CHAPTER XIII

VARIATION AND EVOLUTION

Through the facts of heredity we have reached a new conception of the individual. Hitherto we have been accustomed to distinguish between the members of a family of rabbits like that illustrated on Plate I. by assigning to each an individuality, and by making use of certain external features, such as the coat colour or the markings, as convenient outward signs to express our idea that the individuality of these different animals is different. Apart from this, our notions as to what constituted the individuality in each case were at best but vague. Mendelian analysis has placed in our hands a more precise method of estimating and expressing the variations that are to be found between one individual and another. Instead of looking at the individual as a whole, which is in some vague way endowed with an individuality marking it off from its fellows, we now regard it as an organism built up of definite characters superimposed on a basis beyond which for the moment our analysis will not take us. We have begun to realise that each individual has a definite architecture, and that this architecture depends

primarily upon the number and variety of the factors that existed in the two gametes that went to its building. Now most species exhibit considerable variation and exist in a number, often very large, of more or less well-defined varieties. How far can this great variety be explained in terms of a comparatively small number of factors if the number of possible forms depends upon the number of the factors which may be present or absent?

In the simple case where the homozygous and heterozygous conditions are indistinguishable in appearance the number of possible forms is 2, raised to the power of the number of factors concerned. Thus where one factor is concerned there are only 2¹ = 2 possible forms, where ten factors are concerned there are 2¹⁰ = 1024 possible forms differing from one another in at most ten and at least one character. Where the factors interact upon one another this number will, of course, be considerably increased. If the heterozygous form is different in appearance from the homozygous form, there are three possible forms connected with each factor; for ten such factors the possible number of individuals would be 3¹⁰ = 59,049; for twenty such factors the possible number of different individuals would be 3²⁰ = 3,486,784,401. The presence or absence of a comparatively small number of factors in a species carries with it the possibility of an enormous range of individual variation. But every one of these individuals has a perfectly definite constitution which can

be determined in each case by the ordinary methods of Mendelian analysis. For in every instance the variation depends upon the presence or absence of definite factors carried in by the gametes from whose union the individual results. And as these factors separate out cleanly in the gametes which the individual forms, such variations as depend upon them are transmitted strictly according to the Mendelian scheme. Provided that the constitution of the gametes is unchanged, the heredity of such variation is independent of any change in the conditions of nutrition or environment which may operate upon the individual producing the gametes.

But, as everybody knows, an individual organism, whether plant or animal, reacts, and often reacts markedly, to the environmental conditions under which its life is passed. More especially is this to be seen where such characters as size or weight are concerned. More sunlight or a richer soil may mean stronger growth in a plant, better nutrition may result in a finer animal, superior education may lead to a more intelligent man. But although the changed conditions produce a direct effect upon the individual, we have no indisputable evidence that such alterations are connected with alterations in the nature of the gametes which the individual produces. And without this such variations cannot be perpetuated through heredity, but the conditions which produce the effect must always be renewed in each

successive generation. We are led, therefore, to the conclusion that two sorts of variations exist, those which are due to the presence of specific factors in the organism and those which are due to the direct effect of the environment during its lifetime. The former are known as mutations, and are inherited according to the Mendelian scheme; the latter have been termed fluctuations, and at present we have no valid reason for supposing that they are ever inherited. For though instances may be found in which effects produced during the lifetime of the individual would appear to affect the offspring, this is not necessarily due to heredity. Thus plants which are poorly nourished and grown under adverse conditions may set seed from which come plants that are smaller than the normal although grown under most favorable conditions. It is natural to attribute the smaller size of the offspring to the conditions under which the parents were grown, and there is no doubt that we should be quite right in doing so. Nevertheless, it need have nothing to do with heredity. As we have already pointed out, the seed is a larval plant which draws its nourishment from the mother. The size of the offspring is affected because the poorly nourished parent offered a bad environment to the young plant, and not because the gametes of the parent were changed through the adverse conditions under which it grew. The parent in this case is not only the producer of gametes, but also a part of the environment of the young

plant, and it is in this latter capacity that it affects its offspring. Wherever, as in plants and mammals, the organism is parasitic upon the mother during its earlier stages, the state of nutrition of the latter will almost certainly react upon it, and in this way a semblance of transmitted weakness or vigour is brought about. Such a connection between mother and offspring is purely one of environment, and it cannot be too strongly emphasised that it has nothing to do with the ordinary process of heredity.

The distinction between these two kinds of variation, so entirely different in their causation, renders it possible to obtain a clearer view of the process of evolution than that recently prevalent. As Darwin long ago realised, any theory of evolution must be based upon the facts of heredity and variation. Evolution only comes about through the survival of certain variations and the elimination of others. But to be of any moment in evolutionary change a variation must be inherited. And to be inherited it must be represented in the gametes. This, as we have seen, is the case for those variations which we have termed mutations. For the inheritance of fluctuations, on the other hand, of the variations which result from the direct action of the environment upon the individual, there is no indisputable evidence. Consequently we have no reason for regarding them as playing any part in the production of that succession of temporarily stable forms which we term evolution. In

the light of our present knowledge we must regard the mutation as the basis of evolution—as the material upon which natural selection works. For it is the only form of variation of whose heredity we have any certain knowledge.

It is evident that this view of the process of evolution is in some respects at variance with that generally held during the past half century. There we were given the conception of an abstract type representing the species, and from it most of the individuals diverged in various directions, though, generally speaking, only to a very small extent. It was assumed that any variation, however small, might have a selection value, that is to say, could be transmitted to the offspring. Some of these would possess it in a less and some in a greater degree than the parent. If the variation were a useful one, those possessing to a rather greater extent would be favoured through the action of natural selection at the expense of their less fortunate brethren, and would leave a greater number of offspring, of whom some possessed it in an even more marked degree than themselves. And so it would go on. The process was a cumulative one. The slightest variation in a favourable direction gave natural selection a starting-point to work on. Through the continued action of natural selection on each successive generation the useful variation was gradually worked up, until at last it reached the magnitude of a specific

distinction. Were it possible in such a case to have all the forms before us, they would present the appearance of a long series imperceptibly grading from one extreme to the other.

Upon this view are made two assumptions not unnatural in the absence of any exact knowledge of the nature of heredity and variation. It was assumed, in the first place that variation was a continuous process, and, second, that any variation could be transmitted to the offspring. Both of these assumptions have since been shown to be unjustified. Even before Mendel's work became known Bateson had begun to call attention to the prevalence of discontinuity in variation, and a few years later this was emphasised by the Dutch botanist Hugo de Vries in his great work on The Mutation Theory. The ferment of new ideas was already working in the solution, and under the stimulus of Mendel's work they have rapidly crystallised out. With the advent of heredity as a definite science we have been led to revise our views as to the nature of variation, and consequently in some respects as to the trend of evolution. Heritable variation has a definite basis in the gamete, and it is to the gamete, therefore, not to the individual, that we must look for the initiation of this process. Somewhere or other in the course of their production is added or removed the factor upon whose removal or addition the new variation owes its existence. The new variation springs into being by a

sudden step, not by a process of gradual and almost imperceptible augmentation. It is not continuous but discontinuous, because it is based upon the presence or absence of some definite factor or factors—upon discontinuity in the gametes from which it sprang. Once formed, its continued existence is subject to the arbitrament of natural selection. If of value in the struggle for existence natural selection will decide that those who possess it shall have a better chance of survival and of leaving offspring than those who do not possess it. If it is harmful to the individual natural selection will soon bring about its elimination. But if the new variation is neither harmful nor useful there seems no reason why it should not persist.

In this way we avoid a difficulty that beset the older view. For on that view no new character could be developed except by the piling up of minute variations through the action of natural selection. Consequently any character found in animals and plants must be supposed to be of some definite use to the individual. Otherwise it could not have developed through the action of natural selection. But there are plenty of characters to which it is exceedingly difficult to ascribe any utility, and the ingenuity of the supporters of this view has often been severely taxed to account for their existence. On the more modern view this difficulty is avoided. The origin of a new variation is independent of natural

selection, and provided that it is not directly harmful there is no reason why it should not persist. In this way we are released from the burden of discovering a utilitarian motive behind all the multitudinous characters of living organisms. For we now recognise that the function of natural selection is selection and not creation. It has nothing to do with the formation of the new variation. It merely decides whether it is to survive or to be eliminated.

One of the arguments made use of by supporters of the older view is that drawn from the study of adaptation. Animals and plants are as a rule remarkably well adapted to living the life which their surroundings impose upon them, and in some cases this adaptation is exceedingly striking. Especially is this so in the many instances of what is called protective coloration, where the animal comes to resemble its surroundings so closely that it may reasonably be supposed to cheat even the keenest sighted enemy. Surely, we are told, such perfect adaptation could hardly have arisen through the mere survival of chance sports. Surely there must be some guiding hand moulding the species into the required shape. The argument is an old one. For John Ray that guiding hand was the superior wisdom of the Creator: for the modern Darwinian it is Natural Selection controlling the direction of variation. Mendelism certainly offers no suggestion of any such controlling force. It interprets the

variations of living forms in terms of definite physiological factors, and the diversity of animal and plant life is due to the gain or loss of these factors, to the origination of new ones, or to fresh combinations among those already in existence. Nor is there any valid reason against the supposition that even the most remarkable cases of resemblance, such as that of the leaf insect, may have arisen through a process of mutation. Experience with domestic plants and animals shows that the most bizarre forms may arise as sports and perpetuate themselves. Were such forms, arising under natural conditions, to be favoured by natural selection owing to a resemblance to something in their environment we should obtain a striking case of protective adaptation. And here it must not be forgotten that those striking cases to which our attention is generally called are but a very small minority of the existing forms of life.

For that special group of adaptation phenomena classed under the head of Mimicry, Mendelism seems to offer an interpretation simpler than that at present in vogue. This perhaps may be more clearly expressed by taking a specific case. There is in Africa a genus of Danaine butterflies known as Amauris, and there are reasons for considering that the group to which it belongs possesses properties which render it unpalatable to vertebrate enemies such as birds or monkeys. In the same region is also found the genus Euralia belonging to the entirely

different family of the Nymphalidae, to which there is no evidence for assigning the disagreeable properties of the Danaines. Now the different species of Euralia show remarkably close resemblances to the species of Amauris, which are found flying in the same region, and it is supposed that by "mimicking" the unpalatable forms they impose upon their enemies and thereby acquire immunity from attack. The point at issue is the way in which this seemingly purposeful resemblance has been brought about.

One of the species of Euralia occurs in two very distinct forms (Pl. VI.), which were previously regarded as separate species under the names E. wahlbergi and E. mima. These two forms respectively resemble Amauris dominicanus and A. echeria. For purposes of argument we will assume A. echeria to be the more recent form of the two. On the modern Darwinian view certain individuals of A. dominicanus gradually diverged from the dominicanus type and eventually reached the echeria type, though why this should have happened does not appear to be clear. At the same time those specimens which tended to vary in the direction of A. echeria in places where this species was more abundant than A. dominicanus were encouraged by natural selection, and under its guiding hand the form mima eventually arose from wahlbergi.

According to Mendelian views, on the other hand,

A. echeria arose suddenly from A. dominicanus (or vice versa), and similarly mima arose suddenly from wahlbergi. If mima occurred where A. echeria was common and A. dominicanus was rare, its resemblance to the more plentiful distasteful form would give it the advantage over wahlbergi and allow it to establish itself in place of the latter. On the modern Darwinian view natural selection gradually shapes wahlbergi into the mima form owing to the presence of A. echeria; on the Mendelian view natural selection merely conserves the mima form when once it has arisen. Now this case of mimicry is one of especial interest, because we have experimental evidence that the relation between mima and wahlbergi is a simple Mendelian one, though at present it is uncertain which is the dominant and which the recessive form. The two have been proved to occur in families bred from the same female without the occurrence of any intermediates, and the fact that the two segregate cleanly is strong evidence in favour of the Mendelian view. On this view the genera Amauris and Euralia contain a similar set of pattern factors, and the conditions, whatever they may be, which bring about mutation in the former lead to the production of a similar mutation in the latter. Of the different forms of Euralia produced in any region that one has the best chance of survival, through the operation of natural selection, which resembles the most plentiful Amauris form. Mimetic resemblance is a true phenomenon, but natural selection plays the part of a conservative, not of a formative agent.

It is interesting to recall that in earlier years Darwin was inclined to ascribe more importance to "sports" as opposed to continuous minute variation, and to consider that they might play a not inconsiderable part in the formation of new varieties in nature. This view, however, he gave up later, because he thought that the relatively rare sport or mutation would rapidly disappear through the swamping effects of crossing with the more abundant normal form, and so, even though favoured by natural selection, would never succeed in establishing itself. Mendel's discovery has eliminated this difficulty. For suppose that the sport differed from the normal in the loss of a factor and were recessive. When mated with the normal this character would seem to disappear, though, of course, half of the gametes of its progeny would bear it. By continual crossing with normals a small proportion of heterozygotes would eventually be scattered among the population, and as soon as any two of these mated together the recessive sport would appear in one quarter of their offspring.

A suggestive contribution to this subject was recently made by G. H. Hardy. Considering the distribution of a single factor in a mixed population consisting of the heterozygous and the two homozygous forms he showed that such a population breeding at random rapidly fell into a

stable condition with regard to the proportion of these three forms, whatever may have been the proportion of the three forms to start with. Let us suppose for instance, that the population consists of p homozygotes of one kind, r homozygotes of the other kind, and 2 q heterozygotes. Hardy pointed out that, other things being equal, such a population would be in equilibrium for this particular factor so long as the condition q² = pr was fulfilled. If the condition is fulfilled to start with, the population remains in equilibrium. If the condition is not fulfilled to start with, Hardy showed that a position of equilibrium becomes established after a single generation, and that this position is thereafter maintained. The proportions of the three classes which satisfy the equation q² = pr are exceedingly numerous, and populations in which they existed in the proportions shown in the appended table would remain in stable equilibrium generation after generation:—

p.	2q.	r.
1	2	1
1	4	4
1	6	9
1	8	16
1	20,000	100,000,000
1	2n	n²

This, of course, assumes that all three classes are equally fertile, and that no form of selection is taking place to the

benefit of one class more than of another. Moreover, it makes no difference whether p represents the homozygous dominants or whether it stands for the recessives. A population containing a very small proportion of dominants and one containing a similar proportion of recessives are equally stable. The term dominant is in some respects apt to be misleading, for a dominant character cannot in virtue of its dominance establish itself at the expense of a recessive one. Brown eyes in man are dominant to blue, but there is no reason to suppose that as years go on the population of these islands will become increasingly brown eyed. Given equality of conditions both are on an equal footing. If, however, either dominant or recessive be favoured by selection the conditions are altered, and it can be shown that even a small advantage possessed by the one will rapidly lead to the elimination of the other. Even with but a 5 per cent selection advantage in its favour it can be shown that a rare sport will oust the normal form in a few hundred generations. In this way we are freed from a difficulty inherent in the older view that varieties arose through a long-continued process involving the accumulation of very slight variations. On that view the establishing of a new type was of necessity a very long and tedious business, involving many thousands of generations. For this reason the biologist has been accustomed to demand a very large supply of time, often a great deal more than the physicist is

disposed to grant, and this has sometimes led him to expostulate with the latter for cutting off the supply. On the newer views, however, this difficulty need not arise, for we realise that the origin and establishing of a new form may be a very much more rapid process than has hitherto been deemed possible.

One last question with regard to evolution. How far does Mendelism help us in connection with the problem of the origin of species? Among the plants and animals with which we have dealt we have been able to show that distinct differences, often considerable, in colour, size, and structure, may be interpreted in terms of Mendelian factors. It is not unlikely that most of the various characters which the systematist uses to mark off one species from another, the so-called specific characters, are of this nature. They serve as convenient labels, but are not essential to the conception of species. A systematist who defined the wild sweet pea could hardly fail to include in his definition such characters as the procumbent habit, the tendrils, the form of the pollen, the shape of the flower, and its purple colour. Yet all these and other characters have been proved to depend upon the presence of definite factors which can be removed by appropriate crossing. By this means we can produce a small plant a few inches in height with an erect habit of growth, without tendrils, with round instead of oblong pollen, and with colourless deformed flowers quite different

in appearance from those of the wild form. Such a plant would breed perfectly true, and a botanist to whom it was presented, if ignorant of its origin, might easily relegate it to a different genus. Nevertheless, though so widely divergent in structure, such a plant must yet be regarded as belonging to the species Lathyrus odoratus. For it still remains fertile with the many different varieties of sweet pea. It is not visible attributes that constitute the essential difference between one species and another. The essential difference, whatever it may be, is that underlying the phenomenon of sterility. The visible attributes are those made use of by the systematist in cataloguing the different forms of animal and plant life, for he has no other choice. But it must not be forgotten that they are often misleading. Until they were bred together Euralia wahlbergi and E. mima were regarded as perfectly valid species, and there is little doubt that numbers of recognised species will eventually fall to the ground in the same way as soon as we are in a position to apply the test of breeding. Mendelism has helped us to realise that specific characters may be but incidental to a species—that the true criterion of what constitutes a species is sterility, and that particular form of sterility which prevents two healthy gametes on uniting from producing a zygote with normal powers of growth and reproduction. For there are forms of sterility which are purely mechanical. The pollen of Mirabilis jalapa cannot fertilise M.

longiflora, because the pollen tubes of the former are not long enough to penetrate down to the ovules of the latter. Hybrids can nevertheless be obtained from the reciprocal cross. Nor should we expect offspring from a St. Bernard and a toy terrier without recourse to artificial fertilisation. Or sterility may be due to pathological causes which prevent the gametes from meeting one another in a healthy state. But in most cases it is probable that the sterility is due to some other cause. It is not inconceivable that definite differences in chemical composition render the protoplasm of one species toxic to the gametes of the other, and if this is so it is not impossible that we may some day be able to express these differences in terms of Mendelian factors. The very nature of the case makes it one of extreme difficulty for experimental investigation. At any rate, we realise more clearly than before that the problem of species is not one that can be resolved by the study of morphology or of systematics. It is a problem in physiology.