Evolution without natural selection : or the segregation of species without aid of the Darwinian hypothesis

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Preface

A few words are perhaps necessary by way of preface to the following pages. How were they written ? why were they written ? In the first place, having occasion to thoroughly work out the plumages and distribution of British birds and their allies from an evolutionist’s point of view, I was often sorely puzzled in attempting to harmonize the facts before me with Darwin’s theory of Natural Selection ; and consequently I attempted to explain them by other and, what I believe to be, neglected causes. In the second place, I was led to write up into the present form the result of my investigations and speculations in the belief that they would prove of interest to many naturalists and perhaps, no matter in how humble a way, serve to encourage research and stimulate inquiry in a branch of Biology which is, as yet, almost entirely uninvestigated. If I can convey to the reader even a little of the charm which this subject possesses, I shall be more than repaid.

CHARLES DIXON.

INTRODUCTORY.

“ Whoever is led to believe that species are mutable will do good service by conscientiously expressing his conviction ; for thus only can the load of prejudice by which this subject is overwhelmed be removed.” — Darwin.

Darwin’s theory that existing forms of animals and plants have been evolved by means of Natural Selection, or by the preservation of favoured races in the struggle for life, has been almost universally accepted by scientific men as the true solution of that “ mystery of mysteries ” the origin of species. The great simplicity of the hypothesis in a large measure accounts for its popularity; but even the most sanguine and enthusiastic of its supporters are obliged to admit that there is much that it cannot satisfactorily explain — that it has many weak points which its opponents have not failed to detect and to use to its disadvantage in that conflict of opinion which such a theory must inevitably cause. Darwin, with his accustomed frankness, admits many difficulties; and no doubt his keen perception detected many more which he passed by in silence. The supporters of the theory of Natural Selection have demanded too much from it — have taxed it beyond its strength. Darwin’s disciples have been too enthusiastic, too confident, and so blinded by

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his grand discoveries, that they seem to have overlooked the important fact that there may be many other causes at work aiding the evolution of new forms of life, in addition to that of the survival of the fittest by Natural Selection ; and for these causes they must search if they would solve many difficulties that beset biological investigation. It is only fair to say that Darwin states, in his ‘ Origin of Species/ that he is “ convinced that Natural Selection has been the main, but not the exclusive, means of modification.” But in that celebrated book we find the other “ means of modification ” scarcely ever alluded to ; and the very title of the work is sufficient to show that Darwin regarded Natural Selection as by far the most potent, if not the exclusive, agent in the modification of species. Furthermore, he says (‘ Animals and Plants under Domestication/ ii. p. 192) that “ slight individual differences, however, suffice for the work, and are probably the sole differences which are effective in the production of new species.” It is high time that Natural Selection took its proper place and yielded up the honours which do not rightfully belong to it ; its true value must be ascertained, and the other causes at work in the modification of species must be allowed their proper share of importance : many serious difficulties will then be overcome, and Natural Selection will be freed from much that it is at present asked vainly and most unfairly to explain.

Whilst carefully working at the geographical distribution and the changes and variations of plumage of Palaearctic birds and their allies in various other parts of the world for the past five years, a vast array of facts has been brought to my notice, which appears to suggest some explanation to the many difficulties one meets in attempting to explain the

INTRODUCTORY.

,3 origin of species — difficulties over which the theory of Natural Selection breaks hopelessly down. How the minute differences which separate so many closely allied species can be of benefit in the struggle for life — how the many fine gradations between species and well-marked races can be of sufficient importance, or be in any way profitable to their possessors, and thus be preserved on the Darwinian hypothesis—has always been to me one of the gravest difficulties of the theory of Natural Selection. The Coal Titmouse (Petrus atev) is a remarkably good illustration of the difficulty to which I allude. The birds of this somewhat variable species inhabiting the British Islands are distinguished from those of continental Europe by having the slate-grey of the upper parts more or less suffused with brown. This difference was first pointed out by Sharpe and Dresser, who bestowed upon the British form of Coal Tit the name of P. britannicus. But the variations of the Coal Tit do not end here. Siberian examples have the upper parts a purer grey, and the underparts a purer white, than West-European examples, and an incipient crest is often observable ; whilst in examples from East Siberia, Kamtschatka, and Japan the brown on the flanks extends to the breast and belly. In examples from North China the upper parts are suffused with brown, the underparts are still browner, and the crest is still more developed — differences which have caused the bird to be described as distinct under the name of P. pekinensis. When we get into Turkestan we find that the Coal Tit inhabiting that country has the upper parts suffused with brown, and the underparts have become almost uuiform brown : this race has been described as P. rufipectus. Still further south, in the Caucasus, the difference in colour is about the same,

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but the size is slightly larger : to this form the name P. michalowskii has been applied. In South-Persian examples the upper parts are even browner than in British, and the flanks are also very brown — differences which have caused the bird to be called P. jphceonotus ; whilst in the Himalayas the birds have the buff underparts still more pronounced, the white cheek-patch is not so pure, and the crest is more developed — differences which have caused this race to be named P. cemodius. In the evergreen-oak and cedar forests of Algeria the Coal Titmouse has become so much differentiated (or it would, perhaps, be more correct to say that it is the least changed member of the great Coal-Tit family, seeing that the differences it presents are characters which distinguish the young birds of its European allies) that it has received the name of P. Icdoucii.

A very similar phenomenon is presented in the Marsh - Titmouse (P. palustris ) ; but, in addition to the different shades of colour, the tail varies in shape from being nearly square to very much rounded, the upper parts vary from slate-grey and sandy white to sandy brown, and the flanks from white to brown. This Titmouse has been divided into no less than six species ; but in a sufficiently large series the differences intergrade, and no hard-and-fast line can be drawn between either of them. How is it possible to conceive that these slight differences have been produced by Natural Selection ? How is it possible to show that in the struggle for life one form of Titmouse gains an advantage over its congeners by the possession of such a trifling difference from them? What conceivable benefit is it, for instance, to the British form of Coal Tit to be slightly browner on the upper parts than its congeners in Western

INTRODUCTORY.

Europe, or greyer than those inhabiting South Persia ? I venture to assert that it is no benefit. Natural Selection has not produced this island form of Titmouse by eliminating all birds that did not chance to have a tendency to this peculiarity. Clearly, then, we have here a case where Natural Selection has played no part in the modification of species. But (as I hope presently to clearly demonstrate) this is not a solitary iustance ; hundreds of species present similar facts.

EVOLUTION.

I.— ISOLATION.

To study the subject more clearly, I purpose to divide the present essay into several parts, each dealing with a considerable mass of facts, which show how species may be modified without any Natural Selective process. In the first place, I will deal with the facts that I have collected which appear to show how Isolation has been the agent by means of which many species have been modified.

We must look to other and more universal causes for the origin of many species than to Natural Selection — causes that can work results more rapidly. On the theory of Natural Selection sufficient time (in conjunction with violent changes of the earth’s surface) could not have elapsed to form all the wonderful and complex diversity of animal life ; but if we take Isolation and other at present disregarded causes into consideration, we see how the changes in organic life can act much quicker, and can almost constantly be in progress. This, I think, is proved by the vast number of occurrences of local forms and imperfectly segregated species at the present day, when never probably in the history of the world were conditions of life more tranquil and geological changes less in progress. Nay, it is even possible that the great amount of variation we now see in so many forms of

ISOLATION.

organic life is simply the inevitable result of the comparatively tranquil state of the globe — it is even possible that the one fact is but a natural consequence of the other. Natural Selection is probably the most potent agent in the evolution of new species only at such times when the earth is undergoing violent change — as, for instance, during glacial periods, or during great convulsion of the surface by volcanic action, &c. I also think (and all the evidence I am about to bring forward proves its correctness) that during the comparatively calm periods of the earth’s history — the lulls in the great convulsive strife — many species are evolved purely by Isolation, and by many attendant minor causes, as overcrowding and its consequent emigration, &c. Comparatively uniform conditions of life are, in my opinion, aids to variation, and consequently to the establishment of many local forms or incipient species. At the present day we have countless instances of incipient species in every form of life, both animal and vegetable, which are undoubtedly produced without the interfence of any Natural Selective process ; for the conditions of life of many incipient species and the parent forms are at the present time practically the same throughout the area of their distribution, or ouly slightly different from climatic or other local causes. We can conceive how, as soon as violent changes once more pervade the world, the struggle for life will be infinitely greater than it is now. Then species will be matched against species, race against parent form, or race against race ; all Nature will be thrown into a kind of chaos : and then Natural Selection will adjust the disordered balance — the forms best fitted to cope with the changed conditions live, those not so constituted die ; the law of the survival of the fittest will

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inexorably perform its task, and gradually Nature will settle down into her accustomed course, furnished wdth those forms of life best adapted to the changed conditions of their surroundings. Again, Isolation and its constant attendant, Variation, work hand in hand together — incipient forms, imperfectly segregated species, branch off from the then representative or parental species of the world after her convulsive or glacial change, and similar phenomena to those now before us at the present day have been or will be presented as the cycle of mighty and violent changes are relieved by intervals of tranquillity. We can understand how Natural Selection can probably assist in giving to the butterfly its protective hues, or to the tiger its stripes, and to the leopard its spots ; but no amount of Natural Selection can explain the infinite gradations between so many species, or the shades of colour which denote so many closely allied but perfectly distinct forms of animal life. How, then, have these marked characteristics been produced ?

Let us glance at a few striking instances that plainly show how Isolation has been the means in many cases of modifying species. No better examples can be found than amongst the great cosmopolitan subfamily of the Thrushes (Turdinae). Take, for instance, the species and races of the genus Catharus, or Thrush-Robins — a small group of birds confined to the mountains of South Mexico and Central America, and the Andes as far south as Bolivia. These birds frequent the forests on the mountain-sides, never descending to the lowlands. The genus contains twelve recognized species ; but some of them have evidently only quite recently become differentiated, and the characters on which their specific rank is based are so slight that, were it not for their

ISOLATION.

geographical areas being different-, no reasonable excuse could be found for separating them. Many of the species, however, though obviously most closely allied, are separated from each other just as completely as non-migratory species on many islands, the birds on one chain of mountains being completely isolated from the nearly allied species on another chain. They are all resident birds, so far as is known, and never descend from their own particular zone to the lowlands, where they might interbreed, and thus prevent by intermarriage any slight difference from becoming constant. How have the slight differences which mark many species in this group arisen ? Let us go back to the time when the parent species inhabited the lowlands : we can imagine how the climate changed, or some cause arose which drove the species to the mountains, some birds up one, some up another. Not being migratory, or perhaps gradually becoming resident in their highland haunts through favourable conditions of life, we can understand how Isolation would soon do its work — changes of colour, perhaps changes of structure (one possibly involving the other) arise, and new species are eventually evolved. We can then understand how one form acquires a slate-grey mantle, another a russet one, another a chestnut head, or others a black or grey head — the accidental variations or, perhaps, in some cases, reversions in any one direction being preserved simply because they were isolated.

Take, for instance, Catharus griseiceps and C. phceopleurus , only distinguished by the shade of colour on the back. In the former species it is russet-brown, in the latter it is olive-brown. C. griseiceps has only been obtained on the highlands of Panama, whilst C. phceopleurus is apparently

EVOLUTION.

confined to the mountain-forests of Antioquia in Colombia. Undoubtedly these two species at no very remote period were one. Circumstances arose that divided the area of its distribution, sending one portion up the highlands of Panama, the other up those of Colombia. Our single species is now isolated into two colonies : perhaps C. griseiceps was exposed to a more pluvial climate, causing it to gradually develop a russet-brown back. The two colonies never intermarried ; the slight differences would therefore soon become constant by Isolation, and finally the result is as we see it at the present time — two nearly allied, but apparently perfectly distinct, species inhabiting different areas of distribution. No Natural Selection could possibly have been at work in such a case ; for it could have served no beneficial purpose.

* It was no advantage for either colony to possess those differences that have finally become constant characters ; they gave their fortunate possessors no favour in the struggle for life — a russet back was no more advantage to its possessor than an olive back, though both have been preserved, not by the survival of the fittest, but simply by Isolation.

Let us now glance at a few of the Pacific-Island Thrushes. Not counting Merula albiceps from Formosa, and M. javanica from Java, Sumatra, Borneo, and Timor, which can scarcely be regarded as belonging to this little group of Oceanic Thrushes, we have ten species distributed throughout the Pacific, all obviously closely, some very closely, allied, and evidently all descendants from one or two common ancestors at no very remote period. Of these ten species no less than five are almost exclusively distinguished by the colour of the head and neck, which is in violent contrast to the rest of the

ISOLATION.

plumage — pattern of colour which can only show community of origin. Three of these species are nearly uniform black in colour ; but one of them, M. pritzbueri , has the head and neck creamy grey and is confined to Lifu, one of the Loyalty group of islands ; another, M. bicolor, inhabits Kandavu, one of the Fiji Islands, and has the head and neck orange-buff ; whilst the third, M. tempesti, is distinguished by having those parts brownish grey, and is apparently confined to Taviuni, another island in the Fiji group. The remaining two species are almost uniform black, with the exception that the under tail-coverts have pale shaft-lines and tips; but this may be a mark of immaturity. One of these two species, M. albifrons , is distinguished by its whiter head and neck, and is confined to Eromanga, an island of the New Hebrides group ; whilst M. polioceplutla , having the head and neck brownish grey, inhabits Norfolk Island, between New Caledonia and New Zealand. Of the remaining five species, one, M. xanthopus , inhabits New Caledonia, and is almost uniform dark brown ; another, M. vinitincta, is found on Lord Howe’s Island, and only differs from the preceding species in being lighter in colour and slightly smaller in size. A third, M. vitiensis , is confined to Yiti Levu, one of the Fiji group, and is chiefly distinguished by having the underparts below the breast rich deep chestnut ; M. samoensis is confined to the Samoa Islands, and is characterized by its uniform dull black plumage ; and, lastly, M. mcireensis, from Mare, one of the Loyalty Islands, is almost uniform dark brown, with black wings and with chestnut-brown shafts and tips to the under tail-coverts. What do we infer from these interesting facts ? Probably most, if not all, of the island homes of these Thrushes are of volcanic origin. Large islands have perhaps been split up

EVOLUTION.

into several smaller ones by volcanic action, and by this means the individuals of one or two parent species have been isolated, where in course of time they have become differentiated. Conditions of life throughout this ocean must be very uniform ; the birds on one .island would be exposed to practically the same struggle for existence as their congeners on another island, so that any of the slight differences which the several species now present could never have been preserved by any Selective process. The shade of colour of the head could not possibly be of any benefit in the struggle for life, or have determined which individuals should live and which should die ; streaked under tail-coverts could not be of any advantage over plain ones, and thus be preserved by Natural Selection weeding out all individuals that did not chance to possess them, seeing that birds with both kinds inhabit localities not very far apart.

Or we can suppose that at some remote time the islands of the Pacific were peopled by emigrant birds driven from northern haunts by a glacial period. We can understand how one or two species swept down and located themselves on the islands of the Pacific. Some gained places where they could live and multiply ; others were probably cast on unsuitable islands, where they eventually died out. No cause for any migration arose, the birds on one island did not visit those on another island. In time Isolation and Variation would gradually split up the one or two parent species into several forms, peculiar to certain islands ; and the results are as we now see them — almost every island that can support a Thrush having a species peculiar to it, differing in some cases but very slightly from the species on neighbouring islands.

ISOLATION.

How, then, is it possible to make Natural Selection responsible for the evolution of these island Thrushes? Their conditions of life are similar ; there is no reason why one species should be better fitted to cope with its surroundings. We have no evidence to show that any competition between species took place ; no selection of individuals was required ; still a change occurred — a change that eventually slowly but surely segregated a species into several representative forms. How are these species evolved ? Simply by individuals becoming isolated on certain islands, where no interbreeding could take place, and where the variations any of those individuals might chance to present were preserved, not by the survival of the fittest, but by their isolation, and finally became fixed characters. The case of these Pacific-Island Thrushes is only one amongst thousands of similar cases where islands possess peculiar resident species, nearly allied in most instances to species on neighbouring islands or continents, and which, there can be no reasonable doubt, have become modified bv their isolation.

Further, it seems that when these Thrushes were driven down to the Pacific some were isolated in India and Formosa. Thus we have in India M. ccistanea , a species nearly uniform rich chestnut, with black wings and tail, but with the head and neck pale grey. In Formosa, as previously stated, M. albiceps is found, a species with a white head and neck, nearly black upper parts and chestnut lower parts. Both these species are obviously very closely allied to many of the Pacific-Island Thrushes, and owe their specific distinctness to their isolation. In the same way we can understand how it was, when the common ancestor of M. jcivanicci and M. vitiensis was driven southwards, some of the individuals of that

EVOLUTION.

parent species lingered in Java, Sumatra, Borneo, and Timor, and possibly other islands in the Malay archipelago, whilst others sped onwards to the Fiji Islands. The descendants of the birds that were isolated in the northern islands are known as M. javanica — a species almost uniform brown, except below the breast, which is rich chestnut, and the under tail-coverts, which have broad pale shaft-lines and tips. The birds in the southern islands are known as M. vitiensis , closely resembling their northern cousins, but the brown on the head, neck, and breast is paler.

We have seen how species can be modified and local forms evolved by Isolation on islands and mountains ; now let us see how the same great universal law can work on continents. A most interesting instance is to be found in the common House-Martin ( Hirundo urbica ) and its allies. It is rather doubtful whether the little English House-Martin, with which we are all so familiar, occurs east of the Ural Mountains ; but from the valley of the Yenesay eastwards to the Pacific a nearly allied but perfectly distinct species, H. Icigo- 2)ocla, is found. It only differs from the Western species in having the longest upper tail-coverts white instead of black, and in having a shorter and squarer tail. Besides these, we have another Martin, H. dasypus, confined during the breeding-season to Japan, which is rather larger than our bird, and has the chin, just at the base of the lower mandible, black — a character only perceptible on close examination. It is also characterized by its slightly forked tail and black longest upper tail- coverts. Another ally of our Martin, H. cashmirensis , is confined to the Himalayas, and is a somewhat smaller bird, also with the slightly forked tail of H. lagojioda, but the black longest upper tail-coverts of the

ISOLATION.

English Martin. We have here four species of Martin, all very closely allied, yet presenting constant distinguishing characters. It is, however, impossible to conceive how the trivial differences which characterize them have been acquired by Natural Selection or the survival of the fittest. A minute black spot on the chin, a more or less forked tail, or black longest upper tail-coverts could not possibly have been of sufficient benefit or advantage, in the several localities to which the birds possessing them are peculiar, as to ensure the elimination by Natural Selection of all individuals that did not chance to develop them. There can be scarcely any doubt that Isolation again has been the means of evolving these species. At some remote period all these Martins were the same species, with a continuous area of distribution ; but circumstances arose that eventually isolated the individuals of this species in four districts — in Japan, the Himalayas, East Siberia, and Europe. Not being able to interbreed, these four colonies of Martins gradually developed the slight specific characters which they now possess. The Common Swallow ( H . domestica) and its numerous allies present very similar facts ; and there can be little doubt that all the representative species of this bird are descendants from a common ancestor, not by Natural Selection, but by Isolation preserving the several variations, which have finally become fixed and definite characters.

The Dipper is another remarkable instance of how a species can be modified into several representative forms by Isolation. The Dipper throughout its wide area of distribution is practically a sedentary species, even the birds inhabiting one river-valley comparatively seldom visiting and interbreeding with the birds of the adjoining valleys, so that even

EVOLUTION.

in England slight differences are easily detected by an experienced eye amongst the birds from certain districts. The Irish Dipper is said to be slightly different from the English one. Confining our observations to the West Palaearctic Dippers, we find the following interesting facts. Beginning ith our own islands, we find that the Dippers ( Cinclus aquaticus britannicus ) are slightly different from those on the continent, having the chestnut on the lower parts developed to its greatest extent, and the head and nape very dark brown. Away northwards in Scandinavia the Dipper ( C . melanog aster) inhabiting the wild mountain-streams of that rugged country has no trace of chestnut in its plumage, that colour being replaced by nearly black. On the streams of the Vosges Mountains and the Pyrenees another form of Dipper is found, intermediate in colour between the typical British form and C. cilbicollis , inhabiting the countries bordering the Mediterranean Sea. This latter species is found in the mountains of Spain, Italy, Greece, and Algeria, and is distinguished by its much paler upper parts. Were we to follow the Dipper into Asia similar facts would be presented ; other forms occur in several districts of that enormous continent ; but sufficient has been gathered for our purpose. Erom its habits, and from the nature of the localities it frequents, the Dipper is a bird peculiarly liable to those modifications in colour which Isolation so often produces. But in many localities the ranges of some of these Dippers coalesce, and then interbreeding takes place, so that in a large series no hard-and-fast line can be drawn between them. These extreme forms, however, rarely, if ever intermarry; but the individuals in one valley sometimes mate with their congeners in an adjoining valley, and they in their

ISOLATION.

turn with their neighbours in the next. This interbreeding is most probably the reason why each of these races of Dipper has not become completely differentiated. The differences Isolation produces are being constantly nullified by the intermarriage of individuals ; this free intercrossing keeps in check, by blending together, slight differences in colour which may arise. If from any cause this interbreeding should cease, Isolation would, in the course of time, effectually complete the work it has commenced, and slowly but surely evolve these geographical races into species. Climate may have some influence on the colour of the plumage of these Dippers, developing the chestnut in warm and wet localities, the black in cold and the pale grey in dry ones ; but it seems more probable that Isolation is the most important agent in the work of differentiation. We know that other species inhabit the entire area occupied by the Dipper, and are exposed to precisely the same climatic changes, yet do not present any noticeable difference in colour, probably because their area is continuous and they intermarry throughout that area. That climate does modify species in a most remarkable manner, I am not for one moment going to deny : indeed I hope presently to show that it is one of the most important agents in the modification of species ; but in the case of the Dippers it does not seem probable. We have here another convincing proof that Natural Selection has not been the agent of modification. Such slight differences could not possibly be of any benefit to the birds acquiring them. These variations were not preserved by the survival of the fittest, but purely by Isolation. The cause of the variation is, after all, of only minor importance in such a case.

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The Common Wren {Troglodytes parvulus) is a very similar instance to the foregoing. This little bird in one of its numerous forms is found in the temperate parts of the Palaearctic, North Oriental, and Nearctic Regions. There can be little doubt that formerly, when the Arctic regions were much warmer than they are now, the Wren was circumpolar. Now its range does not extend anywhere above the Arctic circle. Throughout its extensive distribution it has become isolated in some localities, and is exposed to considerable differences of climate, with the result that ornithologists recognize upwards of a dozen races or species. A glance at one or two of these will be sufficient for our purpose. The Wren found in the British Islands is similar to that inhabiting the greater part of continental Europe ; but in St. Kilda, as I had the good fortune to ascertain last year, there is another very distinct race, distinguished by its much greyer colour and barred upper parts. It is known as T. Tiirtensis. In the Faroes and Iceland the Wren {T. borecdis) differs from that in the British Islands in being darker on the upper parts, the underparts are more strongly barred, and the beak is longer. From its habit of perching on rocks rather than on trees and bushes, its feet, like those of the St. -Ivilda Wren, are much stouter. These Wrens are not migratory; the differences they present are so trifling that it is impossible to conceive how they could be of any benefit in the struggle for life, and thus be preserved by Natural Selection. Again we have a case where Isolation has produced the differences above alluded to. We can imagine how, as the arctic cold increased, the Wrens were driven southwards from their circumpolar area : some tarried in the comparatively mild climate of Iceland and the Faroes, others sped onwards