds are a little hardier than the white Cape broccoli, "but they are not to be depended on after the thermometer falls below 24° Fahr.:" the Walcheren broccoli is less tender than the Cape, and there are several varieties which will stand much severer cold than the Walcheren.[787] Cauliflowers seed more freely in India than cabbages.[788] To give one instance with flowers: eleven plants raised from a hollyhock, called the Queen of the Whites,[789] were found to be much more tender than various other seedlings. It may be presumed that all tender varieties would succeed better under a climate warmer than ours. With fruit-trees, it is well known that certain varieties, for instance of the peach, stand forcing in a hot-house better than others; and this shows {311} either pliability of organisation or some constitutional difference. The same individual cherry-tree, when forced, has been observed during successive years gradually to change its period of vegetation.[790] Few pelargoniums can resist the heat of a stove, but Alba multiflora will, as a most skilful gardener asserts, "stand pine-apple top and bottom heat the whole winter, without looking any more drawn than if it had stood in a common greenhouse; and Blanche Fleur seems as if it had been made on purpose for growing in winter, like many bulbs, and to rest all summer."[791] There can hardly be a doubt that the Alba multiflora pelargonium must have a widely different constitution from that of most other varieties of this plant; it would probably withstand even an equatorial climate.
We have seen that according to Labat the vine and wheat require acclimatisation in order to succeed in the West Indies. Similar facts have been observed at Madras: "two parcels of mignonette-seed, one direct from Europe, the other saved at Bangalore (of which the mean temperature is much below that of Madras) were sown at the same time: they both vegetated equally favourably, but the former all died off a few days after they appeared above ground; the latter still survive, and are vigorous healthy plants." So again, "turnip and carrot seed saved at Hyderabad are found to answer better at Madras than seed from Europe or from the Cape of Good Hope."[792] Mr. J. Scott, of the Calcutta Botanic Gardens, informs me that seeds of the sweet-pea (Lathyrus odoratus) imported from England produce plants, with thick, rigid stems and small leaves, which rarely blossom and never yield seed; plants raised from French seed blossom sparingly, but all the flowers are sterile; on the other hand, plants raised from sweet-peas grown near Darjeeling in Upper India, but originally derived from England, can be successfully cultivated on the plains of India; for they flower and seed profusely, and their stems are lax and scandent. In some of the foregoing cases, as Dr. Hooker has remarked to me, the greater success may perhaps be attributed to the seeds having been more fully ripened under a more favourable climate; but this view can hardly be extended to so many cases, including plants, which, from being cultivated under a climate hotter than their native one, become fitted for a still hotter climate. We may therefore safely conclude that plants can to a certain extent become accustomed to a climate either hotter or colder than their own; although these latter cases have been more frequently observed.
We will now consider the means by which acclimatisation may be effected, namely, through the spontaneous appearance of varieties having a different constitution, and through the effects of use or habit. In regard to the first process, there is no evidence that a change in the constitution of the {312} offspring necessarily stands in any direct relation with the nature of the climate inhabited by the parents. On the contrary, it is certain that hardy and tender varieties of the same species appear in the same country. New varieties thus spontaneously arising become fitted to slightly different climates in two different ways; firstly, they may have the power, either as seedlings or when full-grown, of resisting intense cold, as with the Moscow pear, or of resisting intense heat, as with some kinds of Pelargonium, or the flowers may withstand severe frost, as with the Forelle pear. Secondly, plants may become adapted to climates widely different from their own, from flowering and fruiting either earlier or later in the season. In both these cases the power of acclimatisation by man consists simply in the selection and preservation of new varieties. But without any direct intention on his part of securing a hardier variety, acclimatisation may be unconsciously effected by merely raising tender plants from seed, and by occasionally attempting their cultivation further and further northwards, as in the case of maize, the orange, and the peach.
How much influence ought to be attributed to inherited habit or custom in the acclimatisation of animals and plants is a much more difficult question. In many cases natural selection can hardly have failed to have come into play and complicated the result. It is notorious that mountain sheep resist severe weather and storms of snow which would destroy lowland breeds; but then mountain sheep have been thus exposed from time immemorial, and all delicate individuals will have been destroyed, and the hardiest preserved. So with the Arrindy silk-moths of China and India; who can tell how far natural selection may have taken a share in the formation of the two races, which are now fitted for such widely different climates? It seems at first probable that the many fruit-trees, which are so well fitted for the hot summers and cold winters of North America, in contrast with their poor success under our climate, have become adapted through habit; but when we reflect on the multitude of seedlings annually raised in that country, and that none would succeed unless born with a fitting constitution, it is possible that mere habit may have done nothing towards their acclimatisation. On the other hand, when we {313} hear that Merino sheep, bred during no great number of generations at the Cape of Good Hope--that some European plants raised during only a few generations in the cooler parts of India, withstand the hotter parts of that country much better than the sheep or seeds imported directly from England, we must attribute some influence to habit. We are led to the same conclusion when we hear from Naudin[793] that the races of melons, squashes, and gourds, which have long been cultivated in Northern Europe, are comparatively more precocious, and need much less heat for maturing their fruit, than the varieties of the same species recently brought from tropical regions. In the reciprocal conversion of summer and winter wheat, barley, and vetches into each other, habit produces a marked effect in the course of a very few generations. The same thing apparently occurs with the varieties of maize, which, when carried from the Southern to the Northern States of America, or into Germany, soon become accustomed to their new homes. With vine-plants taken to the West Indies from Madeira, which are said to succeed better than plants brought directly from France, we have some degree of acclimatisation in the individual, independently of the production of new varieties by seed.
The common experience of agriculturists is of some value, and they often advise persons to be cautious in trying in one country the productions of another. The ancient agricultural writers of China recommend the preservation and cultivation of the varieties peculiar to each country. During the classical period, Columella wrote, "Vernaculum pecus peregrino longe præstantius est."[794]
I am aware that the attempt to acclimatise either animals or plants has been called a vain chimæra. No doubt the attempt in most cases deserves to be thus called, if made independently of the production of new varieties endowed with a different constitution. Habit, however much prolonged, rarely produces any effect on a plant propagated by buds; it apparently acts only through successive seminal generations. {314} The laurel, bay, laurestinus, &c., and the Jerusalem artichoke, which are propagated by cuttings or tubers, are probably now as tender in England as when first introduced; and this appears to be the case with the potato, which until recently was seldom multiplied by seed. With plants propagated by seed, and with animals, there will be little or no acclimatisation unless the hardier individuals are either intentionally or unconsciously preserved. The kidney-bean has often been advanced as an instance of a plant which has not become hardier since its first introduction into Britain. We hear, however, on excellent authority,[795] that some very fine seed, imported from abroad, produced plants "which blossomed most profusely, but were nearly all but abortive, whilst plants grown alongside from English seed podded abundantly;" and this apparently shows some degree of acclimatisation in our English plants. We have also seen that seedlings of the kidney-bean occasionally appear with a marked power of resisting frost; but no one, as far as I can hear, has ever separated such hardy seedlings, so as to prevent accidental crossing, and then gathered their seed, and repeated the process year after year. It may, however, be objected with truth that natural selection ought to have had a decided effect on the hardiness of our kidney-beans; for the tenderest individuals must have been killed during every severe spring, and the hardier preserved. But it should be borne in mind that the result of increased hardiness would simply be that gardeners, who are always anxious for as early a crop as possible, would sow their seed a few days earlier than formerly. Now, as the period of sowing depends much on the soil and elevation of each district, and varies with the season; and as new varieties have often been imported from abroad, can we feel sure that our kidney-beans are not somewhat hardier? I have not been able, by searching old horticultural works, to answer this question satisfactorily.
On the whole the facts now given show that, though habit does something towards acclimatisation, yet that the spontaneous appearance of constitutionally different individuals is a far more effective agent. As no single instance has been recorded, either with animals or plants, of hardier individuals {315} having been long and steadily selected, though such selection is admitted to be indispensable for the improvement of any other character, it is not surprising that man has done little in the acclimatisation of domesticated animals and cultivated plants. We need not, however, doubt that under nature new races and new species would become adapted to widely different climates, by spontaneous variation, aided by habit, and regulated by natural selection.
Arrests of Development: Rudimentary and Aborted Organs.
These subjects are here introduced because there is reason to believe that rudimentary organs are in many cases the result of disuse. Modifications of structure from arrested development, so great or so serious as to deserve to be called monstrosities, are of common occurrence, but, as they differ much from any normal structure, they require here only a passing notice. When a part or organ is arrested during its embryonic growth, a rudiment is generally left. Thus the whole head may be represented by a soft nipple-like projection, and the limbs by mere papillæ. These rudiments of limbs are sometimes inherited, as has been observed in a dog.[796]
Many lesser anomalies in our domesticated animals appear to be due to arrested development. What the cause of the arrest may be, we seldom know, except in the case of direct injury to the embryo within the egg or womb. That the cause does not generally act at a very early embryonic period we may infer from the affected organ seldom being wholly aborted,--a rudiment being generally preserved. The external ears are represented by mere vestiges in a Chinese breed of sheep; and in another breed, the tail is reduced "to a little button, suffocated, in a manner, by fat."[797] In tailless dogs and cats a stump is left; but I do not know whether it includes at an early embryonic age rudiments of all the caudal vertebræ. In certain breeds of fowls the comb and wattles are reduced to rudiments; in the Cochin-China breed scarcely more than rudiments of spurs exist. With polled Suffolk cattle, "rudiments of horns can often be felt at an early age;"[798] and with species in a state of nature, the relatively greater development of rudimentary organs at an early period of life is highly characteristic of such organs. With hornless breeds of cattle and sheep; another and singular kind of rudiment has been observed, namely, minute dangling horns attached to the skin alone, and which are often shed and grow again. With hornless goats, according to Desmarest,[799] {316} the bony protuberances which properly support the horns exist as mere rudiments.
With cultivated plants it is far from rare to find the petals, stamens, and pistils represented by rudiments, like those observed in natural species. So it is with the whole seed in many fruits; thus near Astrakhan there is a grape with mere traces of seeds, "so small and lying so near the stalk that they are not perceived in eating the grape."[800] In certain varieties of the gourd, the tendrils, according to Naudin, are represented by rudiments or by various monstrous growths. In the broccoli and cauliflower the greater number of the flowers are incapable of expansion, and include rudimentary organs. In the Feather hyacinth (Muscari comosum) the upper and central flowers are brightly coloured but rudimentary; under cultivation the tendency to abortion travels downwards and outwards, and all the flowers become rudimentary; but the abortive stamens and pistils are not so small in the lower as in the upper flowers. In the Viburnum opulus, on the other hand, the outer flowers naturally have their organs of fructification in a rudimentary state, and the corolla is of large size; under cultivation, the change spreads to the centre, and all the flowers become affected; thus the well-known Snow-ball bush is produced. In the Compositæ, the so-called doubling of the flowers consists in the greater development of the corolla of the central florets, generally accompanied with some degree of sterility; and it has been observed[801] that the progressive doubling invariably spreads from the circumference to the centre,--that is, from the ray florets, which so often include rudimentary organs, to those of the disc. I may add, as bearing on this subject, that, with Asters, seeds taken from the florets of the circumference have been found to yield the greatest number of double flowers.[802] In these several cases we have a natural tendency in certain parts to become rudimentary, and this under culture spreads either to, or from, the axis of the plant. It deserves notice, as showing how the same laws govern the changes which natural species and artificial varieties undergo, that in a series of species in the genus Carthamus, one of the Compositæ, a tendency in the seeds to the abortion of the pappus may be traced extending from the circumference to the centre of the disc: thus, according to A. de Jussieu,[803] the abortion is only partial in Carthamus creticus, but more extended in C. lanatus; for in this species two or three alone of the central seeds are furnished with a pappus, the surrounding seeds being either quite naked or furnished with a few hairs; and lastly, in C. tinctorius, even the central seeds are destitute of pappus, and the abortion is complete.
With animals and plants under domestication, when an organ disappears, leaving only a rudiment, the loss has generally been sudden, as with hornless and tailless breeds; and such cases may be ranked as inherited monstrosities. But in some few cases the loss has been gradual, and {317} has been partly effected by selection, as with the rudimentary combs and wattles of certain fowls. We have also seen that the wings of some domesticated birds have been slightly reduced by disuse, and the great reduction of the wings in certain silk-moths, with mere rudiments left, has probably been aided by disuse.
With species in a state of nature, rudimentary organs are so extremely common that scarcely one can be named which is wholly free from a blemish of this nature. Such organs are generally variable, as several naturalists have observed; for, being useless, they are not regulated by natural selection, and they are more or less liable to reversion. The same rule certainly holds good with parts which have become rudimentary under domestication. We do not know through what steps under nature rudimentary organs have passed in being reduced to their present condition; but we so incessantly see in species of the same group the finest gradations between an organ in a rudimentary and perfect state, that we are led to believe that the passage must have been extremely gradual. It may be doubted whether a change of structure so abrupt as the sudden loss of an organ would ever be of service to a species in a state of nature; for the conditions to which all organisms are closely adapted usually change very slowly. Even if an organ did suddenly disappear in some one individual by an arrest of development, intercrossing with the other individuals of the same species would cause it to reappear in a more or less perfect manner, so that its final reduction could only be effected by the slow process of continued disuse or natural selection. It is much more probable that, from changed habits of life, organs first become of less and less use, and ultimately superfluous; or their place may be supplied by some other organ; and then disuse, acting on the offspring through inheritance at corresponding periods of life, would go on reducing the organ; but as most organs could be of no use at an early embryonic period, they would not be affected by disuse; consequently they would be preserved at this stage of growth, and would remain as rudiments. In addition to the effects of disuse, the principle of economy of growth, already alluded to in this chapter, would lead to the still further reduction of all superfluous parts. With respect to the final and total suppression or abortion of any organ, another and distinct principle, which will be discussed in the chapter on pangenesis, probably takes a share in the work.
With animals and plants reared by man there is no severe or recurrent struggle for existence, and the principle of economy will not come into action. So far, indeed, is this from being the case, that in some instances organs, which are naturally rudimentary in the parent-species, become partially redeveloped in the domesticated descendants. Thus cows, like most other ruminants, properly have four active and two rudimentary mammæ; but in our domesticated animals, the latter occasionally become considerably developed and yield milk. The atrophied mammæ, which, in male domesticated animals, including man, have in some rare cases grown to full size and secreted milk, perhaps offer an analogous case. The hind feet of dogs include rudiments of a fifth toe, and in certain large breeds these toes, though still rudimentary, become considerably developed {318} and are furnished with claws. In the common Hen, the spurs and comb are rudimentary, but in certain breeds these become, independently of age or disease of the ovaria, well developed. The stallion has canine teeth, but the mare has only traces of the alveoli, which, as I am informed by the eminent veterinary Mr. G. T. Brown, frequently contain minute irregular nodules of bone. These nodules, however, sometimes become developed into imperfect teeth, protruding through the gums and coated with enamel; and occasionally they grow to a third or even a fourth of the length of the canines in the stallion. With plants I do not know whether the redevelopment of rudimentary organs occurs more frequently under culture than under nature. Perhaps the pear-tree may be a case in point, for when wild it bears thorns, which though useful as a protection are formed of branches in a rudimentary condition, but, when the tree is cultivated, the thorns are reconverted into branches.
Finally, though organs which must be classed as rudimentary frequently occur in our domesticated animals and cultivated plants, these have generally been formed suddenly, through an arrest of development. They usually differ in appearance from the rudiments which so frequently characterise natural species. In the latter, rudimentary organs have been slowly formed through continued disuse, acting by inheritance at a corresponding age, aided by the principle of the economy of growth, all under the control of natural selection. With domesticated animals, on the other hand, the principle of economy is far from coming into action, and their organs, although often slightly reduced by disuse, are not thus almost obliterated with mere rudiments left.
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XXV. LAWS OF VARIATION, continued--CORRELATED VARIABILITY
EXPLANATION OF TERM--CORRELATION AS CONNECTED WITH DEVELOPMENT--MODIFICATIONS CORRELATED WITH THE INCREASED OR DECREASED SIZE OF PARTS--CORRELATED VARIATION OF HOMOLOGOUS PARTS--FEATHERED FEET IN BIRDS ASSUMING THE STRUCTURE OF THE WINGS--CORRELATION BETWEEN THE HEAD AND THE EXTREMITIES--BETWEEN THE SKIN AND DERMAL APPENDAGES--BETWEEN THE ORGANS OF SIGHT AND HEARING--CORRELATED MODIFICATIONS IN THE ORGANS OF PLANTS--CORRELATED MONSTROSITIES--CORRELATION BETWEEN THE SKULL AND EARS--SKULL AND CREST OF FEATHERS--SKULL AND HORNS--CORRELATION OF GROWTH COMPLICATED BY THE ACCUMULATED EFFECTS OF NATURAL SELECTION--COLOUR AS CORRELATED WITH CONSTITUTIONAL PECULIARITIES.
All the parts of the organisation are to a certain extent connected or correlated together; but the connexion may be so slight that it hardly exists, as with compound animals or the buds on the same tree. Even in the higher animals various parts are not at all closely related; for one part may be wholly suppressed or rendered monstrous without any other part of the body being affected. But in some cases, when one part varies, certain other parts always, or nearly always, simultaneously vary; they are then subject to the law of correlated variation. Formerly I used the somewhat vague expression of correlation of growth, which may be applied to many large classes of facts. Thus, all the parts of the body are admirably coordinated for the peculiar habits of life of each organic being, and they may be said, as the Duke of Argyll insists in his 'Reign of Law,' to be correlated for this purpose. Again, in large groups of animals certain structures always co-exist; for instance, a peculiar form of stomach with teeth of peculiar form, and such structures may in one sense be said to be correlated. But these cases have no necessary connexion with the law to be discussed in the present chapter; for we do not know that {320} the initial or primary variations of the several parts were in any way related; slight modifications or individual differences may have been preserved, first in one and then in another part, until the final and perfectly co-adapted structure was acquired; but to this subject I shall presently recur. Again, in many groups of animals the males alone are furnished with weapons, or are ornamented with gay colours; and these characters manifestly stand in some sort of correlation with the male reproductive organs, for when the latter are destroyed these characters disappear. But it was shown in the twelfth chapter that the very same peculiarity may become attached at any age to either sex, and afterwards be exclusively transmitted by the same sex at a corresponding age. In these cases we have inheritance limited by, or correlated with, both sex and age; but we have no reason for supposing that the original cause of the variation was necessarily connected with the reproductive organs, or with the age of the affected being.
In cases of true correlated variation, we are sometimes able to see the nature of the connexion; but in most cases the bond is hidden from us, and certainly differs in different cases. We can seldom say which of two correlated parts first varies, and induces a change in the other; or whether the two are simultaneously produced by some distinct cause. Correlated variation is an important subject for us; for when one part is modified through continued selection, either by man or under nature, other parts of the organisation will be unavoidably modified. From this correlation it apparently follows that, with our domesticated animals and plants, varieties rarely or never differ from each other by some single character alone.
One of the simplest cases of correlation is that a modification which arises during an early stage of growth tends to influence the subsequent development of the same part, as well as of other and intimately connected parts. Isidore Geoffroy St. Hilaire states[804] that this may constantly be observed with monstrosities {321} in the animal kingdom; and Moquin-Tandon[805] remarks, that, as with plants the axis cannot become monstrous without in some way affecting the organs subsequently produced from it, so axial anomalies are almost always accompanied by deviations of structure in the appended parts. We shall presently see that with short-muzzled races of the dog certain histological changes in the basal elements of the bones arrest their development and shorten them, and this affects the position of the subsequently developed molar teeth. It is probable that certain modifications in the larvæ of insects would affect the structure of the mature insects. But we must be very careful not to extend this view too far, for, during the normal course of development, certain members in the same group of animals are known to pass through an extraordinary course of change, whilst other and closely allied members arrive at maturity with little change of structure.
Another simple case of correlation is that with the increased or decreased dimensions of the whole body, or of any particular part, certain organs are increased or diminished in number, or are otherwise modified. Thus pigeon-fanciers have gone on selecting pouters for length of body, and we have seen that their vertebræ are generally increased in number, and their ribs in breadth. Tumblers have been selected for their small bodies, and their ribs and primary wing-feathers are generally lessened in number. Fantails have been selected for their large, widely-expanded tails, with numerous tail-feathers, and the caudal vertebræ are increased in size and number. Carriers have been selected for length of beak, and their tongues have become longer, but not in strict accordance with the length of beak. In this latter breed and in others having large feet, the number of the scutellæ on the toes is greater than in the breeds with small feet. Many similar cases could be given. In Germany it has been observed that the period of gestation is longer in large-sized than in small-sized breeds of cattle. With our highly-improved animals of all kinds the period of maturity has advanced, both with respect to the full growth of the body and the period of reproduction; and, in correspondence with this, the teeth are now developed earlier than formerly, so that, {322} to the surprise of agriculturists, the ancient rules for judging the age of an animal by the state of its teeth are no longer trustworthy.[806]
Correlated Variation of Homologous Parts.--Parts which are homologous tend to vary in the same manner; and this is what might have been expected, for such parts are identical in form and structure during an early period of embryonic development, and are exposed in the egg or womb to similar conditions. The symmetry, in most kinds of animals, of the corresponding or homologous organs on the right and left sides of the body, is the simplest case in point; but this symmetry sometimes fails, as with rabbits having only one ear, or stags with one horn, or with many-horned sheep which sometimes carry an additional horn on one side of their heads. With flowers which have regular corollas, the petals generally vary in the same manner, as we see in the same complicated and elegant pattern, on the flowers of the Chinese pink; but with irregular flowers, though the petals are of course homologous, this symmetry often fails, as with the varieties of the Antirrhinum or snapdragon, or that variety of the kidney-bean (Phaseolus multiflorus) which has a white standard-petal.
In the vertebrata the front and hind limbs are homologous, and they tend to vary in the same manner, as we see in long and short-legged, or in thick and thin-legged races of the horse and dog. Isidore Geoffroy[807] has remarked on the tendency of supernumerary digits in man to appear, not only on the right and left sides, but on the upper and lower extremities. Meckel has insisted[808] that, when the muscles of the arm depart in number or arrangement from their proper type, they almost always imitate those of the leg; and so conversely the varying muscles of the leg imitate the normal muscles of the arm.
In several distinct breeds of the pigeon and fowl, the legs and the two outer toes are heavily feathered, so that in the trumpeter pigeon they appear like little wings. In the feather-legged bantam the "boots" or feathers, which grow from the outside of the leg and generally from the two outer toes, have, {323} according to the excellent authority of Mr. Hewitt,[809] been seen to exceed the wing-feathers in length, and in one case were actually nine and a half inches in length! As Mr. Blyth has remarked to me, these leg-feathers resemble the primary wing-feathers, and are totally unlike the fine down which naturally grows on the legs of some birds, such as grouse and owls. Hence it may be suspected that excess of food has first given redundancy to the plumage, and then that the law of homologous variation has led to the development of feathers on the legs, in a position corresponding with those on the wing, namely, on the outside of the tarsi and toes. I am strengthened in this belief by the following curious case of correlation, which for a long time seemed to me utterly inexplicable, namely, that in pigeons of any breed, if the legs are feathered, the two outer toes are partially connected by skin. These two outer toes correspond with our third and fourth toes. Now, in the wing of the pigeon or any other bird, the first and fifth digits are wholly aborted; the second is rudimentary and carries the so-called "bastard-wing;" whilst the third and fourth digits are completely united and enclosed by skin, together forming the extremity of the wing. So that in feather-footed pigeons, not only does the exterior surface support a row of long feathers, like wing-feathers, but the very same digits which in the wing are completely united by skin become partially united by skin in the feet; and thus by the law of the correlated variation of homologous parts we can understand the curious connection of feathered legs and membrane between the two outer toes.
Andrew Knight[810] has remarked that the face or head and the limbs vary together in general proportions. Compare, for instance, the head and limbs of a dray and race-horse, or of a greyhound and mastiff. What a monster a greyhound would appear with the head of a mastiff! The modern bulldog, however, has fine limbs, but this is a recently-selected character. From the measurements given in the sixth chapter, we clearly see that in all the breeds of the pigeon the length of the beak and the size of the feet are correlated. The view which, as before explained, seems the most probable is, that disuse in all cases tends {324} to diminish the feet, the beak becoming at the same time through correlation shorter; but that in those few breeds in which length of beak has been a selected point, the feet, notwithstanding disuse, have through correlation increased in size.
With the increased length of the beak in pigeons, not only the tongue increases in length, but likewise the orifice of the nostrils. But the increased length of the orifice of the nostrils perhaps stands in closer correlation with the development of the corrugated skin or wattle at the base of the beak; for when there is much wattle round the eyes, the eyelids are greatly increased or even doubled in length.
There is apparently some correlation even in colour between the head and the extremities. Thus with horses a large white star or blaze on the forehead is generally accompanied by white feet.[811] With white rabbits and cattle, dark marks often co-exist on the tips of the ears and on the feet. In black and tan dogs of different breeds, tan-coloured spots over the eyes and tan-coloured feet almost invariably go together. These latter cases of connected colouring may be due either to reversion or to analogous variation,--subjects to which we shall hereafter return,--but this does not necessarily determine the question of their original correlation. If those naturalists are correct who maintain that the jaw-bones are homologous with the limb-bones, then we can understand why the head and limbs tend to vary together in shape and even in colour; but several highly competent judges dispute the correctness of this view.
The lopping forwards and downwards of the immense ears of fancy rabbits is in part due to the disuse of the muscles, and in part to the weight and length of the ears, which have been increased by selection during many generations. Now, with the increased size and changed direction of the ears, not only has the bony auditory meatus become changed in outline, direction, and greatly in size, but the whole skull has been slightly modified. This could be clearly seen in "half-lops"--that is, in rabbits with one ear alone lopping forward--for the opposite sides of their skulls were not strictly symmetrical. This seems to me a curious instance of correlation, between hard {325} bones and organs so soft and flexible, as well as so unimportant under a physiological point of view, as the external ears. The result no doubt is largely due to mere mechanical action, that is, to the weight of the ears, on the same principle that the skull of a human infant is easily modified by pressure.
The skin and the appendages of hair, feathers, hoofs, horns, and teeth, are homologous over the whole body. Every one knows that the colour of the skin and that of the hair usually vary together; so that Virgil advises the shepherd to look whether the mouth and tongue of the ram are black, lest the lambs should not be purely white. With poultry and certain ducks we have seen that the colour of the plumage stands in some connexion with the colour of the shell of the egg,--that is, with the mucous membrane which secretes the shell. The colour of the skin and hair, and the odour emitted by the glands of the skin, are said[812] to be connected, even in the same race of men. Generally the hair varies in the same way all over the body in length, fineness, and curliness. The same rule holds good with feathers, as we see with the laced and frizzled breeds both of fowls and pigeons. In the common cock the feathers on the neck and loins are always of a particular shape, called hackles: now in the Polish breed, both sexes are characterised by a tuft of feathers on the head; but through correlation these feathers in the male always assume the form of hackles. The wing and tail-feathers, though arising from parts not homologous, vary in length together; so that long or short winged pigeons generally have long or short tails. The case of the Jacobin-pigeon is more curious, for the wing and tail feathers are remarkably long; and this apparently has arisen in correlation with the elongated and reversed feathers on the back of the neck, which form the hood.
The hoofs and hair are homologous appendages; and a careful observer, namely Azara,[813] states that in Paraguay horses of various colours are often born with their hair curled and twisted like that on the head of a negro. This peculiarity is strongly inherited. But what is remarkable is that the hoofs of these horses "are absolutely like those of a mule." The hair also of the mane and tail is invariably much shorter than usual, being only from four {326} to twelve inches in length; so that curliness and shortness of the hair are here, as with the negro, apparently correlated.
With respect to the horns of sheep, Youatt[814] remarks that "multiplicity of horns is not found in any breed of much value: it is generally accompanied by great length and coarseness of the fleece." Several tropical breeds of sheep, which are clothed with hair instead of wool, have horns almost like those of a goat. Sturm[815] expressly declares that in different races the more the wool is curled the more the horns are spirally twisted. We have seen in the third chapter, where other analogous facts have been given, that the parent of the Mauchamp breed, so famous for its fleece, had peculiarly shaped horns. The inhabitants of Angora assert[816] that "only the white goats which have horns wear the fleece in the long curly locks that are so much admired; those which are not horned having a comparatively close coat." From these cases we may conclude that the hair or wool and the horns vary in a correlated manner. Those who have tried hydropathy are aware that the frequent application of cold water stimulates the skin; and whatever stimulates the skin tends to increase the growth of the hair, as is well shown in the abnormal growth of hair near old inflamed surfaces. Now, Professor Low[817] is convinced that with the different races of British cattle thick skin and long hair depend on the humidity of the climate which they inhabit. We can thus see how a humid climate might act on the horns--in the first place directly on the skin and hair, and secondly by correlation on the horns. The presence or absence of horns, moreover, both in the case of sheep and cattle, acts, as will presently be shown, by some sort of correlation on the skull.
With respect to hair and teeth, Mr. Yarrell[818] found many of the teeth deficient in three hairless "Ægyptian" dogs, and in a hairless terrier. The incisors, canines, and premolars suffered most, but in one case all the teeth, except the large tubercular molar on each side, were deficient. With man several striking cases have been recorded[819] of inherited baldness with {327} inherited deficiency, either complete or partial, of the teeth. We see the same connexion in those rare cases in which the hair has been renewed in old age, for this has "usually been accompanied by a renewal of the teeth." I have remarked in a former part of this volume that the great reduction in the size of the tusks in domestic boars probably stands in close relation with their diminished bristles, due to a certain amount of protection; and that the reappearance of the tusks in boars, which have become feral and are fully exposed to the weather, probably depends on the reappearance of the bristles. I may add, though not strictly connected with our present point, that an agriculturist[820] asserts that "pigs with little hair on their bodies are most liable to lose their tails, showing a weakness of the tegumental structure. It may be prevented by crossing with a more hairy breed."
In the previous cases deficient hair, and teeth deficient in number or size, are apparently connected. In the following cases abnormally redundant hair, and teeth either deficient or redundant, are likewise connected. Mr. Crawfurd[821] saw at the Burmese Court a man, thirty years old, with his whole body, except the hands and feet, covered with straight silky hair, which on the shoulders and spine was five inches in length. At birth the ears alone were covered. He did not arrive at puberty, or shed his milk teeth, until twenty years old; and at this period he acquired five teeth in the upper jaw, namely four incisors and one canine, and four incisor teeth in the lower jaw; all the teeth were small. This man had a daughter, who was born with hair within her ears; and the hair soon extended over her body. When Captain Yule[822] visited the Court, he found this girl grown up; and she presented a strange appearance with even her nose densely covered with soft hair. Like her father, she was furnished with incisor teeth alone. The King had with difficulty bribed a man to marry her, and of her two children, one, a boy fourteen months old, had hair growing out of his ears, with a beard and moustache. This strange peculiarity had, therefore, been inherited for three generations, with the molar teeth deficient in the grandfather and mother; whether {328} these teeth would likewise fail in the infant could not be told. Here is another case communicated to me by Mr. Wallace on the authority of Dr. Purland, a dentist: Julia Pastrana, a Spanish dancer, was a remarkably fine woman, but she had a thick masculine beard and a hairy forehead; she was photographed, and her stuffed skin was exhibited as a show; but what concerns us is, that she had in both the upper and lower jaw an irregular double set of teeth, one row being placed within the other, of which Dr. Purland took a cast. From the redundancy of the teeth her mouth projected, and her face had a gorilla-like appearance. These cases and those of the hairless dogs forcibly call to mind the fact, that the two orders of mammals--namely, the Edentata and Cetacea--which are the most abnormal in their dermal covering, are likewise the most abnormal either by deficiency or redundancy of teeth.
The organs of sight and hearing are generally admitted to be homologous, both with each other and with the various dermal appendages; hence these parts are liable to be abnormally affected in conjunction. Mr. White Cowper says "that in all cases of double microphthalmia brought under his notice he has at the same time met with defective development of the dental system." Certain forms of blindness seem to be associated with the colour of the hair; a man with black hair and a woman with light-coloured hair, both of sound constitution, married and had nine children, all of whom were born blind; of these children, five "with dark hair and brown iris were afflicted with amaurosis; the four others, with light-coloured hair and blue iris, had amaurosis and cataract conjoined." Several cases could be given, showing that some relation exists between various affections of the eyes and ears; thus Liebreich states that out of 241 deaf-mutes in Berlin, no less than fourteen suffered from the rare disease called pigmentary retinitis. Mr. White Cowper and Dr. Earle have remarked that inability to distinguish different colours, or colour-blindness, "is often associated with a corresponding inability to distinguish musical sounds."[823]
{329}
Here is a more curious case: white cats, if they have blue eyes, are almost always deaf. I formerly thought that the rule was invariable, but I have heard of a few authentic exceptions. The first two notices were published in 1829, and relate to English and Persian cats: of the latter, the Rev. W. T. Bree possessed a female, and he states "that of the offspring produced at one and the same birth, such as, like the mother, were entirely white (with blue eyes) were, like her, invariably deaf; while those that had the least speck of colour on their fur, as invariably possessed the usual faculty of hearing."[824] The Rev. W. Darwin Fox informs me that he has seen more than a dozen instances of this correlation in English, Persian, and Danish cats; but he adds "that, if one eye, as I have several times observed, be not blue, the cat hears. On the other hand, I have never seen a white cat with eyes of the common colour that was deaf." In France Dr. Sichel[825] has observed during twenty years similar facts; he adds the remarkable case of the iris beginning, at the end of four months, to grow dark-coloured, and then the cat first began to hear.
This case of correlation in cats has struck many persons as marvellous. There is nothing unusual in the relation between blue eyes and white fur; and we have already seen that the organs of sight and hearing are often simultaneously affected. In the present instance the cause probably lies in a slight arrest of development in the nervous system in connection with the sense-organs. Kittens during the first nine days, whilst their eyes are closed, appear to be completely deaf; I have made a great clanging noise with a poker and shovel close to their heads, both when they were asleep and awake, without producing any effect. The trial must not be made by shouting close to their ears, for they are, even when asleep, extremely sensitive to a breath of air. Now, as long as the eyes continue closed, the iris is no doubt blue, for in all the kittens which I have seen this colour remains for some time after the eyelids open. Hence, if we suppose the development of the organs of sight and hearing to be arrested at the stage of the closed eyelids, the eyes would {330} remain permanently blue and the ears would be incapable of perceiving sound; and we should thus understand this curious case. As, however, the colour of the fur is determined long before birth, and as the blueness of the eyes and the whiteness of the fur are obviously connected, we must believe that some primary cause acts at an early period.
The instances of correlated variability hitherto given have been chiefly drawn from the animal kingdom, and we will now turn to plants. Leaves, sepals, petals, stamens, and pistils are all homologous. In double flowers we see that the stamens and pistils vary in the same manner, and assume the form and colour of the petals. In the double columbine (Aquilegia vulgaris), the successive whorls of stamens are converted into cornucopias, which are enclosed within each other and resemble the petals. In hose-and-hose flowers the sepals mock the petals. In some cases the flowers and leaves vary together in tint: in all the varieties of the common pea, which have purple flowers, a purple mark may be seen on the stipules. In other cases the leaves and fruit and seeds vary together in colour, as in a curious pale-leaved variety of the sycamore, which has recently been described in France,[826] and as in the purple-leaved hazel, in which the leaves, the husk of the nut, and the pellicle round the kernel are all coloured purple.[827] Pomologists can predict to a certain extent, from the size and appearance of the leaves of their seedlings, the probable nature of the fruit; for, as Van Mons remarks,[828] variations in the leaves are generally accompanied by some modification in the flower, and consequently in the fruit. In the Serpent melon, which has a narrow tortuous fruit above a yard in length, the stem of the plant, the peduncle of the female flower, and the middle lobe of the leaf, are all elongated in a remarkable manner. On the other hand, several varieties of Cucurbita, which have dwarfed stems, all produce, as Naudin remarks with surprise, leaves of the same peculiar shape. Mr. G. Maw informs me that all the varieties of the scarlet Pelargoniums which have contracted or imperfect leaves have contracted flowers: the difference between {331} "Brilliant" and its parent "Tom Thumb" is a good instance of this. It may be suspected that the curious case described by Risso,[829] of a variety of the Orange which produces on the young shoots rounded leaves with winged petioles, and afterwards elongated leaves on long but wingless petioles, is connected with the remarkable change in form and nature which the fruit undergoes during its development.
In the following instance we have the colour and form of the petals apparently correlated, and both dependent on the nature of the season. An observer, skilled in the subject, writes,[830] "I noticed, during the year 1842, that every Dahlia, of which the colour had any tendency to scarlet, was deeply notched--indeed to so great an extent as to give the petals the appearance of a saw; the indentures were, in some instances, more than a quarter of an inch deep." Again, Dahlias which have their petals tipped with a different colour from the rest are very inconstant, and during certain years some, or even all the flowers, become uniformly coloured; and it has been observed with several varieties,[831] that when this happens the petals grow much elongated and lose their proper shape. This, however, may be due to reversion, both in colour and form, to the aboriginal species.
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In this discussion on correlation, we have hitherto treated of cases in which we can partly understand the bond of connexion; but I will now give cases in which we cannot even conjecture, or can only very obscurely see, what is the nature of the bond. Isidore Geoffroy St. Hilaire, in his work on Monstrosities, insists,[832] "que certaines anomalies coexistent rarement entr'elles, d'autres fréquemment, d'autres enfin presque constamment, malgré la différence très-grande de leur nature, et quoiqu'elles puissent paraître complètement indépendantes les unes des autres." We see something analogous in certain diseases: thus I hear from Mr. Paget that in a rare affection of the {332} renal capsules (of which the functions are unknown), the skin becomes bronzed; and in hereditary syphilis, both the milk and the second teeth assume a peculiar and characteristic form. Professor Rolleston, also, informs me that the incisor teeth are sometimes furnished with a vascular rim in correlation with intra-pulmonary deposition of tubercles. In other cases of phthisis and of cyanosis the nails and finger-ends become clubbed like acorns. I believe that no explanation has been offered of these and of many other cases of correlated disease.
What can be more curious and less intelligible than the fact previously given, on the authority of Mr. Tegetmeier, that young pigeons of all breeds, which when mature have white, yellow, silver-blue, or dun-coloured plumage, come out of the egg almost naked; whereas pigeons of other colours when first born are clothed with plenty of down? White Pea-fowls, as has been observed both in England and France,[833] and as I have myself seen, are inferior in size to the common coloured kind; and this cannot be accounted for by the belief that albinism is always accompanied by constitutional weakness; for white or albino moles are generally larger than the common kind.
To turn to more important characters: the niata cattle of the Pampas are remarkable from their short foreheads, upturned muzzles, and curved lower jaws. In the skull the nasal and premaxillary bones are much shortened, the maxillaries are excluded from any junction with the nasals, and all the bones are slightly modified, even to the plane of the occiput. From the analogical case of the dog, hereafter to be given, it is probable that the shortening of the nasal and adjoining bones is the proximate cause of the other modifications in the skull, including the upward curvature of the lower jaw, though we cannot follow out the steps by which these changes have been effected.
Polish fowls have a large tuft of feathers on their heads; and their skulls are perforated by numerous holes, so that a pin can be driven into the brain without touching any bone. That this deficiency of bone is in some way connected with the tuft of feathers is clear from tufted ducks and geese likewise having {333} perforated skulls. The case would probably be considered by some authors as one of balancement or compensation. In the chapter on Fowls, I have shown that with Polish fowls the tuft of feathers was probably at first small; by continued selection it became larger, and then rested on a fleshy or fibrous mass; and finally, as it became still larger, the skull itself became more and more protuberant until it acquired its present extraordinary structure. Through correlation with the protuberance of the skull, the shape and even the relative connexion of the premaxillary and nasal bones, the shape of the orifice of the nostrils, the breadth of the frontal bone, the shape of the post-lateral processes of the frontal and squamosal bones, and the direction of the bony cavity of the ear, have all been modified. The internal configuration of the skull and the whole shape of the brain have likewise been altered in a truly marvellous manner.
After this case of the Polish fowl it would be superfluous to do more than refer to the details previously given on the manner in which the changed form of the comb, in various breeds of the fowl, has affected the skull, causing by correlation crests, protuberances, and depressions on its surface.
With our cattle and sheep the horns stand in close connexion with the size of the skull, and with the shape of the frontal bones; thus Cline[834] found that the skull of a horned ram weighed five times as much as that of a hornless ram of the same age. When cattle become hornless, the frontal bones are "materially diminished in breadth towards the poll;" and the cavities between the bony plates "are not so deep, nor do they extend beyond the frontals."[835]
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It may be well here to pause and observe how the effects of correlated variability, of the increased use of parts, and of the accumulation through natural selection of so-called spontaneous variations, are in many cases inextricably commingled. We may borrow an illustration from Mr. Herbert Spencer, who remarks that, when the Irish elk acquired its gigantic horns, weighing above one hundred pounds, numerous co-ordinated {334} changes of structure would have been indispensable,--namely, a thickened skull to carry the horns; strengthened cervical vertebræ, with strengthened ligaments; enlarged dorsal vertebræ to support the neck, with powerful fore-legs and feet; all these parts being supplied with proper muscles, blood-vessels, and nerves. How then could these admirably co-ordinated modifications of structure have been acquired? According to the doctrine which I maintain, the horns of the male elk were slowly gained through sexual selection,--that is, by the best-armed males conquering the worse-armed, and leaving a greater number of descendants. But it is not at all necessary that the several parts of the body should have simultaneously varied. Each stag presents individual differences, and in the same district those which had slightly heavier horns, or stronger necks, or stronger bodies, or were the most courageous, would secure the greater number of does, and consequently leave a greater number of offspring. The offspring would inherit, in a greater or less degree, these same qualities, would occasionally intercross with each other, or with other individuals varying in some favourable manner; and of their offspring, those which were the best endowed in any respect would continue multiplying; and so onwards, always progressing, sometimes in one direction, and sometimes in another, towards the present excellently co-ordinated structure of the male elk. To make this clear, let us reflect on the probable steps, as shown in the twentieth chapter, by which our race and dray-horses have arrived at their present state of excellence; if we could view the whole series of intermediate forms between one of these animals and an early unimproved progenitor, we should behold a vast number of animals, not equally improved in each generation throughout their entire structure, but sometimes a little more in one point, and sometimes in another, yet on the whole gradually approaching in character to our present race or dray-horses, which are so admirably fitted in the one case for fleetness and in the other for draught.
Although natural selection would thus[836] tend to give to the {335} male elk its present structure, yet it is probable that the inherited influence of use has played an equal or more important part. As the horns gradually increased in weight, the muscles of the neck, with the bones to which they are attached, would increase in size and strength; and these parts would react on the body and legs. Nor must we overlook the fact that certain parts of the skull and the extremities would, judging by analogy, tend from the first to vary in a correlated manner. The increased weight of the horns would also act directly on the skull, in the same manner as, when one bone is removed in the leg of a dog, the other bone, which has to carry the whole weight of the body, increases in thickness. But from the facts given with respect to horned and hornless cattle, it is probable that the horns and skull would immediately act on each other through the principle of correlation. Lastly, the growth and subsequent wear and tear of the augmented muscles and bones would require an increased supply of blood, and consequently an increased supply of food; and this again would require increased powers of mastication, digestion, respiration, and excretion.
Colour as Correlated with Constitutional Peculiarities.
It is an old belief that with man there is a connexion between complexion and constitution; and I find that some of the best authorities believe in this to the present day.[837] Thus Dr. Beddoe by his tables shows[838] that a relation exists between liability to consumption and the colour of the hair, eyes, and skin. It has been affirmed[839] that, in the French army which invaded Russia, soldiers having a dark complexion, from the {336} southern parts of Europe, withstood the intense cold better than those with lighter complexions from the north; but no doubt such statements are liable to error.
In the second chapter on Selection I have given several cases proving that with animals and plants differences in colour are correlated with constitutional differences, as shown by greater or less immunity from certain diseases, from the attacks of parasitic plants and animals, from burning by the sun, and from the action of certain poisons. When all the individuals of any one variety possess an immunity of this nature, we cannot feel sure that it stands in any sort of correlation with their colour; but when several varieties of the same species, which are similarly coloured, are thus characterised, whilst other coloured varieties are not thus favoured, we must believe in the existence of a correlation of this kind. Thus in the United States purple-fruited plums of many kinds are far more affected by a certain disease than green or yellow-fruited varieties. On the other hand, yellow-fleshed peaches of various kinds suffer from another disease much more than the white-fleshed varieties. In the Mauritius red sugar-canes are much less affected by a particular disease than the white canes. White onions and verbenas are the most liable to mildew; and in Spain the green-fruited grapes suffered from the vine-disease more than other coloured varieties. Dark-coloured pelargoniums and verbenas are more scorched by the sun than varieties of other colours. Red wheats are believed to be hardier than white; whilst red-flowered hyacinths were more injured during one particular winter in Holland than other coloured varieties. With animals, white terriers suffer most from the distemper, white chickens from a parasitic worm in their tracheæ, white pigs from scorching by the sun, and white cattle from flies; but the caterpillars of the silk-moth which yield white cocoons suffered in France less from the deadly parasitic fungus than those producing yellow silk.
The cases of immunity from the action of certain vegetable poisons, in connexion with colour, are more interesting, and are at present wholly inexplicable. I have already given a remarkable instance, on the authority of Professor Wyman, of all the hogs, excepting those of a black colour, suffering severely in Virginia from eating the root of the Lachnanthes tinctoria. {337} According to Spinola and others,[840] buckwheat (Polygonum fagopyrum), when in flower, is highly injurious to white or white-spotted pigs, if they are exposed to the heat of the sun, but is quite innocuous to black pigs. By two accounts, the Hypericum crispum in Sicily is poisonous to white sheep alone; their heads swell, their wool falls off, and they often die; but this plant, according to Lecce, is poisonous only when it grows in swamps; nor is this improbable, as we know how readily the poisonous principle in plants is influenced by the conditions under which they grow.
Three accounts have been published in Eastern Prussia, of white and white-spotted horses being greatly injured by eating mildewed and honeydewed vetches; every spot of skin bearing white hairs becoming inflamed and gangrenous. The Rev. J. Rodwell informs me that his father turned out about fifteen cart-horses into a field of tares which in parts swarmed with black aphides, and which no doubt were honeydewed, and probably mildewed; the horses, with two exceptions, were chesnuts and bays with white marks on their faces and pasterns, and the white parts alone swelled and became angry scabs. The two bay horses with no white marks entirely escaped all injury. In Guernsey, when horses eat fools' parsley (Æthusa cynapium) they are sometimes violently purged; and this plant "has a peculiar effect on the nose and lips, causing deep cracks and ulcers, particularly on horses with white muzzles."[841] With cattle, independently of the action of any poison, cases have been published by Youatt and Erdt of cutaneous diseases with much constitutional disturbance (in one instance after exposure to a hot sun) affecting every single point which bore a white hair, but completely passing over other parts of the body. Similar cases have been observed with horses.[842]
We thus see that not only do those parts of the skin which bear white hair differ in a remarkable manner from those bearing {338} hair of any other colour, but that in addition some great, constitutional difference must stand in correlation with the colour of the hair; for in the above-mentioned cases, vegetable poisons caused fever, swelling of the head, as well as other symptoms, and even death, to all the white or white-spotted animals.
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XXVI. LAWS OF VARIATION, continued--SUMMARY
ON THE AFFINITY AND COHESION OF HOMOLOGOUS PARTS--ON THE VARIABILITY OF MULTIPLE AND HOMOLOGOUS PARTS--COMPENSATION OF GROWTH--MECHANICAL PRESSURE--RELATIVE POSITION OF FLOWERS WITH RESPECT TO THE AXIS OF THE PLANT, AND OF SEEDS IN THE CAPSULE, AS INDUCING VARIATION--ANALOGOUS OR PARALLEL VARIETIES--SUMMARY OF THE THREE LAST CHAPTERS.
On the Affinity of Homologous Parts.--This law was first generalised by Geoffroy Saint Hilaire, under the expression of La loi de l'affinité de soi pour soi. It has been fully discussed and illustrated by his son, Isidore Geoffroy, with respect to monsters in the animal kingdom,[843] and by Moquin-Tandon, with respect to monstrous plants. When similar or homologous parts, whether belonging to the same embryo or to two distinct embryos, are brought during an early stage of development into contact, they often blend into a single part or organ; and this complete fusion indicates some mutual affinity between the parts, otherwise they would simply cohere. Whether any power exists which tends to bring homologous parts into contact seems more doubtful. The tendency to complete fusion is not a rare or exceptional fact. It is exhibited in the most striking manner by double monsters. Nothing can be more extraordinary than the manner, as shown in various published plates, in which the corresponding parts of two embryos become intimately fused together. This is perhaps best seen in monsters with two heads, which are united, summit to summit, or face to face, or, Janus-like, back to back, or obliquely side to side. In one instance of two heads united almost face to face, but a little obliquely, four ears were developed, and on one side a perfect face, which was manifestly formed by the union of two {340} half-faces. Whenever two bodies or two heads are united, each bone, muscle, vessel, and nerve on the line of junction seems to seek out its fellow, and becomes completely fused with it. Lereboullet,[844] who carefully studied the development of double monsters in fishes, observed in fifteen instances the steps by which two heads gradually became fused into one. In this and other such cases, no one, I presume, supposes that the two already formed heads actually blend together, but that the corresponding parts of each head grow into one during the further progress of development, accompanied as it always is with incessant absorption and renovation. Double monsters were formerly thought to be formed by the union of two originally distinct embryos developed upon distinct vitelli; but now it is admitted that "their production is due to the spontaneous divarication of the embryonic mass into two halves;"[845] this, however, is effected by different methods. But the belief that double monsters originate from the division of one germ, does not necessarily affect the question of subsequent fusion, or render less true the law of the affinity of homologous parts.
The cautious and sagacious J. Müller,[846] when speaking of Janus-like monsters, says, that "without the supposition that some kind of affinity or attraction is exerted between corresponding parts, unions of this kind are inexplicable." On the other hand, Vrolik, and he is followed by others, disputes this conclusion, and argues from the existence of a whole series of monstrosities, graduating from a perfectly double monster to a mere rudiment of an additional digit, that "an excess of formative power" is the cause and origin of every monstrous duplicity. That there are two distinct classes of cases, and that parts may be doubled independently of the existence of two embryos, is certain; for a single embryo, or even a single adult animal, may produce doubled organs. Thus Valentin, as quoted by Vrolik, injured the caudal extremity of an embryo, and three days afterwards it produced rudiments of a double pelvis and of double hind limbs. {341} Hunter and others have observed lizards with their tails reproduced and doubled. When Bonnet divided longitudinally the foot of the salamander, several additional digits were occasionally formed. But neither these cases, nor the perfect series from a double monster to an additional digit, seem to me opposed to the belief that corresponding parts have a mutual affinity, and consequently tend to fuse together. A part may be doubled and remain in this state, or the two parts thus formed may afterwards through the law of affinity become blended; or two homologous parts in two separate embryos may, through the same principle, unite and form a single part.
The law of the affinity and fusion of similar parts applies to the homologous organs of the same individual animal, as well as to double monsters. Isidore Geoffroy gives a number of instances of two or more digits, of two whole legs, of two kidneys, and of several teeth becoming symmetrically fused together in a more or less perfect manner. Even the two eyes have been known to unite into a single eye, forming a cyclopean monster, as have the two ears, though naturally standing so far apart. As Geoffroy remarks, these facts illustrate in an admirable manner the normal fusion of various organs which during an early embryonic period are double, but which afterwards always unite into a single median organ. Organs of this nature are generally found in a permanently double condition in other members of the same class. These cases of normal fusion appear to me to afford the strongest support in favour of the present law. Adjoining parts which are not homologous sometimes cohere; but this cohesion appears to result from mere juxtaposition, and not from mutual affinity.
In the vegetable kingdom Moquin-Tandon[847] gives a long list of cases, showing how frequently homologous parts, such as leaves, petals, stamens, and pistils, as well as aggregates of homologous parts, such as buds, flowers, and fruit, become blended into each other with perfect symmetry. It is interesting to examine a compound flower of this nature, formed of exactly double the proper number of sepals, petals, stamens, and pistils, with each whorl of organs circular, and with no trace left of the {342} process of fusion. The tendency in homologous parts to unite during their early development, Moquin-Tandon considers as one of the most striking laws governing the production of monsters. It apparently explains a multitude of cases, both in the animal and vegetable kingdoms; it throws a clear light on many normal structures which have evidently been formed by the union of originally distinct parts, and it possesses, as we shall see in a future chapter, much theoretical interest.
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On the Variability of Multiple and Homologous Parts.--Isidore Geoffroy[848] insists that, when any part or organ is repeated many times in the same animal, it is particularly liable to vary both in number and structure. With respect to number, the proposition may, I think, be considered as fully established; but the evidence is chiefly derived from organic beings living under their natural conditions, with which we are not here concerned. When the vertebræ, or teeth, or rays in the fins of fishes, or feathers in the tails of birds, or petals, stamens, pistils, and seeds in plants, are very numerous, the number is generally variable. The explanation of this simple fact is by no means obvious. With respect to the variability in structure of multiple parts, the evidence is not so decisive; but the fact, as far as it may be trusted, probably depends on multiple parts being of less physiological importance than single parts; consequently their perfect standard of structure has been less rigorously enforced by natural selection.
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Compensation of Growth, or Balancement.--This law, as applied to natural species, was propounded by Goethe and Geoffroy St. Hilaire at nearly the same time. It implies that, when much organised matter is used in building up some one part, other parts are starved and become reduced. Several authors, especially botanists, believe in this law; others reject it. As far as I can judge, it occasionally holds good; but its importance has probably been exaggerated. It is scarcely possible to distinguish between the supposed effects of such compensation of growth, and the effects of long-continued selection, which {343} may at the same time lead to the augmentation of one part and the diminution of another. There can be no doubt that an organ may be greatly increased without any corresponding diminution in the adjoining parts. To recur to our former illustration of the Irish elk, it may be asked what part has suffered in consequence of the immense development of the horns?
It has already been observed that the struggle for existence does not bear hard on our domesticated productions; consequently the principle of economy of growth will seldom affect them, and we ought not to expect to find frequent evidence of compensation. We have, however, some such cases. Moquin-Tandon describes a monstrous bean,[849] in which the stipules were enormously developed, and the leaflets apparently in consequence completely aborted; this case is interesting, as it represents the natural condition of Lathyrus aphaca, with its stipules of great size, and its leaves reduced to mere threads, which act as tendrils. De Candolle[850] has remarked that the varieties of Raphanus sativus which have small roots yield numerous seed, valuable from containing oil, whilst those with large roots are not productive in this latter respect; and so it is with Brassica asperifolia. The varieties of the potato which produce tubers very early in the season rarely bear flowers; but Andrew Knight,[851] by checking the growth of the tubers, forced the plants to flower. The varieties of Cucurbita pepo which produce large fruit yield, according to Naudin, few in number; whilst those producing small fruit yield a vast number. Lastly, I have endeavoured to show in the eighteenth chapter that with many cultivated plants unnatural treatment checks the full and proper action of the reproductive organs, and they are thus rendered more or less sterile; consequently, in the way of compensation, the fruit becomes greatly enlarged, and, in double flowers, the petals are greatly increased in number.
With animals, it has been found difficult to produce cows which should first yield much milk, and afterwards be capable of {344} fattening well. With fowls which have large topknots and beards the comb and wattles are generally much reduced in size. Perhaps the entire absence of the oil-gland in fantail pigeons may be connected with the great development of their tails.
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Mechanical Pressure as a Cause of Modifications.--In some few cases there is reason to believe that mere mechanical pressure has affected certain structures. Every one knows that savages alter the shape of their infants' skulls by pressure at an early age; but there is no reason to believe that the result is ever inherited. Nevertheless Vrolik and Weber[852] maintain that the shape of the human head is influenced by the shape of the mother's pelvis. The kidneys in different birds differ much in form, and St. Ange[853] believes that this is determined by the form of the pelvis, which again, no doubt, stands in close relation with their various habits of locomotion. In snakes, the viscera are curiously displaced, in comparison with their position in other vertebrates; and this has been attributed by some authors to the elongation of their bodies; but here, as in so many previous cases, it is impossible to disentangle any direct result of this kind from that consequent on natural selection. Godron has argued[854] that the normal abortion of the spur on the inner side of the flower in Corydalis, is caused by the buds being closely pressed at a very early period of growth, whilst under ground, against each other and against the stem. Some botanists believe that the singular difference in the shape both of the seed and corolla, in the interior and exterior florets in certain compositous and umbelliferous plants, is due to the pressure to which the inner florets are subjected; but this conclusion is doubtful.
The facts just given do not relate to domesticated productions, and therefore do not strictly concern us. But here is a more appropriate case: H. Müller[855] has shown that in {345} short-faced races of the dog some of the molar teeth are placed in a slightly different position from that which they occupy in other dogs, especially in those having elongated muzzles; and as he remarks, any inherited change in the arrangement of the teeth deserves notice, considering their classificatory importance. This difference in position is due to the shortening of certain facial bones, and the consequent want of space; and the shortening results from a peculiar and abnormal state of the basal cartilages of the bones.
Relative Position of Flowers with respect to the Axis, and of Seeds in the Capsule, as inducing Variation.
In the thirteenth chapter various peloric flowers were described, and their production was shown to be due either to arrested development, or to reversion to a primordial condition. Moquin-Tandon has remarked that the flowers which stand on the summit of the main stem or of a lateral branch are more liable to become peloric than those on the sides;[856] and he adduces, amongst other instances, that of Teucrium campanulatum. In another Labiate plant grown by me, viz. the Galeobdolon luteum, the peloric flowers were always produced on the summit of the stem, where flowers are not usually borne. In Pelargonium, a single flower in the truss is frequently peloric, and when this occurs I have during several years invariably observed it to be the central flower. This is of such frequent occurrence that one observer[857] gives the names of ten varieties flowering at the same time, in every one of which the central flower was peloric. Occasionally more than one flower in the truss is peloric, and then of course the additional ones must be lateral. These flowers are interesting as showing how the whole structure is correlated. In the common Pelargonium the upper sepal is produced into a nectary which coheres with the flower-peduncle; the two upper petals differ a little in shape from the three lower ones, and are marked with dark shades of colour; the stamens are graduated in length and upturned. In the peloric flowers, the nectary aborts; all the petals become alike both in shape and colour; the stamens are generally reduced in number and become straight, so that the whole flower resembles that of the allied genus Erodium. The correlation between these changes is well shown when one of the two upper petals alone loses its dark mark, for in this case the nectary does not entirely abort, but is usually much reduced in length.[858]
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Morren has described[859] a marvellous flask-shaped flower of the Calceolaria, nearly four inches in length, which was almost completely peloric; it grew on the summit of the plant, with a normal flower on each side; Prof. Westwood also has described[860] three similar peloric flowers, which all occupied a central position on the flower-branches. In the Orchideous genus, Phalænopsis, the terminal flower has been seen to become peloric.
In a Laburnum-tree I observed that about a fourth part of the racemes produced terminal flowers which had lost their papilionaceous structure. These were produced after almost all the other flowers on the same racemes had withered. The most perfectly pelorised examples had six petals, each marked with black striæ like those on the standard-petal. The keel seemed to resist the change more than the other petals. Dutrochet has described[861] an exactly similar case in France, and I believe these are the only two instances of pelorism in the laburnum which have been recorded. Dutrochet remarks that the racemes on this tree do not properly produce a terminal flower, so that, as in the case of the Galeobdolon, their position as well as their structure are both anomalies, which no doubt are in some manner related. Dr. Masters has briefly described another leguminous plant,[862] namely, a species of clover, in which the uppermost and central flowers were regular or had lost their papilionaceous structure. In some of these plants the flower-heads were also proliferous.
Lastly, Linaria produces two kinds of peloric flowers, one having simple petals, and the other having them all spurred. The two forms, as Naudin remarks,[863] not rarely occur on the same plant, but in this case the spurred form almost invariably stands on the summit of the spike.
The tendency in the terminal or central flower to become peloric more frequently than other flowers, probably results from "the bud which stands on the end of a shoot receiving the most sap; it grows out into a stronger shoot than those situated lower down."[864] I have discussed the connection between pelorism and a central position, partly because some few plants are known normally to produce a terminal flower different in structure from the lateral ones; but chiefly on account of the following case, in which we see a tendency to variability or to reversion connected with the same position. A great judge of Auriculas[865] states that when an Auricula throws up a side bloom it is pretty sure to keep its character; but that if it grows from the centre or heart of the plant, whatever the colour of the edging ought to be, "it is just as likely to come in any other class as in the one to which it properly belongs." This is so notorious a {347} fact, that some florists regularly pinch off the central trusses of flowers. Whether in the highly improved varieties the departure of the central trusses from their proper type is due to reversion, I do not know. Mr. Dombrain insists that, whatever may be the commonest kind of imperfection in each variety, this is generally exaggerated in the central truss. Thus one variety "sometimes has the fault of producing a little green floret in the centre of the flower," and in central blooms these become excessive in size. In some central blooms, sent to me by Mr. Dombrain, all the organs of the flower were rudimentary in structure, of minute size, and of a green colour, so that by a little further change all would have been converted into small leaves. In this case we clearly see a tendency to prolification--a term which, I may explain to those who have never attended to botany, means the production of a branch or flower, or head of flowers, out of another flower. Now Dr. Masters[866] states that the central or uppermost flower on a plant is generally the most liable to prolification. Thus, in the varieties of the Auricula, the loss of their proper character and a tendency to prolification, and in other plants a tendency to prolification and pelorism, are all connected together, and are due either to arrested development, or to reversion to a former condition.
The following is a more interesting case; Metzger[867] cultivated in Germany several kinds of maize brought from the hotter parts of America, and he found, as has been previously described, that in two or three generations the grains became greatly changed in form, size, and colour; and with respect to two races he expressly states that in the first generation, whilst the lower grains on each head retained their proper character, the uppermost grains already began to assume that character which in the third generation all the grains acquired. As we do not know the aboriginal parent of the maize, we cannot tell whether these changes are in any way connected with reversion.
In the two following cases, reversion, as influenced by the position of the seed in the capsule, evidently acts. The Blue Imperial pea is the offspring of the Blue Prussian, and has larger seed and broader pods than its parent. Now Mr. Masters, of Canterbury, a careful observer and a raiser of new varieties of the pea, states[868] that the Blue Imperial always has a strong tendency to revert to its parent-stock, and the reversion "occurs in this manner: the last (or uppermost) pea in the pod is frequently much smaller than the rest; and if these small peas are carefully collected and sown separately, very many more, in proportion, will revert to their origin, than those taken from the other parts of the pod." Again M. Chaté[869] says that in raising seedling stocks he succeeds in getting eighty per cent. to bear double flowers, by leaving only a few of the secondary branches to seed; but in addition to this, "at the time of extracting the seeds, the upper portion of the pod is separated and {348} placed aside, because it has been ascertained that the plants coming from the seeds situated in this portion of the pod, give eighty per cent. of single flowers." Now the production of single-flowering plants from the seed of double-flowering plants is clearly a case of reversion. These latter facts, as well as the connection between a central position and pelorism and prolification, show in an interesting manner how small a difference--namely a little greater freedom in the flow of sap towards one part of the same plant--determines important changes of structure.
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Analogous or Parallel Variation.--By this term I wish to express that similar characters occasionally make their appearance in the several varieties or races descended from the same species, and more rarely in the offspring of widely distinct species. We are here concerned, not as hitherto with the causes of variation, but with the results; but this discussion could not have been more conveniently introduced elsewhere. The cases of analogous variation, as far as their origin is concerned, may be grouped, disregarding minor subdivisions, under two main heads; firstly, those due to unknown causes having acted on organic beings with nearly the same constitution, and which consequently vary in an analogous manner; and secondly, those due to the reappearance of characters which were possessed by a more or less remote progenitor. But these two main divisions can often be only conjecturally separated, and graduate, as we shall presently see, into each other.
Under the first head of analogous variations, not due to reversion, we have the many cases of trees belonging to quite different orders which have produced pendulous and fastigate varieties. The beech, hazel, and barberry have given rise to purple-leaved varieties; and as Bernhardi has remarked,[870] a multitude of plants, as distinct as possible, have yielded varieties with deeply-cut or laciniated leaves. Varieties descended from three distinct species of Brassica have their stems, or so-called roots, enlarged into globular masses. The nectarine is the offspring of the peach; and the varieties of both these trees offer a remarkable parallelism in the fruit being white, red, or yellow fleshed--in being clingstones or freestones--in the flowers being large or small--in the leaves being serrated or crenated, furnished with globose or reniform glands, or quite destitute of glands. It should be remarked that each variety of the nectarine has not derived its character from a corresponding variety of the peach. The several varieties also of a closely allied genus, namely the apricot, differ from each other in nearly the same parallel manner. There is no reason {349} to believe that in any of these cases long-lost characters have reappeared, and in most of them this certainly has not occurred.
Three species of Cucurbita have yielded a multitude of races, which correspond so closely in character that, as Naudin insists, they may be arranged in an almost strictly parallel series. Several varieties of the melon are interesting from resembling in important characters other species, either of the same genus or of allied genera; thus, one variety has fruit so like, both externally and internally, the fruit of a perfectly distinct species, namely, the cucumber, as hardly to be distinguished from it; another has long cylindrical fruit twisting about like a serpent; in another the seeds adhere to portions of the pulp; in another the fruit, when ripe, suddenly cracks and falls into pieces; and all these highly remarkable peculiarities are characteristic of species belonging to allied genera. We can hardly account for the appearance of so many unusual characters by reversion to a single ancient form; but we must believe that all the members of the family have inherited a nearly similar constitution from an early progenitor. Our cereal and many other plants offer similar cases.
With animals we have fewer cases of analogous variation, independently of direct reversion. We see something of the kind in the resemblance between the short-muzzled races of the dog, such as the pug and bulldog; in feather-footed races of the fowl, pigeon, and canary-bird; in horses of the most different races presenting the same range of colour; in all black-and-tan dogs having tan-coloured eye-spots and feet, but in this latter case reversion may possibly have played a part. Low has remarked[871] that several breeds of cattle are "sheeted,"--that is, have a broad band of white passing round their bodies like a sheet; this character is strongly inherited and sometimes originates from a cross; it may be the first step in reversion to an original or early type, for, as was shown in the third chapter, white cattle with dark ears, feet, and tip of tail formerly existed, and now exist in a feral or semi-feral condition in several quarters of the world.
Under our second main division, namely, of analogous variations due to reversion, the best cases are afforded by animals, and by none better than by pigeons. In all the most distinct breeds sub-varieties occasionally appear coloured exactly like the parent rock-pigeon, with black wing-bars, white loins, banded tail, &c.; and no one can doubt that these characters are simply due to reversion. So with minor details; turbits properly have white tails, but occasionally a bird is born with a dark-coloured and banded tail; pouters properly have white primary wing-feathers, but not rarely a "sword-flighted" bird, that is, one with the few first primaries dark-coloured, appears; and in these cases we have characters proper to the rock-pigeon, but new to the breed, evidently appearing from reversion. In some domestic varieties the wing-bars, instead of being simply black, as in the rock-pigeon, are beautifully edged with different zones of colour, and they then present a striking analogy with the wing-bars in certain natural species of the same family, such as Phaps chalcoptera; and this may probably be accounted for by {350} all the forms descended from the same remote progenitor having a tendency to vary in the same manner. Thus also we can perhaps understand the fact of some Laugher-pigeons cooing almost like turtle-doves, and of several races having peculiarities in their flight, for certain natural species (viz. C. torquatrix and palumbus) display singular vagaries in this respect. In other cases a race, instead of imitating in character a distinct species, resembles some other race; thus certain runts tremble and slightly elevate their tails, like fantails; and turbits inflate the upper part of their oesophagus, like pouter-pigeons.
It is a common circumstance to find certain coloured marks persistently characterising all the species of a genus, but differing much in tint; and the same thing occurs with the varieties of the pigeon: thus, instead of the general plumage being blue with the wing-bars black, there are snow-white varieties with red bars, and black varieties with white bars; in other varieties the wing-bars, as we have seen, are elegantly zoned with different tints. The Spot pigeon is characterised by the whole plumage being white, excepting the tail and a spot on the forehead; but these parts may be red, yellow, or black. In the rock-pigeon and in many varieties the tail is blue, with the outer edges of the outer feathers white; but in one sub-variety of the monk-pigeon we have a reversed variation, for the tail is white, except the outer edges of the outer feathers, which are black.[872]
With some species of birds, for instance with gulls, certain coloured parts appear as if almost washed out, and I have observed exactly the same appearance in the terminal dark tail-bar in certain pigeons, and in the whole plumage of certain varieties of the duck. Analogous facts in the vegetable kingdom could be given.
Many sub-varieties of the pigeon have reversed and somewhat lengthened feathers on the back part of their heads, and this is certainly not due to reversion to the parent-species, which shows no trace of such structure; but when we remember that sub-varieties of the fowl, turkey, canary-bird, duck, and goose, all have topknots or reversed feathers on their heads; and when we remember that scarcely a single large natural group of birds can be named, in which some members have not a tuft of feathers on their heads, we may suspect that reversion to some extremely remote form has come into action.
Several breeds of the fowl have either spangled or pencilled feathers; and these cannot be derived from the parent-species, the Gallus bankiva; though of course it is possible that an early progenitor of this species may have been spangled, and a still earlier or a later progenitor may have been pencilled. But as many gallinaceous birds are spangled or pencilled, it is a more probable view that the several domestic breeds of the fowl have acquired this kind of plumage from all the members of the family inheriting a tendency to vary in a like manner. The same principle may account for the ewes in certain breeds of sheep being hornless, like the females of some other hollow-horned ruminants; it may account for certain domestic cats having slightly-tufted ears, like those of the lynx; and for the skulls of domestic rabbits often differing from each {351} other in the same characters by which the skulls of the various species of the genus Lepus differ.
I will only allude to one other case, already discussed. Now that we know that the wild parent of the ass has striped legs, we may feel confident that the occasional appearance of stripes on the legs of the domestic ass is due to direct reversion; but this will not account for the lower end of the shoulder-stripe being sometimes angularly bent or slightly forked. So, again, when we see dun and other coloured horses with stripes on the spine, shoulders, and legs, we are led, from reasons formerly given, to believe that they reappear from direct reversion to the wild parent-horse. But when horses have two or three shoulder-stripes with one of them occasionally forked at the lower end, or when they have stripes on their faces, or as foals are faintly striped over nearly their whole bodies, with the stripes angularly bent one under the other on the forehead, or irregularly branched in other parts, it would be rash to attribute such diversified characters to the reappearance of those proper to the aboriginal wild horse. As three African species of the genus are much striped, and as we have seen that the crossing of the unstriped species often leads to the hybrid offspring being conspicuously striped--bearing also in mind that the act of crossing certainly causes the reappearance of long-lost characters--it is a more probable view that the above-specified stripes are due to reversion, not to the immediate wild parent-horse, but to the striped progenitor of the whole genus.
I have discussed this subject of analogous variation at considerable length, because, in a future work on natural species, it will be shown that the varieties of one species frequently mock distinct species--a fact in perfect harmony with the foregoing cases, and explicable only on the theory of descent. Secondly, because these facts are important from showing, as remarked in a former chapter, that each trifling variation is governed by law, and is determined in a much higher degree by the nature of the organisation, than by the nature of the conditions to which the varying being has been exposed. Thirdly, because these facts are to a certain extent related to a more general law, namely, that which Mr. B. D. Walsh[873] has called the "Law of Equable Variability," or, as he explains it, "if any given character is very variable in one species of a group, it will tend to be variable in allied species; and if any given character is perfectly constant in one species of a group, it will tend to be constant in allied species."
This leads me to recall a discussion in the chapter on Selection, in which it was shown that with domestic races, which are {352} now undergoing rapid improvement, those parts or characters which are the most valued vary the most. This naturally follows from recently selected characters continually tending to revert to their former less improved standard, and from their being still acted on by the same agencies, whatever these may be, which first caused the characters in question to vary. The same principle is applicable to natural species, for, as stated in my 'Origin of Species,' generic characters are less variable than specific characters; and the latter are those which have been modified by variation and natural selection, since the period when all the species belonging to the same genus branched off from a common progenitor, whilst generic characters are those which have remained unaltered from a much more remote epoch, and accordingly are now less variable. This statement makes a near approach to Mr. Walsh's law of Equable Variability. Secondary sexual characters, it may be added, rarely serve to characterise distinct genera, for they usually differ much in the species of the same genus, and are highly variable in the individuals of the same species; we have also seen in the earlier chapters of this work how variable secondary sexual characters become under domestication.
Summary of the three previous Chapters, on the Laws of Variation.
In the twenty-third chapter we have seen that changed conditions occasionally act in a definite manner on the organisation, so that all, or nearly all, the individuals thus exposed become modified in the same manner. But a far more frequent result of changed conditions, whether acting directly on the organisation or indirectly through the reproductive system being affected is indefinite and fluctuating variability. In the three latter chapters we have endeavoured to trace some of the laws by which such variability is regulated.
Increased use adds the size of a muscle, together with the blood-vessels, nerves, ligaments, the crests of bone to which these are attached, the whole bone and other connected bones. So it is with various glands. Increased functional activity strengthens the sense-organs. Increased and intermittent pressure thickens the epidermis; and a change in the nature of the food sometimes modifies the coats of the stomach, and increases or {353} decreases the length of the intestines. Continued disuse, on the other hand, weakens and diminishes all parts of the organisation. Animals which during many generations have taken but little exercise, have their lungs reduced in size, and as a consequence the bony fabric of the chest, and the whole form of the body, become modified. With our anciently domesticated birds, the wings have been little used, and they are slightly reduced; with their decrease, the crest of the sternum, the scapulæ, coracoids, and furcula, have all been reduced.
With domesticated animals, the reduction of a part from disuse is never carried so far that a mere rudiment is left, but we have good reason to believe that this has often occurred under nature. The cause of this difference probably is that with domestic animals not only sufficient time has not been granted for so profound a change, but that, from not being exposed to a severe struggle for life, the principle of the economy of organisation does not come into action. On the contrary, we sometimes see that structures which are rudimentary in the parent-species become partially redeveloped in their domesticated progeny. When rudiments are formed or left under domestication, they are the result of a sudden arrest of development, and not of long-continued disuse with the absorption of all superfluous parts; nevertheless they are of interest, as showing that rudiments are the relics of organs once perfectly developed.
Corporeal, periodical, and mental habits, though the latter have been almost passed over in this work, become changed under domestication, and the changes are often inherited. Such changed habits in any organic being, especially when living a free life, would often lead to the augmented or diminished use of various organs, and consequently to their modification. From long-continued habit, and more especially from the occasional birth of individuals with a slightly different constitution, domestic animals and cultivated plants become to a certain extent acclimatised, or adapted to a climate different from that proper to the parent-species.
Through the principle of correlated variability, when one part varies other parts vary,--either simultaneously, or one after the other. Thus an organ modified during an early embryonic period affects other parts subsequently developed. When an {354} organ, such as the beak, increases or decreases in length, adjoining or correlated parts, as the tongue and the orifice of the nostrils, tend to vary in the same manner. When the whole body increases or decreases in size, various parts become modified; thus with pigeons the ribs increase or decrease in number and breadth. Homologous parts, which are identical during their early development and are exposed to similar conditions, tend to vary in the same or in some connected manner,--as in the case of the right and left sides of the body, of the front and hind limbs, and even of the head and limbs. So it is with the organs of sight and hearing; for instance, white cats with blue eyes are almost always deaf. There is a manifest relation throughout the body between the skin and its various appendages of hair, feathers, hoofs, horns, and teeth. In Paraguay, horses with curly hair have hoofs like those of a mule; the wool and the horns of sheep vary together; hairless dogs are deficient in their teeth; men with redundant hair have abnormal teeth, either deficient or in excess. Birds with long wing-feathers usually have long tail-feathers. When long feathers grow from the outside of the legs and toes of pigeons, the two outer toes are connected by membrane; for the whole leg tends to assume the structure of the wing. There is a manifest relation between a crest of feathers on the head and a marvellous amount of change in the skull of various fowls; and in a lesser degree, between the greatly elongated, lopping ears of rabbits and the structure of their skulls. With plants, the leaves, various parts of the flower, and the fruit, often vary together in a correlated manner.
In some cases we find correlation without being able even to conjecture what is the nature of the connexion, as with various correlated monstrosities and diseases. This is likewise the case with the colour of the adult pigeon, in connexion with the presence of down on the young bird. Numerous curious instances have been given of peculiarities of constitution, in correlation with colour, as shown by the immunity of individuals of some one colour from certain diseases, from the attacks of parasites, and from the action of certain vegetable poisons.
Correlation is an important subject; for with species, and in a lesser degree with domestic races, we continually find that {355} certain parts have been greatly modified to serve some useful purpose; but we almost invariably find that other parts have likewise been more or less modified, without our being able to discover any advantage in the change. No doubt great caution is necessary in coming to this conclusion, for it is difficult to overrate our ignorance on the use of various parts of the organisation; but from what we have now seen, we may believe that many modifications are of no direct service, having arisen in correlation with other and useful changes.
Homologous parts during their early development evince an affinity for each other,--that is, they tend to cohere and fuse together much more readily than other parts. This tendency to fusion explains a multitude of normal structures. Multiple and homologous organs are especially liable to vary in number and probably in form. As the supply of organised matter is not unlimited, the principle of compensation sometimes comes into action; so that, when one part is greatly developed, adjoining parts or functions are apt to be reduced; but this principle is probably of much less importance than the more general one of the economy of growth. Through mere mechanical pressure hard parts occasionally affect soft adjoining parts. With plants the position of the flowers on the axis, and of the seeds in the capsule, sometimes leads, through a freer flow of sap, to changes of structure; but these changes are often due to reversion. Modifications, in whatever manner caused, will be to a certain extent regulated by that co-ordinating power or nisus formativus, which is in fact a remnant of one of the forms of reproduction, displayed by many lowly organised beings in their power of fissiparous generation and budding. Finally, the effects of the laws, which directly or indirectly govern variability, may be largely influenced by man's selection, and will so far be determined by natural selection that changes advantageous to any race will be favoured and disadvantageous changes checked.
Domestic races descended from the same species, or from two or more allied species, are liable to revert to characters derived from their common progenitor, and, as they have much in common in their constitutions, they are also liable under changed conditions to vary in the same manner; from these {356} two causes analogous varieties often arise. When we reflect on the several foregoing laws, imperfectly as we understand them, and when we bear in mind how much remains to be discovered, we need not be surprised at the extremely intricate manner in which our domestic productions have varied, and still go on varying.
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XXVII. PROVISIONAL HYPOTHESIS OF PANGENESIS
PRELIMINARY REMARKS.--FIRST PART:--THE FACTS TO BE CONNECTED UNDER A SINGLE POINT OF VIEW, NAMELY, THE VARIOUS KINDS OF REPRODUCTION--THE DIRECT ACTION OF THE MALE ELEMENT ON THE FEMALE--DEVELOPMENT--THE FUNCTIONAL INDEPENDENCE OF THE ELEMENTS OR UNITS OF THE BODY--VARIABILITY--INHERITANCE--REVERSION.
SECOND PART:--STATEMENT OF THE HYPOTHESIS--HOW FAR THE NECESSARY ASSUMPTIONS ARE IMPROBABLE--EXPLANATION BY AID OF THE HYPOTHESIS OF THE SEVERAL CLASSES OF FACTS SPECIFIED IN THE FIRST PART--CONCLUSION.
In the previous chapters large classes of facts, such as those bearing on bud-variation, the various forms of inheritance, the causes and laws of variation, have been discussed; and it is obvious that these subjects, as well as the several modes of reproduction, stand in some sort of relation to each other. I have been led, or rather forced, to form a view which to a certain extent connects these facts by a tangible method. Every one would wish to explain to himself, even in an imperfect manner, how it is possible for a character possessed by some remote ancestor suddenly to reappear in the offspring; how the effects of increased or decreased use of a limb can be transmitted to the child; how the male sexual element can act not solely on the ovule, but occasionally on the mother-form; how a limb can be reproduced on the exact line of amputation, with neither too much nor too little added; how the various modes of reproduction are connected, and so forth. I am aware that my view is merely a provisional hypothesis or speculation; but until a better one be advanced, it may be serviceable by bringing together a multitude of facts which are at present left disconnected by any efficient cause. As Whewell, the historian of the inductive sciences, remarks:--"Hypotheses may often be of service to science, when they involve a certain portion of incompleteness, and even of error." Under this point of view I venture to advance the hypothesis of Pangenesis, which {358} implies that the whole organisation, in the sense of every separate atom or unit, reproduces itself. Hence ovules and pollen-grains,--the fertilised seed or egg, as well as buds,--include and consist of a multitude of germs thrown off from each separate atom of the organism.
In the First Part I will enumerate as briefly as I can the groups of facts which seem to demand connection; but certain subjects, not hitherto discussed, must be treated at disproportionate length. In the Second Part the hypothesis will be given; and we shall see, after considering how far the necessary assumptions are in themselves improbable, whether it serves to bring under a single point of view the various facts.
Part I
Reproduction may be divided into two main classes, namely, sexual and asexual. The latter is effected in many ways--by gemmation, that is by the formation of buds of various kinds, and by fissiparous generation, that is by spontaneous or artificial division. It is notorious that some of the lower animals, when cut into many pieces, reproduce so many perfect individuals: Lyonnet cut a Nais or freshwater worm into nearly forty pieces, and these all reproduced perfect animals.[874] It is probable that segmentation could be carried much further in some of the protozoa, and with some of the lowest plants each cell will reproduce the parent-form. Johannes Müller thought that there was an important distinction between gemmation and fission; for in the latter case the divided portion, however small, is more perfectly organised; but most physiologists are now convinced that the two processes are essentially alike.[875] Prof. Huxley remarks, "fission is little more than a peculiar {359} mode of budding," and Prof. H. J. Clark, who has especially attended to this subject, shows in detail that there is sometimes "a compromise between self-division and budding." When a limb is amputated, or when the whole body is bisected, the cut extremities are said to bud forth; and as the papilla, which is first formed, consists of undeveloped cellular tissue like that forming an ordinary bud, the expression is apparently correct. We see the connection of the two processes in another way; for Trembley observed that with the hydra the reproduction of the head after amputation was checked as soon as the animal began to bud.[876]
Between the production, by fissiparous generation, of two or more complete individuals, and the repair of even a very slight injury, we have, as remarked in a former chapter, so perfect and insensible a gradation, that it is impossible to doubt that they are connected processes. Between the power which repairs a trifling injury in any part, and the power which previously "was occupied in its maintenance by the continued mutation of its particles," there cannot be any great difference; and we may follow Mr. Paget in believing them to be the selfsame power. As at each stage of growth an amputated part is replaced by one in the same state of development, we must likewise follow Mr. Paget in admitting "that the powers of development from the embryo are identical with those exercised for the restoration from injuries: in other words, that the powers are the same by which perfection is first achieved, and by which, when lost, it is recovered."[877] Finally, we may conclude that the several forms of gemmation, and of fissiparous generation, the repair of injuries, the maintenance of each part in its proper state, and the growth or progressive development of the whole structure of the embryo, are all essentially the results of one and the same great power.
Sexual Generation.--The union of the two sexual elements seems to make a broad distinction between sexual and asexual reproduction. But the well-ascertained cases of Parthenogenesis prove that the distinction is not really so great as it at first appears; for ovules occasionally, and even in some cases {360} frequently, become developed into perfect beings, without the concourse of the male element. J. Müller and others admit that ovules and buds have the same essential nature. Certain bodies, which during their early development cannot be distinguished by any external character from true ovules, nevertheless must be classed as buds, for though formed within the ovarium they are incapable of fertilisation. This is the case with the germ-balls of the Cecidomyide larvæ, as described by Leuckart.[878] Ovules and the male element, before they become united, have, like buds, an independent existence.[879] Both have the power of transmitting every single character possessed by the parent-form. We see this clearly when hybrids are paired inter se, for the characters of either grandparent often reappear, either perfectly or by segments, in the progeny. It is an error to suppose that the male transmits certain characters and the female other characters; though no doubt, from unknown causes, one sex sometimes has a stronger power of transmission than the other.
It has been maintained by some authors that a bud differs essentially from a fertilised germ, by always reproducing the perfect character of the parent-stock; whilst fertilised germs become developed into beings which differ, in a greater or less degree, from each other and from their parents. But there is no such broad distinction as this. In the eleventh chapter, numerous cases were given showing that buds occasionally grow into plants having new and strongly marked characters; and varieties thus produced can be propagated for a length of time by buds, and occasionally by seed. Nevertheless, it must be admitted that beings produced sexually are much more liable to vary than those produced asexually; and of this fact a partial explanation will hereafter be attempted. The variability in both cases is determined by the same general causes, and is governed by the same laws. Hence new varieties arising from buds cannot be distinguished from those arising from seed. Although bud-varieties usually retain their character during {361} successive bud-generations, yet they occasionally revert, even after a long series of bud-generations, to their former character. This tendency to reversion in buds is one of the most remarkable of the several points of agreement between the offspring from bud and seminal reproduction.
There is, however, one difference between beings produced sexually and asexually, which is very general. The former usually pass in the course of their development from a lower to a higher grade, as we see in the metamorphoses of insects and in the concealed metamorphoses of the vertebrata; but this passage from a lower to a higher grade cannot be considered as a necessary accompaniment of sexual reproduction, for hardly anything of the kind occurs in the development of Aphis amongst insects, or with certain crustaceans, cephalopods, or with any of the higher vascular plants. Animals propagated asexually by buds or fission are on the other hand never known to undergo a retrogressive metamorphosis; that is, they do not first sink to a lower, before passing on to their higher and final stage of development. But during the act of asexual production or subsequently to it, they often advance in organisation, as we see in the many cases of "alternate generation." In thus speaking of alternate generation, I follow those naturalists who look at the process as essentially one of internal budding or of fissiparous generation. Some of the lower plants, however, such as mosses and certain algæ, according to Dr. L. Radlkofer,[880] when propagated asexually, do undergo a retrogressive metamorphosis. We can to a certain extent understand, as far as the final cause is concerned, why beings propagated by buds should so rarely retrogress during development; for with each organism the structure acquired at each stage of development must be adapted to its peculiar habits. Now, with beings produced by gemmation,--and this, differently from sexual reproduction, may occur at any period of growth,--if there were places for the support of many individuals at some one stage of development, the simplest plan would be that they should be multiplied by gemmation at that stage, and not that they should first retrograde in their development to an earlier or simpler structure, which might not be fitted for the surrounding conditions.
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From the several foregoing considerations we may conclude that the difference between sexual and asexual generation is not nearly so great as it at first appears; and we have already seen that there is the closest agreement between gemmation, fissiparous generation, the repair of injuries, and ordinary growth or development. The capacity of fertilisation by the male element seems to be the chief distinction between an ovule and a bud; and this capacity is not invariably brought into action, as in the cases of parthenogenetic reproduction. We are here naturally led to inquire what the final cause can be of the necessity in ordinary generation for the concourse of the two sexual elements.
Seeds and ova are often highly serviceable as the means of disseminating plants and animals, and of preserving them during one or more seasons in a dormant state; but unimpregnated seeds or ova, and detached buds, would be equally serviceable for both purposes. We can, however, indicate two important advantages gained by the concourse of the two sexes, or rather of two individuals belonging to opposite sexes; for, as I have shown in a former chapter, the structure of every organism appears to be especially adapted for the concurrence, at least occasionally, of two individuals. In nearly the same manner as it is admitted by naturalists that hybridism, from inducing sterility, is of service in keeping the forms of life distinct and fitted for their proper places; so, when species are rendered highly variable by changed conditions of life, the free intercrossing of the varying individuals will tend to keep each form fitted for its proper place in nature; and crossing can be effected only by sexual generation, but whether the end thus gained is of sufficient importance to account for the first origin of sexual intercourse is very doubtful. Secondly, I have shown, from the consideration of a large body of facts, that, as a slight change in the conditions of life is beneficial to each creature, so, in an analogous manner, is the change effected in the germ by sexual union with a distinct individual; and I have been led, from observing the many widely-extended provisions throughout nature for this purpose, and from the greater vigour of crossed organisms of all kinds, as proved by direct experiments, as well as from the evil effects of close interbreeding when long {363} continued, to believe that the advantage thus gained is very great. Besides these two important ends, there may, of course, be others, as yet unknown to us, gained by the concourse of the two sexes.
Why the germ, which before impregnation undergoes a certain amount of development, ceases to progress and perishes, unless it be acted on by the male element; and why conversely the male element, which is enabled to keep alive for even four or five years within the spermatheca of a female insect, likewise perishes, unless it acts on or unites with the germ, are questions which cannot be answered with any certainty. It is, however, possible that both sexual elements perish, unless brought into union, simply from including too little formative matter for independent existence and development; for certainly they do not in ordinary cases differ in their power of giving character to the embryo. This view of the importance of the quantity of formative matter seems probable from the following considerations. There is no reason to suspect that the spermatozoa or pollen-grains of the same individual animal or plant differ from each other; yet Quatrefages has shown in the case of the Teredo,[881] as did formerly Prevost and Dumas with other animals, that more than one spermatozoon is requisite to fertilise an ovule. This has likewise been clearly proved by Newport,[882] who adds the important fact, established by numerous experiments, that, when a very small number of spermatozoa are applied to the ova of Batrachians, they are only partially impregnated and the embryo is never fully developed: the first step, however, towards development, namely, the partial segmentation of the yelk, does occur to a greater or less extent, but is never completed up to granulation. The rate of the segmentation is likewise determined by the number of the spermatozoa. With respect to plants, nearly the same results were obtained by Kölreuter and Gärtner. This last careful observer found,[883] after making successive trials on a Malva with more and more pollen-grains, that even thirty grains did not fertilise a single seed; but when forty grains were applied to the {364} stigma, a few seeds of small size were formed. The pollen-grains of Mirabilis are extraordinarily large, and the ovarium contains only a single ovule; and these circumstances led Naudin[884] to make the following interesting experiments: a flower was fertilised by three grains and succeeded perfectly; twelve flowers were fertilised by two grains, and seventeen flowers by a single grain, and of these one flower alone in each lot perfected its seed; and it deserves especial notice that the plants produced by these two seeds never attained their proper dimensions, and bore flowers of remarkably small size. From these facts we clearly see that the quantity of the peculiar formative matter which is contained within the spermatozoa and pollen-grains is an all-important element in the act of fertilisation, not only in the full development of the seed, but in the vigour of the plant produced from such seed. We see something of the same kind in certain cases of parthenogenesis, that is, when the male element is wholly excluded; for M. Jourdan[885] found that, out of about 58,000 eggs laid by unimpregnated silk-moths, many passed through their early embryonic stages, showing that they were capable of self-development, but only twenty-nine out of the whole number produced caterpillars. Therefore it is not an improbable view that deficient bulk or quantity in the formative matter, contained within the sexual elements, is the main cause of their not having the capacity of prolonged separate existence and development. The belief that it is the function of the spermatozoa to communicate life to the ovule seems a strange one, seeing that the unimpregnated ovule is already alive and continues for a considerable time alive. We shall hereafter see that it is probable that the sexual elements, or possibly only the female element, include certain primordial cells, that is, such as have undergone no differentiation, and which are not present in an active state in buds.
Graft-hybrids.--When discussing in the eleventh chapter the curious case of the Cytisus adami, facts were given which render it to a certain degree probable, in accordance with the belief of some distinguished botanists, that, when the tissues of two plants {365} belonging to distinct species or varieties are intimately united, buds are afterwards occasionally produced which, like hybrids, combine the characters of the two united forms. It is certain that when trees with variegated leaves are grafted or budded on a common stock, the latter sometimes produces buds bearing variegated leaves; but this may perhaps be looked at as a case of inoculated disease. Should it ever be proved that hybridised buds can be formed by the union of two distinct vegetative tissues, the essential identity of sexual and asexual reproduction would be shown in the most interesting manner; for the power of combining in the offspring the characters of both parents, is the most striking of all the functions of sexual generation.
Direct Action of the Male Element on the Female.--In the chapter just referred to, I have given abundant proofs that foreign pollen occasionally affects the mother-plant in a direct manner. Thus, when Gallesio fertilised an orange-flower with pollen from the lemon, the fruit bore stripes of perfectly characterised lemon-peel: with peas, several observers have seen the colour of the seed-coats and even of the pod directly affected by the pollen of a distinct variety; so it has been with the fruit of the apple, which consists of the modified calyx and upper part of the flower-stalk. These parts in ordinary cases are wholly formed by the mother-plant. We here see the male element affecting and hybridising not that part which it is properly adapted to affect, namely the ovule, but the partially developed tissues of a distinct individual. We are thus brought half-way towards a graft-hybrid, in which the cellular tissue of one form, instead of its pollen, is believed to hybridise the tissues of a distinct form. I formerly assigned reasons for rejecting the belief that the mother-plant is affected through the intervention of the hybridised embryo; but even if this view were admitted, the case would become one of graft-hybridism, for the fertilised embryo and the mother-plant must be looked at as distinct individuals.
With animals which do not breed until nearly mature, and of which all the parts are then fully developed, it is hardly possible that the male element should directly affect the female. But we have the analogous and perfectly well-ascertained case of the male element of a distinct form, as with the {366} quagga and Lord Morton's mare, affecting the ovarium of the female, so that the ovules and offspring subsequently produced by her when impregnated by other males are plainly affected and hybridised by the first male.
Development.--The fertilised germ reaches maturity by a vast number of changes: these are either slight and slowly effected, as when the child grows into the man, or are great and sudden, as with the metamorphoses of most insects. Between these extremes we have, even within the same class, every gradation: thus, as Sir J. Lubbock has shown,[886] there is an Ephemerous insect which moults above twenty times, undergoing each time a slight but decided change of structure; and these changes, as he further remarks, probably reveal to us the normal stages of development which are concealed and hurried through, or suppressed, in most other insects. In ordinary metamorphoses, the parts and organs appear to become changed into the corresponding parts in the next stage of development; but there is another form of development, which has been called by Professor Owen metagenesis. In this case "the new parts are not moulded upon the inner surface of the old ones. The plastic force has changed its course of operation. The outer case, and all that gave form and character to the precedent individual, perish and are cast off; they are not changed into the corresponding parts of the new individual. These are due to a new and distinct developmental process," &c.[887] Metamorphosis, however, graduates so insensibly into metagenesis, that the two processes cannot be distinctly separated. For instance, in the last change which Cirripedes undergo, the alimentary canal and some other organs are moulded on pre-existing parts; but the eyes of the old and the young animal are developed in entirely different parts of the body; the tips of the mature limbs are formed within the larval limbs, and may be said to be metamorphosed from them; but their basal portions and the whole thorax are developed in a plane actually at right angles to the limbs and thorax of the larva; and this {367} may be called metagenesis. The metagenetic process is carried to an extreme degree in the development of some Echinoderms, for the animal in the second stage of development is formed almost like a bud within the animal of the first stage, the latter being then cast off like an old vestment, yet sometimes still maintaining for a short period an independent vitality.[888]
If, instead of a single individual, several were to be thus developed metagenetically within a pre-existing form, the process would be called one of alternate generation. The young thus developed may either closely resemble the encasing parent-form, as with the larvæ of Cecidomyia, or may differ to an astonishing degree, as with many parasitic worms and with jelly-fishes; but this does not make any essential difference in the process, any more than the greatness or abruptness of the change in the metamorphoses of insects.
The whole question of development is of great importance for our present subject. When an organ, the eye for instance, is metagenetically formed in a part of the body where during the previous stage of development no eye existed, we must look at it as a new and independent growth. The absolute independence of new and old structures, which correspond in structure and function, is still more obvious when several individuals are formed within a previous encasing form, as in the cases of alternate generation. The same important principle probably comes largely into play even in the case of continuous growth, as we shall see when we consider the inheritance of modifications at corresponding ages.
We are led to the same conclusion, namely, the independence of parts successively developed, by another and quite distinct group of facts. It is well known that many animals belonging to the same class, and therefore not differing widely from each other, pass through an extremely different course of development. Thus certain beetles, not in any way remarkably different from others of the same order, undergo what has been called a hyper-metamorphosis--that is, they pass through an early stage wholly different from the ordinary grub-like larva. In the same sub-order of crabs, namely, the Macroura, as Fritz {368} Müller remarks, the river cray-fish is hatched under the same form which it ever afterwards retains; the young lobster has divided legs, like a Mysis; the Palæmon appears under the form of a Zoea, and Peneus under the Nauplius-form; and how wonderfully these larval forms differ from each other, is known to every naturalist.[889] Some other crustaceans, as the same author observes, start from the same point and arrive at nearly the same end, but in the middle of their development are widely different from each other. Still more striking cases could be given with respect to the Echinodermata. With the Medusæ or jelly-fishes Professor Allman observes, "the classification of the Hydroida would be a comparatively simple task if, as has been erroneously asserted, generically-identical medusoids always arose from generically-identical polypoids; and on the other hand, that generically-identical polypoids always gave origin to generically-identical medusoids." So, again, Dr. Strethill Wright remarks, "in the life-history of the Hydroidæ any phase, planuloid, polypoid, or medusoid, may be absent."[890]
According to the belief now generally accepted by our best naturalists, all the members of the same order or class, the Macrourous crustaceans for instance, are descended from a common progenitor. During their descent they have diverged much in structure, but have retained much in common; and this divergence and retention of character has been effected, though they have passed and still pass through marvellously different metamorphoses. This fact well illustrates how independent each structure must be from that which precedes and follows it in the course of development.
The Functional Independence of the Elements or Units of the Body.--Physiologists agree that the whole organism consists of a multitude of elemental parts, which are to a great extent independent of each other. Each organ, says Claude Bernard,[891] {369} has its proper life, its autonomy; it can develop and reproduce itself independently of the adjoining tissues. The great German authority, Virchow,[892] asserts still more emphatically that each system, as the nervous or osseous system, or the blood, consists of an "enormous mass of minute centres of action.... Every element has its own special action, and even though it derive its stimulus to activity from other parts, yet alone effects the actual performance of its duties.... Every single epithelial and muscular fibre-cell leads a sort of parasitical existence in relation to the rest of the body.... Every single bone-corpuscle really possesses conditions of nutrition peculiar to itself." Each element, as Mr. Paget remarks, lives its appointed time, and then dies, and, after being cast off or absorbed, is replaced.[893] I presume that no physiologist doubts that, for instance, each bone-corpuscle of the finger differs from the corresponding corpuscle in the corresponding joint of the toe; and there can hardly be a doubt that even those on the corresponding sides of the body differ, though almost identical in nature. This near approach to identity is curiously shown in many diseases in which the same exact points on the right and left sides of the body are similarly affected; thus Mr. Paget[894] gives a drawing of a diseased pelvis, in which the bone has grown into a most complicated pattern, but "there is not one spot or line on one side which is not represented, as exactly as it would be in a mirror, on the other."
Many facts support this view of the independent life of each minute element of the body. Virchow insists that a single bone-corpuscle or a single cell in the skin may become diseased. The spur of a cock, after being inserted into the eye of an ox, lived for eight years, and acquired a weight of 306 grammes, or nearly fourteen ounces.[895] The tail of a pig has been grafted into the middle of its back, and reacquired sensibility. Dr. Ollier[896] inserted a piece of periosteum from the bone of a young dog under the skin of a rabbit, and true bone was developed. A multitude of similar facts could be given. The {370} frequent presence of hairs and of perfectly developed teeth, even teeth of the second dentition, in ovarian tumours,[897] are facts leading to the same conclusion.
Whether each of the innumerable autonomous elements of the body is a cell or the modified product of a cell, is a more doubtful question, even if so wide a definition be given to the term, as to include cell-like bodies without walls and without nuclei.[898] Professor Lionel Beale uses the term "germinal matter" for the contents of cells, taken in this wide acceptation, and he draws a broad distinction between germinal matter and "formed material" or the various products of cells.[899] But the doctrine of omnis cellula e cellulâ is admitted for plants, and is a widely prevalent belief with respect to animals.[900] Thus Virchow, the great supporter of the cellular theory, whilst allowing that difficulties exist, maintains that every atom of tissue is derived from cells, and these from pre-existing cells, and these primarily from the egg, which he regards as a great cell. That cells, still retaining the same nature, increase by self-division or proliferation, is admitted by almost every one. But when an organism undergoes a great change of structure during development, the cells, which at each stage are supposed to be directly derived from previously-existing cells, must likewise be greatly changed in nature; this change is apparently attributed by the supporters of the cellular doctrine to some inherent power which the cells possess, and not to any external agency.
Another school maintains that cells and tissues of all kinds may be formed, independently of pre-existing cells, from plastic lymph or blastema; and this it is thought is well exhibited in the repair of wounds. As I have not especially attended to histology, it would be presumptuous in me to express an opinion on the two opposed doctrines. But every one appears to admit that the body consists of a multitude of "organic units,"[901] {371} each of which possesses its own proper attributes, and is to a certain extent independent of all others. Hence it will be convenient to use indifferently the terms cells or organic units or simply units.
Variability and Inheritance.--We have seen in the twenty-second chapter that variability is not a principle co-ordinate with life or reproduction, but results from special causes, generally from changed conditions acting during successive generations. Part of the fluctuating variability thus induced is apparently due to the sexual system being easily affected by changed conditions, so that it is often rendered impotent; and when not so seriously affected, it often fails in its proper function of transmitting truly the characters of the parents to the offspring. But variability is not necessarily connected with the sexual system, as we see from the cases of bud-variation; and although we may not be able to trace the nature of the connexion, it is probable that many deviations of structure which appear in sexual offspring result from changed conditions acting directly on the organisation, independently of the reproductive organs. In some instances we may feel sure of this, when all, or nearly all the individuals which have been similarly exposed are similarly and definitely affected--as in the dwarfed and otherwise changed maize brought from hot countries when cultivated in Germany; in the change of the fleece in sheep within the tropics; to a certain extent in the increased size and early maturity of our highly-improved domesticated animals; in inherited gout from intemperance; and in many other such cases. Now, as such changed conditions do not especially affect the reproductive organs, it seems mysterious on any ordinary view why their product, the new organic being, should be similarly affected.
How, again, can we explain to ourselves the inherited effects of the use or disuse of particular organs? The domesticated duck flies less and walks more than the wild duck, and its limb-bones have become in a corresponding manner diminished and increased in comparison with those of the wild duck. A horse is trained to certain paces, and the colt inherits similar consensual movements. The domesticated rabbit becomes tame from close confinement; the dog intelligent from associating with man; the retriever is taught to fetch and carry: and these {372} mental endowments and bodily powers are all inherited. Nothing in the whole circuit of physiology is more wonderful. How can the use or disuse of a particular limb or of the brain affect a small aggregate of reproductive cells, seated in a distant part of the body, in such a manner that the being developed from these cells inherits the characters of either one or both parents? Even an imperfect answer to this question would be satisfactory.
Sexual reproduction does not essentially differ, as we have seen, from budding or self-division, and these processes graduate through the repair of injuries into ordinary development and growth; it might therefore be expected that every character would be as regularly transmitted by all the methods of reproduction as by continued growth. In the chapters devoted to inheritance it was shown that a multitude of newly-acquired characters, whether injurious or beneficial, whether of the lowest or highest vital importance, are often faithfully transmitted--frequently even when one parent alone possesses some new peculiarity. It deserves especial attention that characters appearing at any age tend to reappear at a corresponding age. We may on the whole conclude that in all cases inheritance is the rule, and non-inheritance the anomaly. In some instances a character is not inherited, from the conditions of life being directly opposed to its development; in many instances, from the conditions incessantly inducing fresh variability, as with grafted fruit-trees and highly cultivated flowers. In the remaining cases the failure may be attributed to reversion, by which the child resembles its grandparents or more remote progenitors, instead of its parents.
This principle of Reversion is the most wonderful of all the attributes of Inheritance. It proves to us that the transmission of a character and its development, which ordinarily go together and thus escape discrimination, are distinct powers; and these powers in some cases are even antagonistic, for each acts alternately in successive generations. Reversion is not a rare event, depending on some unusual or favourable combination of circumstances, but occurs so regularly with crossed animals and plants, and so frequently with uncrossed breeds, that it is evidently an essential part of the principle of inheritance. We know that {373} changed conditions have the power of evoking long-lost characters, as in the case of some feral animals. The act of crossing in itself possesses this power in a high degree. What can be more wonderful than that characters, which have disappeared during scores, or hundreds, or even thousands of generations, should suddenly reappear perfectly developed, as in the case of pigeons and fowls when purely bred, and especially when crossed; or as with the zebrine stripes on dun-coloured horses, and other such cases? Many monstrosities come under this same head, as when rudimentary organs are redeveloped, or when an organ which we must believe was possessed by an early progenitor, but of which not even a rudiment is left, suddenly reappears, as with the fifth stamen in some Scrophulariaceæ. We have already seen that reversion acts in bud-reproduction; and we know that it occasionally acts during the growth of the same individual animal, especially, but not exclusively, when of crossed parentage,--as in the rare cases described of individual fowls, pigeons, cattle, and rabbits, which have reverted as they advanced in years to the colours of one of their parents or ancestors.
We are led to believe, as formerly explained, that every character which occasionally reappears is present in a latent form in each generation, in nearly the same manner as in male and female animals secondary characters of the opposite sex lie latent, ready to be evolved when the reproductive organs are injured. This comparison of the secondary sexual characters which are latent in both sexes, with other latent characters, is the more appropriate from the case recorded of the Hen, which assumed some of the masculine characters, not of her own race, but of an early progenitor; she thus exhibited at the same time the redevelopment of latent characters of both kinds and connected both classes. In every living creature we may feel assured that a host of lost characters lie ready to be evolved under proper conditions. How can we make intelligible, and connect with other facts, this wonderful and common capacity of reversion,--this power of calling back to life long-lost characters? {374}