Recent progress in the study of variation, heredity, and evolution

Preface

The idea of writing this little book occurred to me whilst reading Mr. W. C. D. Whetham's volume on 1 The Recent Development of Physical Science/ I found the story of the modern progress of physics so interesting as to encourage the belief that a similar account of the subjects with which I was myself more particularly familiar might prove of a like interest to other people. I did not, indeed, suppose for a moment that I could vie with Mr. Whetham in the power of literary expression which renders his book so eminently readable. I rather hoped that the peculiar interest and importance of the theme might outweigh the present author's deficiencies in this respect.

For the group of subjects of which I intended to give a brief account Mr. W. Bateson has recently proposed the term * genetics/ an expression which sufficiently indicates their scope to the initiated. Since, however, the meaning of the word ' genetics ' is not yet clearly understood by everybody, it seemed better to adopt in the present instance a somewhat more descriptive title.

vii viii

PREFACE

The rediscovery of Mendel's law some seven years ago led to a complete change in our attitude towards the problems of variation, heredity, and evolution ; and the new method of study thus introduced has rendered possible a renewal of the attack upon these problems with increased vigour and with remarkable results. At the present time this activity may be said to have its centre in the school of genetic research founded at Cambridge by the independent energy of Mr. Bateson. So far-reaching are the results already arrived at by Mr. Bateson and others, both in their scientific interest and in their probable influence upon human affairs, that it seemed desirable to give an immediate account of these and of kindred lines of recent study, even though the rapid progress which is a characteristic manifestation of this department of science must render any such attempt a more or less transitory one.

Whilst I was still engaged upon my task, the first volume of Dr. Lotsy's admirable ' Vorlesungen iiber Descendenztheorien ' made its appearance. But for the fact that most of the following pages had then already been written, I might have hesitated to pursue my project, since a book not altogether unlike the present might be produced by the comparatively simple process of making a series of judicious extracts from Dr. Lotsy's work. The latter is, however, in the German language, and on a considerable scale, so that there seemed still to be room for an introduction to

PREFACE ix

v.

the science of genetics of the more modest dimensions which I had contemplated. I should wish, however, particularly to recommend Dr. Lotsy's lectures to any reader who wishes to go further into these matters.

I am indebted to several friends for assistance during the course of my work. Mr. R. P. Gregory kindly read through the proof of the chapter on cytology ; and T wish here to record my thanks to Mr. J. Stanley Gardiner, to Mr. C. T. Regan, to Mr. W. S. Perrin, and to Mr. R. C. Punnett for information on special points. To the last-named I owe the photograph which appears as Fig. 15. I am particularly grateful to Mr. R. H. Biffen and to Mr. G. Udny Yule for access to work which has not hitherto appeared in print.

Adequately to acknowledge Mr. Bateson's influence upon these pages is a more difficult matter, and not the less so because I have deliberately refrained as far as possible from consulting him whilst the book was in course of preparation, in order that it might retain if possible some traces of individuality. It is therefore clear that he is in no way responsible for its deficiencies. But, apart from the fact that I am conscious of having quoted his ideas at more points than could possibly be acknowledged seriatim, I owe to Mr. Bateson both my first introduction to the science of genetics, and a continual fund of encouragement in the prosecution of studies connected with it.

I have to thank Mr. Francis Darwin for kind permission to reproduce a portrait of his father ; Professor

X

PREFACE de Vries for the present of an excellent portrait ; and Mr. Francis Galton for the loan of a photograph well known from earlier reproductions. The portrait of Mendel is reproduced from the frontispiece to Mr. Bateson's 1 Defence,' by the permission of the Syndics of the Cambridge University Press. Messrs. Macmillan and Co. have kindly allowed the reproduction of the diagram which occupies p. 80, and of the table and figure on pp. 82 and 83. The figures facing pp. 136 and 143 are from de Vries' ' Mutationstheorie,' published by Messrs. Veit.

The attempt has been made to render the following pages intelligible to the general reader, as well as to the more scientific public, to which they are primarily addressed. A short glossary has been added, which may be found useful by those who have no previous acquaintance with biological terms.

Cambridge,

Chapter I

Subject-matter — The study of evolution — Dependence on other sciences — The problem of the nature and origin of species — Linnaean species — Jordan's species — The discontinuity of species — Methods of study — Variation — Different theories as to the origin of species : Lamarck's theory ; Darwin's theory ; the mutation theory — Recent study of species ; Biometry ; Mendelism -

Chapter II Evolution

Definition — Evolution of matter — Evolution in geology — The age of the earth — Organic evolution — Evidences of organic evolution — The theory of Lamarck ------

Chapter III

Historical — Darwin's attitude to variation — Artificial selection — Malthus and the geometrical rate of increase of living things — Natural selection — Adaptation— Protective resemblance and mimicry — Parallel evolution — Regeneration — Sexual selection — The inheritance of acquired characters — Herbert Spencer's arguments — Weismann and the purity of the germ cells -

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CONTENTS

Chapter IV Biometry Pack

Biology and statistics — Normal variations — Definite differences — Biometrical methods — The normal curve — Probable error — The biometrical treatment of heredity — Correlation — Structural and mental characters — The law of ancestral heredity — The effect of selection — Johannsen's theory of the pure line — Selection in pure lines and in populations - 73

Chapter V

Objections to the theories of Lamarck and of Darwin — Aristotle and Dr. Thomas Brown on mutation — Galton and organic stability — Bateson and discontinuous variation — Symmetry — Merism — Similar and simultaneous variation — Homceosis — The views of de Vries — The effect of selection — Mutation in (Enothera — Eversporting varieties — Different methods of mutation — The survival of useless structures - - - - - -113

CHAPTER VI Kolreuter — Knight — Herbert — Gaertner — Naudin

Millardet - - - - - 148

Chapter VII

The double nature of animals and of the higher plants — Unit characters — The inheritance of definite characters in cross-bred maize — Mono-hybrids and di-hybrids — Mendel's law — Gregor Johann Mendel — Experiments with peas — Dominance — Heterozygote forms — The case of the Andalusian fowl — The nature of allelomorphs - - - - - 163

CONTENTS xiii

CHAPTER VIII MENDELISM {continued) PAGE

Coupling : complete and partial — Reversion on crossing : latent characters — Examples in peas, mice, and sweet-peas — Various Mendelian characters — Doubtful cases ; normal fertilization — The supposed ' swamping ' of new species on crossing with the old — Summary of Mendelian ideas — Mendelism and biometry : researches of Yule and Pearson — Practical applications : BifFen's work with cereals ; ' strength ' of wheat-flour and rust-immunity - - 185

Chapter IX

The cell — Cell-division and cell-fusion — The nucleus — Nuclear division — The nature and properties of chromosomes — The reducing division — The study of the chromosomes in the light of Mendel's discovery — Chromomeres — The heredity of sex — The germ-plasm theory ----- 222

Chapter X

The science of genetics — Mendel's law — The ^-generation and the 2#-generation — Discontinuous variation — The mutation theory — Continuous variations — Genetic variations and acquired variations — Johannsen's theory of the pure line — The modern view of the method of origin of new species — The science of genetics applied to human affairs — George Bernard Shaw's view of the situation - 264

LIST OF ILLUSTRATIONS

PORTRAITS

Charles Darwin - Frontispiece Francis Galton To face page 73

Hugo de Vries

Joseph Gottlieb Kolreuter

Gregor Johann Mendel

DIAGRAMS

PAGE

PAGE

Fig. 1 -

Fig. 14. Mendelian Pro-

Fig. 2 - portions in Maize

Fig. 3 -

To face

Fig. 4 -

Fig. 15. Inheritance in

Fig. 5 -

Primula - To face

Figs. 6 and 7

Fig. 8 -

Figs. 17 and 18 -

Fig. 9 -

Figs. 19 to 26 -

Figs. 27 to 31 -

Figs. 32 to 37 -

Fig. 12. Mutation in

Figs. 38 to 42 -

(Enothera - To face

Fig. 13. Trifolium Pra-

Figs. 44 and 45 - tense Quinquefolium

To face

RECENT PROGRESS IN THE STUDY OF VARIATION, HEREDITY, AND EVOLUTION

Chapter I

The present volume deals with variation and inheritance in plants and animals, especially in so far as those subjects bear upon the problem of the origin of species. By inheritance we mean those methods and processes by which the constitution and characteristics of an animal or plant are handed on to its offspringi this transmission of characters being, of course, associated with the fact that the offspring is developed by the processes of growth out of a small fragment detached from the parent organism. The term 4 variation,' on the other hand, includes a number of different phenomena which will be described at greater length as the work proceeds ; but, broadly speaking, we may say that the study of variation is concerned with the circumstance that members of the same species are not all alike, and more particularly with the fact that differences are to be found between different members of the same family. Some of these differences arise comparatively late in life, and may be the result of circumstances or of education. It is the first duty of the student of variation to distinguish as far as may be possible between differences of this kind on the one hand, and those differences on the other which depend upon the fact that the different detached fragments, as we have termed them, of the parent organism — its germ-cells, in fact — show greater or smaller differences among themselves.

The facts of variation have this very special importance, that the whole theory of organic evolution is based upon them. The fact that members of the same species are not all alike, depending upon the further fact that offspring may differ from their parents, makes it possible in the course of generations for progressive changes to take place, so that from the offspring of different members of the same species different new species may arise. But for this fact of variation it would have been quite impossible for Darwin to have overthrown the former crude belief in a special creation of each separate species, since there would have been no material for his great factor — natural selection — to work upon. It is with variation, then, and with the manner in which characters appear in the successive generations of living things, that we are here concerned.

Ever since the publication of Darwin's 'Origin of Species ' in 1859, these subjects, and especially the theoretical aspects of them, have been received even

THE STUDY OF EVOLUTION 3 by the general public with all the signs of a genuine enthusiasm ; and none,; moreover, can be more fascinating to the professional naturalist. But since the time of Darwin the more popular accounts have dealt almost exclusively with theoretical considerations and with matters of opinion. Highly abstruse controversies have raged freely between Neo-Lamarckians and Neo-Darwinians, and these have found a place in the pages of works ostensibly intended for the instruction of all and sundry ; whilst only a bare residuum of actual matters of fact has seen the light of popular publication. If the truth must be told,, the experimental method was given up for a long time by the majority of specialists themselves in favour of the controversial, and, indeed, this tendency has by no means yet died out from among the habits of some professed evolutionists. On the other hand, during the last fifteen to twenty years, a few scattered workers have diligently applied themselves to a study of the facts of variation and inheritance, with results which already more than justify the anticipation in which their work was begun — namely, that by such methods alone can any real progress in our knowledge of the processes of evolution be brought about.

The science of organic evolution is by no means the simple and isolated study it might be supposed to be from a perusal of some of the more popular accounts. Its footing rests immediately upon the widest foundations which zoology, botany, and physiology can afford ; and these in their turn are ultimately based upon the results of chemical and physical science. But some

1 — 2 of the most fundamental parts of physical science, as I think we may fairly call the branches of electricity and molecular physics, seem at present to be undergoing modifications which bid fair to bring about a complete revolution in current ideas upon these subjects. It is highly probable that these results will ultimately lead to a considerable modification in prevailing notions about living things ; but the new developments have yet to reach biology through the channels of organic chemistry, physiology, cytology, and the like, and at present we do not know what the result of this influx is likely to be. These considerations need not, however, detain us, for the new knowledge of variation and inheritance, of which it is proposed to give some account, is largely concerned with the grosser characters of organisms, so that ultraminute structures may be left alone for the present until the stream of physical knowledge stirs them into greater prominence. So much is this the case with the study of variation and inheritance by experimental methods at the present day, that this science is treated by some with a fine contempt,, because its tools are those of the breeder and gardener, and because the assistance of the compound microscope may often be laid aside for days together. Yet this applies only to one aspect of the subject, and the microscopic study of the embryonic rudiments of organisms, going hand in hand with the experimental observation of adult structures, is rapidly leading to a clearer understanding of the processes of heredity.

The problem which those who are engaged in this

THE PROBLEM OF SPECIES 5 kind of work have set themselves for solution is that of the nature and method of origin of the existing differences between certain groups of organic beings — namely, species. Basing their studies on the doctrine that the present species have arisen through the modification of pre-existing species, they endeavour to observe how modifications of existing species do actually arise in Nature, as well as under domestication ; and they watch the hereditary transmission of the modified forms when like is bred with like, and when different types are crossed together. For the theory of uniformity, now universally accepted, teaches us that the organisms with which we are now familiar owe their present characteristics to the accumulation of a series of changes similar to those which are still in progress. It has, therefore, appeared likely to a few that a further understanding of the processes of evolution might best be obtained by a closer study, firstly, of variation, or the ways in which offspring differ from their parents ; and, secondly, of inheritance, or the ways in which the resemblances between parents and their offspring are perpetuated from one generation to another.

It may be well to point out at once that the further study of the method of origin of new species, admitting, as it does, that this process is not yet by any means fully understood, does not for this reason imply that the theory of organic evolution itself is open to criticism. The evidence that new species arise by the modification of pre-existing species is quite independent of the evidence that this process invariably occurs by the action of natural selection upon minute differences, in the manner which Darwin described, and which has been claimed by others as the sole means by which the origin of new forms takes place. The evidences of evolution are much more numerous and more weighty than the evidences of the survival of the fittest. The theory of evolution, as opposed to the creation hypothesis, is supported by innumerable facts of classification, of morphology, and of embryology, by the geographical distribution of animals and plants, and by their succession in the geological strata, as well as by direct observation of the actual occurrence of changes in the case of domestic productions as well as under Nature, and many of these facts have no direct bearing upon the theory of natural selection.

Before discussing the problem of the origin of species, it is necessary to arrive at some idea as to what the term ' species ' means. And this is not altogether an easy matter, since a precise definition has not been, and cannot be, agreed upon. The idea of species is, indeed, of great antiquity and very gradual growth. Primitive men doubtless recognised certain plants or animals as being like one another, and different from others, and they gradually came to distinguish such forms by giving a different name to each. The names first used must have applied as a rule to genera rather than to species. Thus, such common names of plants as rose, bramble, vetch, nettle, dock, crowfoot, are names of genera — groups of greater extent than species, and often more easily defined than the latter. Later

LINN^EAN SPECIES on civilized men paid closer attention to tne different kinds of plants, and the old herbalists discovered and described a number of different sorts of roses, of buttercups, and of other plants,, and distinguished each by a descriptive sentence.

As more and more species came to be described, this method of designation became very cumbersome, until Linnaeus, about the middle of the eighteenth century, adopted the idea of a binomial nomenclature (originally suggested by Bachmann), in which every species of each known genus received a separate name of its own to distinguish it, so that the different kinds of buttercups were now known as Ranunculus acris, R. bulbosust R. sceleratus, and so on.

Linnaeus himself appears to have had a very definite idea of what constituted a species, and in accordance with the view then current, he defined a species as being a group of organisms which owed its origin to a separate act of creation. From the nature of the case this definition could be of little use in practice. Practically,, then, species were defined as groups of animals or plants, the members of which resembled one another in definite morphological characteristics — that is to say,, in constant features of form and structure. This definition has survived the downfall of the dogma of the constancy of species, and at the present day species as defined by Linnaeus are found to be groups of much merit both for naturalness and for convenience — at any rate in the case of plants. The fact that intermediate forms and minor groups do sometimes and to some extent bridge over the gap which separates a pair of species thus defined seems to have caused disquiet in the mind of Linnaeus himself, and he recommended his disciples to have no dealings with these inferior varieties, as being beneath the dignity of a botanist to notice. Of late years these minor species have excited much attention, and it is to a study of this kind of species in particular that the mutation theory of de Vries owes its origin, as will be told in a later chapter.

Such minor groups, occurring within the limits of a single Linnaean species, and subdividing it into smaller collections of individuals, were made the object of special study in the case of plants by the French botanist, Jordan ; and for this reason they are sometimes referred to as Jordan's species. Jordan, for example — though the example is indeed an extreme one — described more than two hundred different types, all of which would formerly have been included in the single Linnaean species, Draba verna. To take a more familiar instance. We find in the ' British Flora ' of Bentham and Hooker the primrose* the cowslip, and the true oxlip, all described as varieties of one and the same species ; yet these three kinds of plants are now almost universally recognised to be as good species as any in nature.* In a similar way, on closer investigation, it has been found necessary to split up a considerable number of Linnaean species, and to subdivide each into several species of smaller range.

* A contrary opinion is, however, expressed in the Journal of Botany for July, 1906.

JORDAN'S SPECIES

It has already been pointed out that Linnaeus himself distinctly deprecated this process of splitting. * Varietates levissimas non curat botanicus,' said Linnaeus. Jordan, however, applied the method of experiment to many of the species of his own definition, and having transplanted them from a variety of localities to the uniform soil of a garden, found that they preserved their distinctive characters and came perfectly true to seed.

It appears then that Jordan's species are just such true and constant groups as those of Linnaeus. They are separated from one another by definite features of form and structure, only these differences are not so wide as those which separate Linnaean species. The latter are, indeed, to be looked upon as more or less artificial groups or aggregates of these physiological species, as Jordan's species have also been called. The problem of the origin of the smaller groups is clearly to be placed before that of the origin of the larger species.

It is true that in the case of certain groups of animals and plants there would appear to be no possibility of drawing hard and fast lines between the species, which thus seem to shade gradually one into the other. There is, however, a great difference between the admission that certain nearly related species are difficult or impossible to separate definitely, and the statement that there is no true distinction between them, and the latter statement is one which few are bold enough to make. The case stands thus. We know that great numbers of large groups (classes and families) of animals and plants exist, in which the most nearly related species are quite definitely distinct from one another. In other classes systematists have so far found great difficulty in framing definitions of specific groups. We shall see later on, though at first sight it may appear almost paradoxical, that it is quite possible for groups to be perfectly distinct, although individual members of them may have deviated so far, each from its proper type, as to render impossible the task of deciding from their appearance which group any of these individuals belong to.

Let us next consider a particular example of a class of animals in which the discrimination of species is difficult or impossible. This is said to be the fact with the majority of sessile animals — such animals as resemble plants in their stationary habit, and in no case are the problems of species separation more difficult than in the class of the stony corals. Now, attempts to determine the species of corals have so far been made almost entirely from a study of what may be called vegetative characters — usually from details of the shape and structure of the stony skeleton of the animals. How far these features may be affected by external circumstances has not been determined, but it must be noted that the so-called skeleton is entirely external to the living organism. Now we know that in the case of many of the higher plants vegetative characters are extremely liable to become modified owing to the action of the environment. Differences of moisture, light, soil, climate, and alti-

DISCONTINUITY OF SPECIES n tude, are all capable of changing the general appearance of a plant so as to render it scarcely recognisable. Fortunately, in the case of the higher plants, the floral organs, which are the ones chiefly made use of for purposes of specific discrimination, are very little liable to modification by external conditions ; but in the corals a similarly stable set of organs does not appear to have been discovered. It seems, therefore, hardly fair to regard the example of the corals as affording an established exception to what we must look upon as the general rule — namely, that species are on the whole definite and discontinuous groups.

As a rule, then, the species riddle presents itself definitely as the problem of the existence of a series of discontinuous groups of creatures, sharply marked off the one from the other, and often, too, existing among surroundings which afford no corresponding discontinuity, though each is well enough fitted for the life which it has to lead.

The problem which we have to face has been enunciated by Bateson in the form of the two following propositions :

1 1. The forms of living things are various, and on the whole are discontinuous or specific.

' 2. The specific forms on the whole fit the places they have to live in.

' How,' he continues, ' have these discontinuous forms been brought into existence, and how is it they are thus adapted ? This is the question the naturalist is to answer. To answer it completely he must find

(i) the modes and (2) the causes by which these things have come to pass.'

The differences between existing species are open to study in more than one way. By way of limiting the discussion for the present,; we shall consider the < case of plant species only ; but the methods of study which are applicable to animal species are of quite similar kinds.

Four methods at least are available. Firstly, that of comparison ; secondly, the method of statistical examination ; thirdly, the method of cultural experiment ; and lastly, that of cross-breeding.

The method of comparison is the one to which the ordinary worker in descriptive botany is almost of necessity confined. In this way plants which closely resemble one another are grouped together as belonging to the same species, and separated from others, the appearance of which is different. By appearance is not meant simply the general habit of the plants ; aU morphological features whatever may be used for purposes of comparison, and the most minute are often of the greatest importance. But the systematist who works only in this way knows nothing of the real relationships between the plants with which he is dealing.

When a sufficient number of specimens is available, the methods of statistics can be applied. These involve the making of a series of accurate measurements or countings of the parts upon which depend the supposed differences or resemblances of the plants

METHODS OF STUDY under consideration. The resulting numbers are then ranged in order so that a precise view of the numerical characters of a large number of specimens can be readily obtained. By the use of such methods valuable information is often to be arrived at. But the same limitation affects them as in the preceding case.

So that the only way in which we can come to a definite decision as to whether a given putative species does or does not represent a definite and constant type is by resorting to our third method, that of sowing its seeds and actually rearing its progeny. And this is not so simple a matter as might appear at first sight, for a great many precautions have to be taken. Thus we must separately sow the seeds of many different individuals of the species which we are examining, so as not to base our conclusions upon a few experiments only. But in many cases, even when this has been done, we should only know one of the parents of our seedlings — that is to say, in cases where the pollen for fertilization may possibly have been conveyed by natural agencies from a different plant. In such a case we must either ensure selfpollination by isolating our plants, or we must artificially provide pollen from a separate known parent. If under these circumstances a particular group of plants preserves the characteristic differences which distinguish it from another group which has also been grown for a number of generations under the same conditions, we have at last reasonable grounds upon which to base the opinion that we are dealing with two distinct physiological species, even though the visible differences between them may seem very small to an untrained eye.

Lastly, evidence of a confirmatory nature may be obtained by observing the results of cross-fertilization between a pair of closely allied species. Much, too, may be made out from the failure of such experiments, since the refusal of two plants to breed together is generally regarded as clear evidence of their specific distinctness. But for this reason the method of cross-breeding is more particularly adapted for the examination of forms somewhat nearly related to one another — for example, different members of the same species.

As the result of the methods presently to be described, the fact has been established that two entirely distinct sorts of divergencies may appear among members of a single family. Variations, that is to say, may be of at least two different kinds. In the first place we have those slight differences which invariably distinguish all the members of every family — individual variations which affect every part and every character. Such differences are known as fluctuating, normal, or continuous variations. As an example we may cite the variations in size or stature shown by the various members of any purely-bred race. When a large number of individuals are compared in respect of a character of this kind, they are found to fall into a continuous series ranging from a certain extreme of shortness on the one hand to an extreme of tallness on the other. Individuals of a medium height,, how ever t are usually more numerous than either