Mendelism Part 6

s.e.x (_continued_)

The cases which we have considered in the last chapter belong to a group in which the peculiarities of inheritance are most easily explained by supposing that the female is heterozygous for some factor that is not found in the male. Femaleness is an additional character superposed upon a basis of maleness, and as we imagine that there is a separate factor for each the full const.i.tutional formula for a female is FfMM, and for a male ffMM. Both s.e.xes are h.o.m.ozygous for the male element, and the difference between them is due to the presence or absence of the female element F.

There are, however, other cases for which the explanation will not suffice, but can be best interpreted on the view that the male is heterozygous for a factor which is not found in the female. Such a case is that recently described by Morgan in America for the pomace fly (_Drosophila ampelophila_). Normally this little insect has a red eye, but white eyed individuals are known to occur as rare sports. Red eye is dominant to white. In their relation to s.e.x the eye colours of the pomace fly {116} are inherited on the same lines as the _grossulariata_ and _lacticolor_ patterns of the currant moth, but with one essential difference. The factor which repels the red-eye factor is in this case to be found in the male, and here consequently it is the male which must be regarded as heterozygous for a s.e.x factor that is lacking in the female.

[Ill.u.s.tration]

In order to bring these cases and others into line an interesting suggestion has recently been put forward by Bateson. On this suggestion each s.e.x is heterozygous for its own s.e.x factor only, and does not contain the factor proper to the opposite s.e.x. The male is of the const.i.tution, Mmff and the female Ffmm. Each s.e.x produces two sorts of gametes, Mf and mf in the case of the male, and Fm, fm in that of the female. But on this view a further supposition is necessary. If each of the two kinds of spermatozoa were capable of fertilising each of the two kinds of ova, we should get individuals of the const.i.tution MmFf and mmff, as well as the normal males and females, Mmff and Ffmm. As the facts of ordinary bis.e.xual reproduction afford us no grounds for a.s.suming the existence of these two cla.s.ses of individuals, whatever they may be, we must suppose that fertilisation. is productive only between the spermatozoa carrying M and the ova without F, or between the spermatozoa {117} without M and the ova containing F. In other words we must on this view suppose that fertilisations between certain forms of gametes, even if they can occur, are incapable of giving rise to zygotes with the capacity for further development. If we admit this supposition, the scheme just given will cover such cases as those of the currant moth and the fowl, equally as well as that of the pomace fly. In the former there is repulsion between either the _grossulariata_ factor and F, or else between the pigment inhibitor factor and F, while in the latter there is repulsion between the factor for red eye and M.

[Ill.u.s.tration: FIG. 26.

Scheme to ill.u.s.trate the probable mode of inheritance of colour-blindness.

The dark signs represent affected individuals. A black dot in the centre denotes an unaffected female who is capable of transmitting the condition to her sons.]

Whatever the merits or demerits of such a scheme it certainly does offer an explanation of a peculiar form of s.e.x limited inheritance in man. It has long been a matter of common knowledge that colour-blindness is much more common among men than among women, and also that unaffected women can transmit it to their sons. At first sight the case is not unlike that of the sheep, where the horned character is apparently dominant in the male but recessive in the female. The hypothesis that the colour-blind condition is due to the presence of an extra factor as compared with the normal, and that a single dose of it will produce {118} colour-blindness in the male but not in the female, will cover a good many of the observed facts (cf.

Fig. 26). Moreover, it serves to explain the remarkable fact that _all_ the sons of colour-blind women are also colour-blind. For a woman cannot be colour-blind unless she is h.o.m.ozygous for the colour-blind factor, in which case all her children must get a single dose of it even if she marries a normal male. And this is sufficient to produce colour-blindness in the male, though not in the female.

But there is one notable difference in this case as compared with that of the sheep. When crossed with pure hornless ewes the heterozygous horned ram transmits the horned character to half his male offspring (cf. p. 71). But the heterozygous colour-blind man does not behave altogether like a sheep, for he apparently does not transmit the colour-blind condition to any of his male offspring. If, however, we suppose that the colour-blind factor is repelled by the factor for maleness, the amended scheme will cover the observed facts. For, denoting the colour-blind factor by X, the gametes produced by the colour-blind male are of two sorts only, viz. Mfx and mfX.

If he marries a normal woman (Ffmmxx), the spermatozoa Mfx unite with ova fmx to give normal males, while the spermatozoa mfX unite with ova Fmx to give females which are heterozygous for the colour-blind factor. These daughters are themselves normal, but transmit the condition to about half their sons. {119}

The attempt to discover a simple explanation of the nature of s.e.x has led us to a.s.sume that certain combinations between gametes are incapable of giving rise to zygotes which can develop further. In the various cases. .h.i.therto considered there is no reason to suppose that anything of the sort occurs, or that the different gametes are otherwise than completely fertile one with another. One peculiar case, however, has been known for several years in which some of the gametes are apparently incapable of uniting to produce offspring. Yellow in the mouse is dominant to agouti, but hitherto a h.o.m.ozygous yellow has never been met with. The yellows from families where only yellows and agoutis occur produce, when bred together, yellows and agoutis in the ratio 2 : 1. If it were an ordinary Mendelian case the ratio should be 3 : 1, and one out of every three yellows so bred should be h.o.m.ozygous and give only yellows when crossed with agouti. But Cuenot and others have shown that _all_ of the yellows are heterozygous, and when crossed with agoutis give both yellows and agoutis. We are led, therefore, to suppose that an ovum carrying the yellow factor is unproductive if fertilised by a spermatozoon which also bears this factor. In this way alone does it seem possible to explain the deficiency of yellows and the absence of h.o.m.ozygous ones in the families arising from the mating of yellows together. At present, however, it remains the only definite instance among animals in which we have {120} grounds for a.s.suming that anything in the nature of unproductive fertilisation takes place.[8]

If we turn from animals to plants we find a more complicated state of affairs. Generally speaking, the higher plants are hermaphrodite, both ovules and pollen grains occurring on the same flower. Some plants, however like most animals, are of separate s.e.xes, a single plant bearing only male or female flowers. In other plants the separate flowers are either male or female, though both are borne on the same individual. In others, again, the conditions are even more complex, for the same plant may bear flowers of three kinds, viz. male, female, and hermaphrodite. Or it may be that these three forms occur in the same species but in different individuals--female and hermaphrodites in one species; males, females, and hermaphrodites in another. One case, however, must be mentioned as it suggests a possibility which we have not hitherto encountered. In the common English bryony (_Bryonia dioica_) the s.e.xes are separate, some plants having only male and others only female flowers. In another European species, _B. alba_, both male and female flowers occur on the same plant. Correns crossed these two species reciprocally, and also fertilised _B. dioica_ by its own male with the following results:--

{121}

dioica [female] dioica [male] gave [female] [female] and [male] [male]

" alba [male] " [female] [female] only alba [female] dioica [male] " [female] [female] and [male] [male].

The point of chief interest lies in the striking difference shown by the reciprocal crosses between _dioica_ and _alba_. Males appear when _alba_ is used as the female parent but not when the female _dioica_ is crossed by male _alba_. It is possible to suggest more than one scheme to cover these facts, but we may confine ourselves here to that which seems most in accord with the general trend of other cases. We will suppose that in _dioica_ femaleness is dominant to maleness, and that the female is heterozygous for this additional factor. In this species, then, the female produces equal numbers of ovules with and without the female factor, while this factor is absent in all the pollen grains. _Alba_ [female] _dioica_ [male] gives the same result as _dioica_ [female] _dioica_ [male], and we must therefore suppose that alba produces male and female ovules in equal numbers. _Alba_ [male] x _dioica_ [female], however, gives nothing but females. Unless, therefore, we a.s.sume that there is selective fertilisation we must suppose that all the pollen grains of alba carry the female factor--in other words, that so far as the s.e.x factors are concerned there is a difference between the ovules and pollen grains borne by the same plant. Unfortunately further investigation of this case is rendered impossible owing to the complete sterility of the F_1 plants. {122}

[Ill.u.s.tration: FIG. 27.

Single and double stocks raised from the same single parent.]

That the possibility of a difference between the ovules and pollen grains of the same individual must be taken into account in future work there is evidence from quite a different source. The double stock is an old horticultural favourite, and for centuries it has been known that of itself it sets no seed, but must be raised from special strains of the single variety. "You must understand withall," wrote John Parkinson of his gilloflowers,[9] "that those plants that beare double flowers, doe beare no seed at all ... but the onely way to have double flowers any yeare is to save the seedes of those plants of this kinde that beare single flowers, for from that seede will rise some that will beare single, and some double flowers." With regard to the nature of these double-throwing strains of singles, Miss Saunders has recently brought out some interesting facts. She crossed the double-throwing singles with pure singles belonging to strains in which doubles never occur. The cross was made both ways, and in both cases all the F_1 plants were single. A distinction, however, appeared when a further generation was raised from the F_1 plants. All the F_1 plants from the pollen of the double-throwing single behaved like double-throwing singles, but of the F_1 plants from the ovules of the double throwers some behaved as double throwers, and some as pure singles. We are led to infer, therefore, that the ovules and pollen grains {123} of the double throwers, though both produced by the same plant, differ in their relation to the factor (or factors) for doubleness. Doubleness is apparently carried by all the pollen grains of such plants, but only by some of the ovules. Though the nature of doubleness in stocks is not yet clearly understood, the facts discovered by Miss Saunders suggest strongly that the ovules and pollen grains of the same plant may differ in their transmitting properties, probably owing to some process of segregation in the growing plant which leads to an unequal distribution of some or other factors to the cells which give rise to the ovules as compared with those from which {124} the pollen grains eventually spring. Whether this may turn out to be the true account or not, the possibility must not be overlooked in future work.

Single +-------------+ Single Double / Pollen of Ovule Pollen Ovule of pure single pure single +----------+ Single Single Single +-------+ +---------+ Single Single Double Single Double +-------+ +---------+ Single Single Double Single Double

From all this it is clear enough that there is much to be done before the problem of s.e.x is solved even so far as the biologist can ever expect to solve it. The possibilities are many, and many a fresh set of facts is needed before we can hope to decide among them. Yet the occasional glimpses of clear-cut and orderly phenomena, which Mendelian spectacles have already enabled us to catch, offer a fair hope that some day they may all be brought into focus, and a.s.signed their proper places in a general scheme which shall embrace them all. Then, though not till then, will the problem of the nature of s.e.x pa.s.s from the hands of the biologist into those of the physicist and the chemist.

{125}

CHAPTER XII

INTERMEDIATES

So far as we have gone we have found it possible to express the various characters of animals and plants in terms of definite factors which are carried by the gametes, and are distributed according to a definite scheme.

Whatever may be the nature of these factors it is possible for purposes of a.n.a.lysis to treat them as indivisible ent.i.ties which may or may not be present in any given gamete. When the factor is present it is present as a whole. The visible properties developed by a zygote in the course of its growth depend upon the nature and variety of the factors carried in by the two gametes which went to its making, and to a less degree upon whether each factor was brought in by both gametes or by one only. If the given factor is brought in by one gamete only, the resulting heterozygote may be more or less intermediate between the h.o.m.ozygous form with a double dose of the factor and the h.o.m.ozygous form which is entirely dest.i.tute of the factor. Cases in point are those of the primula flowers and the Andalusian fowls. Nevertheless these intermediates produce only pure gametes, as is {126} shown by the fact that the pure parental types appear in a certain proportion of their offspring. In such cases as these there is but a single type of intermediate, and the simple ratio in which this and the two h.o.m.ozygous forms appear renders the interpretation obvious. But the nature of the F_2 generation may be much more complex, and, where we are dealing with factors which interact upon one another, may even present the appearance of a series of intermediate forms grading from the condition found in one of the original parents to that which occurred in the other.

As an ill.u.s.tration we may consider the cross between the Brown Leghorn and Silky fowls which we have already dealt with in connection with the inheritance of s.e.x. The offspring of a Silky hen mated with a Brown Leghorn are in both s.e.xes birds with but a trace of the Silky pigmentation. But when such birds are bred together they produce a generation consisting of chicks as deeply pigmented as the original Silky parent, chicks devoid of pigment like the Brown Leghorn, and chicks in which the pigmentation shows itself in a variety of intermediate stages. Indeed from a hundred chicks bred in this way it would be possible to pick out a number of individuals and arrange them in an apparently continuous series of gradually increasing pigmentation, with the completely unpigmented at one end and the most deeply pigmented at the other. Nevertheless, the case is one in which complete segregation of the different factors takes {127} [Ill.u.s.tration]place, place, and the apparently continuous series of intermediates is the result of the interaction of the different factors upon one another. The const.i.tution of the F_1 [male] is a ffPpIi, and such a bird produces in equal numbers the four sorts of gametes fPI, fPi, fpI, fpi. The const.i.tution of the F_1 [female] in this case is FfPpIi. Owing to the repulsion between F and I she produces the four kinds of gametes FPi, Fpi, fPI, fpi, and produces them in equal numbers. The result of bringing two such series of gametes together is shown in Fig. 28. Out of the sixteen types of zygote formed one (FfPPii) is h.o.m.ozygous for the pigmentation factor, and does not contain the inhibitor factor. Such a bird is as deeply pigmented as the pure Silky parent. Two, again, contain a single dose of P in the absence of I. These are nearly as dark as the pure Silky. Four zygotes are dest.i.tute of P, though they may or may not contain I. These birds are completely devoid of pigment like the Brown Leghorn. The remaining nine zygotes show {128} various combinations of the two factors P and I, being either PPIi, PPII, PpII, or PpIi, and in each of these cases the pigment is more or less intense according to the const.i.tution of the bird. Thus a bird of the const.i.tution PPIi approaches in pigmentation a bird of the const.i.tution Ppii, while a bird of the const.i.tution PpII has but little more pigment than the unpigmented bird. In this way we have seven distinct grades of pigmentation, and the series is further complicated by the fact that these various grades exhibit a rather different amount of pigmentation according as they occur in a male or a female bird, for, generally speaking, the female of a given grade exhibits rather more pigment than the corresponding male. The examination of a number of birds bred in this way might quite well suggest that in this case we were dealing with a character which could break up, as it were, to give a continuous series of intergrading forms between the two extremes. With the constant handling of large numbers it becomes possible to recognise most of the different grades, though even so it is possible to make mistakes. Nevertheless, as breeding tests have amply shown, we are dealing with but two interacting factors which segregate cleanly from one another according to the strict Mendelian rule. The approach to continuity in variation exhibited by the F_2 generation depends upon the fact that these two factors interact upon one another, and to different degrees according as the zygote is for one {129} or other or both of them in a h.o.m.ozygous or a heterozygous state. Moreover, certain of these intermediates will breed true to an intermediate condition of the pigmentation. A male of the const.i.tution ffPPII when bred with females of the const.i.tution FfPPIi will produce only males like itself and females like the maternal parent. We have dealt with this case in some detail, because the existence of families showing a series of intermediate stages between two characters has sometimes been brought forward in opposition to the view that the characters of organisms depend upon specific factors which are transmitted according to the Mendelian rule. But, as this case from poultry shows clearly, neither the existence of such a continuous series of intermediates, nor the fact that some of them may breed true to the intermediate condition, are incompatible with the Mendelian principle of segregation.

[Ill.u.s.tration: FIG. 28.

Diagram to ill.u.s.trate the nature and composition of the F_2 generations arising from the cross of Silky hen with Brown Leghorn c.o.c.k.]

In connection with intermediates a more cogent objection to the Mendelian view is the case of the first cross between two definite varieties thenceforward breeding true. The case that will naturally occur to the reader is that of the mulatto, which results from the cross between the negro and the white. According to general opinion, these mulattos, of intermediate pigmentation, continue to produce mulattos. Unfortunately this interesting case has never been critically investigated, and the statement that the mulatto breeds true rests almost entirely upon {130} information that is general and often vague. It may be that the inheritance of skin pigmentation in this instance is a genuine exception to the normal rule, but at the same time it must not be forgotten that it may be one in which several interacting factors are concerned, and that the pure white and the pure black are the result of combinations which from their rarity are apt to be overlooked. But until we are in possession of accurate information it is impossible to p.r.o.nounce definitely upon the nature of the inheritance in this case.

[Ill.u.s.tration: FIG. 29.

Pedigree of a family which originated from a cross between a Hindu and a European. Black signs denote individuals as dark as average Hindus. Plain signs denote quite-fair members, while those with a dot in the centre are intermediate.]

{131}

On the other hand, from the cross between the darkly pigmented Eastern races and the white segregation seems to occur in subsequent generations.

Families are to be found in which one parent is a pure white, while the other has arisen from the cross between the dark and light in the first or some subsequent generation. Such families may contain children indistinguishable from pure blonds as well as children of very dark and of intermediate shades. As an example, I may give the following pedigree, which was kindly communicated to me by an Anglo-Indian friend (Fig. 29).

The family had resided in England for several generations, so that in this case there was no question of a further admixture of black. Most noticeable is the family produced by a very dark lady who had married a white man.

Some of the children were intermediate in colour, but two were fair whites and two were dark as dark Hindus. This sharp segregation or splitting out of blacks and whites in addition to intermediates strongly suggests that the nature of the inheritance is Mendelian, though it may be complicated by the existence of several factors which may also react upon one another. Nor must it be forgotten that in so far as these different factors are concerned the whites themselves may differ in const.i.tution without showing any trace of it in their appearance. Before the case can be regarded as settled all these different possibilities will have to be definitely tested. With the dark Eastern races as with the negro we cannot {132} hope to come to any conclusion until we have evidence collected by critical and competent observers.

Though for the present we must regard the case of the negro as not proven, there are nevertheless two others in which the heredity would appear not to follow the Mendelian rule. Castle in America crossed the lop-eared rabbit with the normal form, and found that the F_1 animals were intermediate with respect to their ears. And subsequent experiment showed that, on the whole, they bred true to this intermediate condition. The other case relates to Lepidoptera. The speckled wood b.u.t.terfly (_Pararge egeria_) has a southern form which differs from the northern one in the greater brightness and depth of its yellow-brown markings. The northern form is generally distinguished as var. _egeriades_. Bateson crossed the southern form from the south of France with the paler British form, and found that the offspring were more or less intermediate in colour, and that in subsequent generations the parental types did not recur. These cases at present stand alone. It is possible that further research may reveal complications which mask or interfere with an underlying process of segregation. Or it may be that segregation does not occur owing to some definite physiological reason which at present we do not understand.

And here it is impossible not to recall Mendel's own experiences with the Hawkweeds (_Hieracium_). This {133} genus of plants exhibits an extraordinary profusion of forms differing from one another sometimes in a single feature, sometimes in several. The question as to how far these numerous forms were to be cla.s.sified as distinct species, how far as varieties, and how far as products of chance hybridisation, was even at that time a source of keen controversy among botanists. There is little doubt that Mendel undertook his experiments on the Hawkweeds in the hope that the conception of unit-characters so brilliantly demonstrated for the pea would serve to explain the great profusion of forms among the Hieraciums. Owing to the minute size of their florets, these plants offer very considerable technical difficulties in the way of cross fertilisation.

By dint of great perseverance and labour, however, Mendel succeeded in obtaining a few crosses between different forms. These hybrids were reared and a further generation produced from them, and, no doubt somewhat to Mendel's chagrin, every one of them proved to breed true. There was a complete absence of that segregation of characters which he had shown to exist in peas and beans, and had probably looked forward with some confidence to finding in _Hieracium_. More than thirty years pa.s.sed before the matter was cleared up. To-day we know that the peculiar behaviour of the hybrid Hieraciums is due to the fact that they normally produce seed by a peculiar process of parthenogenesis. It is possible to take an unopened flower and to shear off with a {134} razor all the male organs together with the stigmata through which the pollen reaches the ovules. The flower, nevertheless, sets perfectly good seed. But the cells from which the seeds develop are not of the same nature as the normal ovules of a plant. They are not gametes but retain the double structure of the maternal cells. They are rather to be regarded as of the nature of buds which early become detached from the parent stock to lead an independent existence, and, like buds, they reproduce exactly the maternal characteristics. The discovery of the true nature of this case was only rendered possible by the development of the study of cytology, and it was not given to Mendel to live long enough to learn why his hybrid Hieraciums all bred true.

{135}

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 ill.u.s.trated on Plate I. by a.s.signing 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 const.i.tuted the individuality in each case were at best but vague. Mendelian a.n.a.lysis 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 a.n.a.lysis will not take us. We have begun to realise that each individual has a definite architecture, and that this architecture depends {136} 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 h.o.m.ozygous 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 2^1 = 2 possible forms, where ten factors are concerned there are 2^{10} = 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 h.o.m.ozygous form, there are three possible forms connected with each factor; for ten such factors the possible number of individuals would be 3^{10} = 59,049; for twenty such factors the possible number of different individuals would be 3^{20} = 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 const.i.tution which can {137} be determined in each case by the ordinary methods of Mendelian a.n.a.lysis. 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 const.i.tution 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 pa.s.sed. 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 {138} 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 {139} 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 {140} 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 a.s.sumed 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 c.u.mulative 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 {141} 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 a.s.sumptions not unnatural in the absence of any exact knowledge of the nature of heredity and variation. It was a.s.sumed, in the first place that variation was a continuous process, and, second, that any variation could be transmitted to the offspring. Both of these a.s.sumptions 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 {142} 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.