The growth of groups in the animal kingdom

Front matter

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Preface

This book deals with the subject of evolution in a somewhat discursive manner, but its main theme is indicated by the title. It points out that groups of like animals, such as are small in membership and temporary in duration, are common in nature. These groups are not considered to be equivalent to species, they are referred to as family groups. A large part of the book deals with the origin, fate, and significance of such groups. Here and there the question. What is a species ? is raised, but not answered. The tenth chapter is an attempt to show that the Mutation Theory is important to the physician, and the last chapter borders on philosophy. The whole appears more compact, if it be regarded as an attempt to help on the new movement in biology, to spread those ideas referred to by some as the Mutation Theory, by others as Mendelism, ideas that are permanently associated with the names of Mendel, Bateson, and De Vries.

The writer’s warrant for asserting himself lies not in the bulk, but in the variety of his experience, which has been derived from several sources, but from none in particular.

At the present day so many books are written by specialists for specialists that it is well to point out that

VI

PREFACE this one, though purely biological in scope, is the opposite of a specialist’s book. It is hoped that if it comes into the hands of a taxonomist, a Mendelian, a pathologist, or an alienist, that he will take it as offered, not as an attempt to teach him his particular subject, but as an assortment of opinions which may be of suggestive value.

It is necessary to warn the reader, who may be inexperienced in biological matters, that the book is not in the least a review of the present position of biology, but an assortment of personal opinions, most of which are borrowed from well-known sources.

It may seem to the experienced reader that the main idea expressed in the last chapter is taken from M. Henri Bergson’s “ Creative Evolution,” but most of the last chapter, including the attempted illustration of things varying in all directions, was written in 1910 before I made my first acquaintance with M. Bergson’s enlightening philosophy. I wrote the passage “ Variation is chaotic or ‘ in all directions.’ Is it possible to know whether it is so or not ? What does the phrase mean ? ” before reading the words of Bergson, “ It is unquestionable that, when ordinarily we speak of disorder, we are thinking of something. But of what ? ” The matter is of little importance, but it must be referred to here, in mentioning the sources from which I have drawn.

My thanks are due to Mr. W. L. Tower of Chicago for permission to reproduce the illustrations on Plate II. It may seem that I have abused this kindness, by suggesting that his judgment of his own work cannot be considered final. It was, however, necessary for me to take this step, for if there is no evidence of the efficacy of mutants in the facts collected by Mr. Tower, there is less than none

PREFACE

Vll in those collected by myself. My opinion, whatever its value, is derived more from his facts than my own. If I have misrepresented any of his facts to suit my own opinions, I have done so unintentionally and I offer my sincere apology.

The frontispiece is the work of Mr. B. L. Das. The figure on the left is of a dead rat lying upon its side, and is very true to nature. Of the others, one is from a stuffed specimen and the other from an animal living in a cage, consequently, these two have a less natural appearance, but in all the colour is a true representation. The other drawings are mostly traced from photographs which have already appeared in the publications of the Indian Museum. (Rec. Vol. III. Pt. I., Mem. Vol. II. No. I.)

The map of Poona was kindly provided by Captain Kundart, I.M.S., to whom I am otherwise much indebted.

I must also express my warmest thanks to Captain G. I. Davys, I.M.S., and to many other friends, for the most part my fellow-workers in the Indian Medical Service, for much help and hospitality during the progress of the investigations which form the chief subject of the book.

COLOURED PLATES

PLATE

I. Varieties of Mus Rattus Frontispiece

II. Sports and Mutants of Leptinotarsa . . facing

PAGE

THE

GROWTH OF GROUPS

Chapter I Introductory

The method of the origin of species is still an open question. Although most biologists regard Natural Selection as a creative agency, yet there are dissenters from the accepted opinion who cannot justly be disregarded as inexperienced. Dissent is the outcome of a difference of judgment which is inherent in the dissenter. We do not know which of the two conflicting opinions will survive the other. Time will show.

A book dealing with this question may be written in one of three ways. It may support one or other opinion, or it may support neither. The author may approach the subject with an open mind and describe the various observations which have been made, the various opinions which have been held. Books of that kind are of value to the student, with their help he may be able to form an opinion for himself ; but they do less to hasten the survival of one or other opinion than those of the factional kind.

It is easy to be deceived as to what constitutes factional writing. An author may claim to have examined the evidence relating to a disputed question with an open mind and to have formed a decided opinion in one

B direction ; but in so doing he deceives only himself, to the critical reader his personal conviction or bias is obvious from the first. It is well, therefore, to admit at the outset that this book is written in the factional spirit as an expression of opinion, derived mainly from the writings of others but supported by personal observation.

Darwin has said, “ We have no evidence of the appearance or at least of the continued procreation, under nature, of abrupt modifications of structure.” These words call attention to the fact that abrupt modifications may occur ; in other words, that an offspring may possess some definite and obvious quality or qualities which were not possessed by its parents. Furthermore, the words “ under nature ” remind us that such modifications may be propagated continuously under man’s protection. Several races of domestic animals and plants are known to have arisen abruptly from sports.

It seems, however, that Darwin’s conviction as to the inefhcacy of sports was unsettled, for in the sixth edition of “ The Origin of Species ” there appears the following well-known passage. “ . . . variations which seem to us in our ignorance to arise spontaneously. It appears that I formerly underrated the frequency and value of these latter forms of variation, as leading to permanent modifications of structure independently of Natural Selection.” Besides the admission they contain, these words are interesting as showing that gradual change should be looked upon as the essential condition of Evolution by Natural Selection. A race arising from a sport, in captivity or under nature, would occur independently of selection, as that process was conceived by Darwin. To speak of selection among mutants is to change the common conception of that agency. In its new role it does not appear as a creative process.

Several writers have urged the importance of sudden and obvious variations such as are called sports, or mutants. Huxley expressed his belief that races were occasionally derived from sports. In 1892 Bateson pointed to the discontinuity of species and explained it as the natural outcome of discontinuous variation. If by species we mean those groups which are being defined as such at the present day, and if by discontinuous variation we mean obvious differences which may be seen, exceptionally, when comparing offspring with their parents ; then it seems to me that Bateson’s proposition is indisputable. The evidence to be described in this book is in support of it.

In the Mutation Theory of De Vries we find the same opinions in a more decided shape, amply supported by evidence from the vegetable kingdom, indeed so complete is the evidence that botanists more generally than zoologists regard mutation as the method of evolution. Above all, through Mendel’s work and all that has followed from it in the last decade, we have gained an insight into the nature of living things, compelling us to recognize that the change which comes over them is not comparable to a gliding movement, but to a series of little steps, not always of the same width but often wide enough to be appreciated with certainty.

DIFFICULTIES ARISING FROM THE USE OF THE WORD

SPECIES.

Discussions on the origin of species are usually unsatisfactory because the term species has no decided meaning. The word may call up a different picture in the minds of independent persons. De Vries and other writers have emphasized the fact that most of the Linnaean species are divisible, that they are groups of lesser groups. No zoologist will deny that this is also true of the animal kingdom. The tendency at the present day is to split up those species defined by the older naturalists into lesser groups and to regard the latter as species. But there is much difference of opinion in regard to the reality of a species. There are two opinions, namely, that they are real, and that they are conventional. De Vries was unable to distinguish between the members of certain groups of plants and considered those groups as elementary or indivisible. He therefore regards species as real. An opinion on this question can only be formed by examining and comparing the attributes of mature organisms, and the opinion will vary according to the part of the living kingdom which is examined. An observer, while regarding the higher plants or simpler kinds of animals, may be sure that indivisible groups are appreciable ; but if he turns his attention to vertebrate animals he will doubt whether his former conclusion is applicable to them. When dealing with such complex forms, he may feel inclined to say, “ Give me a thousand individuals of any of your so-called elementary species and I will split them into lesser groups by taking into account certain minute characteristics/ * The structure and function of a vertebrate animal are so complex that it appears as an illimitable field for observation, in which one can explore and describe up to a point, depending upon the acuteness of the observer and the means of discovery. When dealing with a collection of mammalian animals we may say that they are alike as regards certain characteristics which may be enumerated, but we can never be sure that the enumeration is final as regards each individual. In defining any such collection as a specific group, we close our description at a point which is chosen in an arbitrary manner. Species defined in this manner are merely conventional. On the other hand, those botanists who speak of elementary species, must believe, in regard to any particular species, that they have explored the whole field of observation afforded by it, that no one could see in it more than they have seen. The botanist is more likely to arrive at this opinion than the zoologist because the field for observation is simpler in plants than in animals, there is less scope for disagreement in observation ; he feels that he can see and describe all that there is to see, that no man could see more, hence to him species are indivisible, or real and not merely conventional.

If the reader is of the opinion that species of mammals are also indivisible let him take another step higher and regard mankind, the most complex of living things and the one with which we are best acquainted. Among mankind we see such a variety of inheritable characteristics, present in some individuals but not in others, that we cannot possibly regard any group as indivisible. The individual alone is indivisible.

When two persons discuss the origin of species they will misunderstand one another if the one holds species to be real and the other regards them as conventional. If they differ on this preliminary point it is idle to proceed further.

In this book, species are assumed to be conventional.

There is, in my opinion, no such thing as an indivisible group of vertebrate animals.

HOW ARE SPECIES DEFINED?

What is a species among vertebrate animals ? Species are groups that have been defined as such, but these groups are not equivalent things ; some are characterized by many obvious attributes, others by a few less obvious ones. Those groups, which are being described as species at the present day, are for the most part of less weight than those described in the past. But although the conception has changed, species are merely groups that have been defined and the problem is to describe how they came to be as they are. In defining a group we are at liberty to regard any characters we choose. Naturally, we select the most obvious ones first. We select a number of obvious characters and set up a convention in the form of an assertion thus, “ Every animal in the world which possesses characters A, B, C, etc., is of the species X.” The systematist defines species in this manner, but in doing so he does not claim to have described every attribute which can be perceived by any means in every individual of the species, he has merely selected a number of obvious characters and defined the species by declaring that every animal possessing them is of the species. In this sense a species is a verbal description. In referring to a species it is considered necessary to add the name of its author ; by the species we mean the author’s description of it. The author of any vertebrate species must feel sure that he has overlooked a number of microscopic or unobtrusive characters which are also present in every individual of his species.

There is no essential difference between these unobtrusive characters and the more obvious ones chosen. Some characters are visible to the naked eye ; others come into view when we use a hand lens ; others again can only be seen properly with the help of a microscope. Thus in appearance they are gradually less and less perceptible. Those that are scarcely perceptible cannot as a rule become more evident by accumulation. We also know that specific differences exist beyond the point where they cease to become visible with the help of the microscope. The precipitin test has shown us that there are specific differences in the quality of the living substance. Therefore, in describing a species we must close our description at a point. Microscopic characters are often considered negligible. The only assertion implied in the definition of a species is that all the characters enumerated are present in every individual of it. The systematist then, must overlook certain characters because they are difficult to appreciate, although they may be present in every individual of the species he is defining. But, besides this, he deliberately overlooks certain perceptible and perhaps obvious characters, because they are not present in every individual of the species as he has chosen to define it, but only in some individuals. Such negligible characters are said to be individual and not specific, but none the less they may be perceptible and heritable attributes, although they are not present in every member of the species as defined.

What is the usual fate of a species X which has been defined in terms of certain chosen characters A, B, C ? Let us suppose that it has been defined by a naturalist whom we may call broad-minded or old-fashioned according as his methods seem to us to be good or bad. Sooner or later species X will come under the notice of another naturalist, who may be referred to approvingly as one of the modern school or disapprovingly as a splitter of hairs. He may express himself thus in regard to the species X. “ I have examined a large number of the species X as defined by my predecessor, I can recognize them plainly because they all possess the characters A,B,C as defined, but I do not propose to overlook those features which have been regarded hitherto as individual variations ; thus, among the old species X, there are a number of individuals which possess the slight but perceptible character a, also there are a number of others possessing characters a, b : I will therefore split up the old species into three new ones x, y, z, which henceforth will be defined respectively as A, B, C, as A, B, C, a, and A, B, C, a, b.” This splitting process is being carried on at the present day; there is no doubt that it depends on the existence of characters which, although unobtrusive, can be perceived, and it seems that there is no natural limit to the process. We can never be sure that any particular group of vertebrate animals is indivisible.

Let us also consider some other opinions which may be held in regard to the splitting up of the old-fashioned species. The older school of systematists who take the broad view, feel that the splitting process is harmful to their science, the aim of which is to trace the distribution of species. They have confidence in their belief that old-fashioned species such as X, which were defined in terms of many clear characters, arose in one place and spread from that place ; but they have no confidence in the belief that the new-fashioned species which perhaps have only one or two special characters of their own, arose in one place only ; in fact they feel pretty sure that such groups may arise on many occasions in widely separate places, that is to say at various points of the area which is occupied by the species X. They therefore object to x, y, and z being known as species, for if a species may arise in more than one place, all arguments concerning its distribution are futile, hence they object to any group being known as a species unless it has many clear characters of its own. The justice of their contention cannot be disputed. There is good reason to believe that groups such as x, which by some are spoken of as varieties, arise in separate parts of the area inhabited by the parent species, but there is little or no evidence that groups such as X arise in more than one place.

This discussion about groups X and x, will be clearer if in place of the letters we substitute known groups of animals. Three animals are seen in the frontispiece. All are alike in many ways, in their bodily proportions, in the form of their skulls and in most other respects ; in a broad sense all three belong to the species Musrattus, a species such as X, for they have many characters in common ; but as regards a single character which determines their coat colour they are different, one has a special character which finds its expression in the whiteness of the underparts, another has the melanic character. There is evidence that groups such as these arise from the parent species in separate places on various occasions, the method of origin being growth from unity. There is no evidence to show how the parent group Mus r a tins originated. These so-called varietal groups with their one special character cannot be regarded as different from a species, it is true that they are clearly marked by one character only, but there are many other groups which are marked off by the possession of two, three, or more characters of their own.

A TACIT ASSUMPTION IN THE TERM SPECIES

Before inquiring how animals came to be in their present state, it is well to know that state. We cannot of course know this, for we cannot see the whole of the species at a moment, we can only form an idea of it, and this idea may be and often is erroneous. Every time that a species is defined and the type specimen laid down in a museum, the tacit assumption is made that there are in the world a very large though unknown number of animals like that type specimen. Since the number is unknown we should not assume that it is very large in every case. My opinion concerning t}'pe specimens is as follows. The type specimens in our museums are representatives of groups, but the groups are of variable numerical size. Many of them no doubt contain at the moment millions of millions of individuals, others contain tens of millions, others only thousands or hundreds. A considerable number of them have become extinct since the time of their definition ten or twenty years ago. By virtue of its specific name a small group of animals which occupies merely the corner of a field is made to appear equivalent to a vast multitude of another kind which is spread over most of the earth. Similarly, a group which made its first appearance a few years ago and has since become extinct is made equivalent to a ii group which has come down unchanged from the Tertiary epoch. It is important to know whether this opinion is correct. It must be admitted that there is no reason for the assumption that every species contains an infinite and therefore negligible number of individuals.

The museum worker who defines most of the species has little opportunity of forming an opinion as to the number of individuals which are represented by a given type specimen. The collector who supplies material to the museum is often able to form an opinion, but usually his attitude is humble. If the edict has gone forth that such and such a group is a species then henceforth it is and the matter is at an end, at least for the time. But when an inquirer fixes his attention on a group such as is called generic, and examines thousands of individuals belonging to that group extended over a wide area, he will come to know that groups called specific are very variable in the number of their component individuals, that some of them contain only a few thousands which are confined to a limited area. Few groups have been examined in this manner. The most complete inquiry of the kind, in the writer’s knowledge, is Mr. W. L. Tower’s investigation of the potato beetle, Leptinotarsa, in America, which extended over ten years. Most important are the results of the breeding experiments which were undertaken, but these will be referred to later on. The question to be considered here is — how were the beetles found in nature ? A rare form called pallida was found, generally as single specimens isolated among great numbers of a commoner species, but on one occasion six specimens of pallida were found together ; six is a group, but it is not what we call a species. Another form called rubicunda composed a group which was much larger than this, but still it was limited.

Thus, in August, 1903, it was found over an area of some seven or eight square miles, but in September, 1906, it was limited to a tract of about ten acres in extent, where only a very few specimens were found, hence the group was waning. The better known form decemlineata was first recorded in Nebraska in 1859, in all probability it was then occupying a limited area, at all events it was limited in comparison with the immense area which it came to occupy in the following fifty years. Taking the potato plant as its chief food it spread from Nebraska eastward as far as the Atlantic border, travelling at about the rate of sixty miles a year. The potato beetle is of great economic interest, consequently its distribution was carefully recorded. The relative importance of pallida, rubicunda , and decemlineata are known, but in the case of other generic groups which are not so well known the component species may have the same relative importance as those mentioned and yet be considered as of equal value, since each is represented by a type specimen and a verbal description. The only difference in our regard being that a group such as pallida would be considered rare, while a group such as decemlineata would be common. Now, it is well known that “ rare ” species are very plentiful. The verbal paradox, why are rare species so common, has occurred to many naturalists both of the past and present. It seems that pallida, rubicunda, and decemlineata are all typical of species so-called, and if we wish to explain how animals came to be as they are we must not shut our eyes to such groups as pallida and rubicunda, on the grounds that they are not exactly species since we know too much about them.

A species is usually regarded as a number of like animals ; the number, being indeterminable in almost every case, is considered, perhaps for that reason, to be infinitely great in every case. When therefore we find a group containing so few individuals that the number of them can be determined, we say at once — this is not a species.

The conception of species as groups, which are invariably infinite in their component numbers, is not a true representation of nature and it hinders our understanding the origin of groups. There is no doubt that groups, such as are called specific, are variable in the number of their component individuals, and there is little doubt that many of them are quite small in this respect.

THE TERM GROUP

The word group appears in the title of this work and will be found throughout the text. It is used in place of the more usual terms species, sub-species, and variety. Whatever meanings may be applied to these terms it is evident that they are commonly used to indicate groups of like animals, hence the term group may be used instead of them. Set up any definition of the term species, or of the term variety ; it will be possible to find groups of like animals which do not satisfy the requirements of either of those definitions. Take for example a common definition of the term species. A species is a group of animals which are fertile among themselves but sterile with all others. Take also a common definition of a variety as a kind of animal which is known to have been derived from a species. Let us now turn to nature. The potato beetle, Leptinotarsa rubicunda, has appeared as the offspring of the commoner L. multitceniata and hence is derived from that species. But it is “ almost completely ” sterile with the parent species though perfectly fertile in itself. It is therefore almost completely a species in the terms of the first definition and entirely a variety in terms of the second.

Let us consider some other definitions. Species are groups which differ from one another in all their characteristics, whereas varieties differ from one another or from the parent species in only one or two well-marked characters. If we employ these definitions, we may often be sure that this group is a species and that a variety ; but frequently we shall be in difficulty, since there are many groups in the animal kingdom which appear to have two, three, or more characters of their own, but yet do not differ from allied groups in all respects. Such groups are common among the higher animals, and although they are neither species nor varieties in the terms of the last-mentioned definitions, yet they are always regarded as species by naturalists. It is necessary to define the term group as used in this book, and in doing so a certain conception will be employed which has found wide but not perhaps general approval. Mainly as a result of Mendel's researches, we perceive that organisms contain certain attributes, which are distinct in their relation to the rest of the organism. These attributes or characters appear as separate entities, since each as a unit may be present in the body or absent from it without affecting the remainder of the body. This is assumed here to be true without discussing the physiological proof, the segregation of characters in the germ cells.

Some biologists, however, do not regard characters as perceptible things. Others, while admitting that characters are perceptible, deny that they are units. In the absence of a standard definition of the word unit, it is difficult to discuss such a question. In a strict sense there are no units in the organic world. At any rate, I assume that living bodies contain these separate and distinct attributes called characters. We may now define the term group.

A group is a number of individuals (more than one), each possessing some particular character or characters which are chosen arbitrarily as the distinguishing marks of that group.

OPINION AS TO CHARACTERS

Since characters are to play a part in that which is to follow, my conception of them must be described, although it is somewhat uncertain.

1. Every living body contains a large number of distinct attributes or characters. The colour of a flower, the brown pigment in the front layer of the human iris, the beard of wheat, are characters in that they may be present in or absent from the organism, apparently without affecting the rest of the organism.