Chapter XVII: Part 17

We need not wonder, therefore, that specific types occur in such varying degrees of definiteness. If a species is distributed over a wide connected territory, sporadically, not uniformly, it will depend partly upon the mutual degree of isolation of the sporadic areas whether the individual colonies will exhibit the same specific type or will diverge from one another. If the animal in question is a slow-moving one like a snail, the intermingling from neighbouring sporadic colonies will be much slower than in the case of a resident bird such as a woodpecker. Many interesting results would undoubtedly be gained if the numerous careful investigations into the geographical distributions of species and their local races were studied with special reference to this point, and much light would undoubtedly be thrown upon the evolution of the specific type. But it would be absolutely necessary to study carefully all the biological relations of the animals concerned, to trace back the history of the species as far as possible, and to decide the period of immigration, the mode and direction of distribution, and so on.

Nothing shows more plainly the enormous duration of the period of constancy in species than the wide distribution of the same specific type on scattered areas or even over different areas absolutely isolated from one another. If, as we saw, the same diurnal butterflies live in the Alps and the far North, they must have remained unvaried since the Glacial period, for it was the close of that period that brought them to their present habitats, and while other diurnal butterflies now living on the Alps differ from their relatives in the Arctic zone (Lapland, Siberia, and Labrador) in some unimportant spot or line, and must therefore have diverged from one another in the course of the long period since the Glacial epoch, they have done so only to a minimal degree, and in characters which possibly depend solely upon germinal selection and can hardly be regarded as adaptations.

I should like, however, to cite one of the few cases known to me in which a slight deviation from the specific type undoubtedly depending upon adaptation has occurred on an isolated region. The nut-jay (_Caryocatactes nucifraga_) lives not only upon our Alps and in the Black Forest, but also in the forests of Siberia, and the birds there differ from those with us in small peculiarities of the bill, which is longer and thinner in them, shorter and more powerful in ours. Ornithologists associate this difference with the fact that in this country the birds feed chiefly on hard hazel nuts, which they break open with their bill, and on acorns, beechmast, and, in the Alps, on cembra-cones, while in Siberia, where there are no hazel nuts, they feed chiefly upon the seeds of the Siberian cedar, which are concealed deep down in the cones. Thus we find that in Siberia the bill is slender, and that the upper jaw protrudes awl-like beyond the lower, for about 2.5 mm., and probably serves chiefly to pick out the cedar nuts from behind the cone-scales. In the Alps the birds (var. _pachyrhynchus_) break up the whole cone of the cembra-pine with their thick, hard bill, and in the Upper Engadine, where the nut-jay is abundant, I have often seen the ground underneath the cembra-pines covered with the débris of its meal. In addition to these differences between the two races, the Alpine form is stronger in build, the Siberian form is daintier; in the former the white terminal band on the tail is narrow (about 18 mm.), in the Siberian form it is broader (about 27 mm.).

Such cases of variation of individual parts in different areas seem to me very important theoretically, because they furnish us with an answer to the view which represents the species as a 'life-crystal,' which must be as it is or not at all, and which therefore cannot vary as regards its individual parts. The case of the nut-jay has the further interest that it is one of the few in which we find the new adaptation of a single character without variation of most of the other characters.

It is only in an essentially different sense that we can compare the species, like any other vital unit, to a crystal, in so far as its parts are harmoniously related one to another, or, as I expressed myself years ago, are in a state of equilibrium, which must be brought about by means of intra-selection. This analogy, however, only applies to the actual adjustment of the parts to a whole, and not to their casual adjustment. Species are variable crystals; the constancy of a species in all its parts must be regarded as something quite relative, which may vary at any time, and which is sure to vary at some time in the course of a long period. But the longer the adaptation of a species to new conditions persists the more constant, _ceteris paribus_, and the more slowly variable will it become, and this for two reasons: first, because the determinants which are varying in a suitable direction are being more and more strictly selected, more and more precisely adapted, and are thus becoming more like each other; and secondly, because, according to our theory, the homologous determinants of all the ids do not vary in the required direction, and a portion of the unvaried ancestral ids is always carried on through the course of the phylogeny, and only gradually set aside by the chances of reducing division. But the more completely these unvaried ids are eliminated from the germ-plasm the less likely will they be to find expression in reversions or in impurities of the new specific characters. I may recall the reversions of the various breeds of pigeons to the rock-dove, those of the white species of _Datura_ to the blue form, and the _Hipparion_-like three-toed horse of Julius Cæsar, and so on. The unvaried ancestral ids, which in these cases find only quite exceptional expression, will make the new 'specific character' fluctuating, as long as they are contained in the germ-plasm in considerable numbers, but they must become more and more infrequent in the germ-plasm as successive generations are passed through the sieve of natural selection, and the oftener these germ-plasms, to which the chances of reducing division and amphimixis have assigned a majority of the old determinants, are expelled from the ranks of the species by personal selection. The oftener this has occurred in a species the less frequently will it recur, and the more constant, _ceteris paribus_, will the 'type' of the species become.

If we add to this idea the fact that adaptations take place very slowly, and that every variation of the germ-plasm in an appropriate direction has time to spread over countless hosts of individuals, we gain some idea of the way in which new adaptations gradually bring about the evolution of a more and more sharply-defined specific type.

So far, however, we have only explained the morphological aspect of the problem of the nature of species, but there is also a physiological side, and for a long time this played an important part in the definition of the conception of species. Until the time of Darwin it was regarded as certain that species do not intermingle in the natural state, and that, though they could be crossed in rare cases, the progeny would be infertile.

Although we now know that these statements are only relatively correct, and that in particular there are many higher plants which yield perfectly fertile hybrids, it is nevertheless a striking phenomenon that among the higher animals, mammals, and birds the old law holds good, and hybrids between two species are very rarely fertile. The two products of crossing between the horse and the ass, the mule and the hinny, are never fertile _inter se_, and very rarely with a member of the parent stock.

We have to ask, therefore, what is the reason of this mutual sterility of species; whether it is a necessary outcome of the morphological differences between the species, or only a chance accessory phenomenon, or perhaps an absolutely necessary preliminary condition to the establishment of species.

The last was the view held by Romanes. He believed that a species could only divide into two when it was separated into isolated groups either geographically or physiologically, that is, when sexual segregation in some form is established within the species, so that all the individuals can no longer pair with one another, but groups arise which are mutually sterile. It is only subsequently, he maintained, that these groups come to differ from one another in structure. To this hypothetical process he gave the name of 'physiological selection.' This view depends--it seems to me--upon an underestimate of the power of natural selection. Romanes believed that when a species began to split up, even the adaptive variations would always disappear again because of the continual crossing, and that only geographical isolation or sexual alienation, that is, physiological selection, would be able to prevent this. But even the fact that there are dimorphic and polymorphic species proves sufficiently that adaptation to two or even several sets of conditions can go on on the same area. In many ants we find many kinds of individuals--the two sexual forms, the workers, and the soldiers, and these last are undoubtedly distinguished by adaptive characters which must be referred to selection. The same is true of the caterpillars, whose coloration is adapted to their surroundings in two different ways. If the individuals of one and the same species can be broken up into two or more different forms and combinations of adaptations, while they are mingling uninterruptedly with one another, natural selection must undoubtedly be able, notwithstanding the continual intermingling of divergent types, to discriminate between them and to separate them sharply from one another. Assuredly then a species can not only exhibit uniform variation on a single area, but may also split up into two without the aid of physiological selection. Theoretically it is indisputable that of two varieties which are both equally well suited to the struggle for existence, a mixed form arising through crossing may not be able to survive. Let us recall, for instance, the caterpillars, of which some individuals are green and some brown, and let us assume that the brown colour is as effective a protection as the green, then the two forms would occur with equal frequency; but though a mixed hybrid form which was adapted neither to the green leaves nor to the brown might occasionally crop up, it would always be eliminated. It would occur because the butterflies themselves are alike, whether they owe their origin to green caterpillars or to brown, and thus at first, at least, all sexual combinations would be equally probable.

I do not believe therefore in a 'physiological selection,' in Romanes's sense, as an indispensable preliminary condition to the splitting of species, but it is a different question whether the mutual sterility so frequently observable between species has not conversely been produced by natural selection in order to facilitate the separation of incipient species. For there can be no doubt that the process of separating two new forms, or even of separating one new form from an old one, would be rendered materially easier if sexual antipathy or diminished fertility of the crossings could be established simultaneously with the other variations. This would be useful, since pure and well-defined variations would be better adapted to their life-conditions than hybrids, and would become increasingly so in the course of generations. But as soon as it is useful it must actually come about, if that is possible at all. It may be, however, as we have said before, that the two divergent forms depend merely upon quantitative variations of the already existing characters; sexual attraction, whether it depends upon very delicate chemical substances, or on odours, or on mutual complementary tensions unknown to us, will always fluctuate upwards and downwards, and plus or minus determinants, which lie at the root of these unknown characters in the germ-plasm, must continually present themselves and form the starting-point for selection-processes of a germinal and personal kind, which may bring about sexual antipathy and mutual sterility between the varieties. I therefore consider Romanes's idea correct in so far that separation between species is in many cases accompanied by increasing sexual antipathy and mutual sterility. While Romanes supposed that 'natural selection could in no case have been the cause' of the sterility, I believe, on the contrary, that it could only have been produced by natural selection; it arises simply, as all adaptations do, through personal selection on a basis of germinal selection, and it is not a preliminary condition of the separation of species, but an adaptation for the purpose of making as pure and clean a separation as possible. It is obviously an advantage for both the divergent tendencies of variation that they should intermingle as little as possible. This is corroborated by the fact that by no means all the marked divergences of species are accompanied by sexual alienation, and that the mutual sterility so frequently seen is not an inevitable accompaniment of differences in the rest of the organism.

That this is not the case is very clearly proved by our domesticated animals. The differences in structure between the various breeds of pigeon and poultry are very great, and breeds of dog also diverge from one another very markedly, especially in shape and size of body. Yet all these are fertile with one another, and they yield fertile offspring. But they are products of artificial selection by man, and he has no interest in making them mutually sterile, so that they have not been selected with a view to sexual alienation, but in reference to the other characters. The segregation of animal species into several sub-species on the same area is probably usually accompanied by sexual antipathy, since in this case it would be useful although not indispensable. But the matter is different in the case of the transformation of a colony upon a geographically isolated region. 'Amiktic' forms, such as _Vanessa ichnusa_ of Corsica, are hardly likely to be sexually alienated from the parent form; we have here to do only with the preponderance of a fortuitous and biologically valueless variation and its consequent elevation to the rank of a variety. The new form was not an adaptation, but only a variation, and as it was of no use, it was not in a position to incite any process of selection favouring its advancement.

But even adaptive transformations on isolated regions from which the parent species is excluded are not likely to develop rapidly any sexual antipathy as regards the parent stock, and I should not be surprised if experiments showed that there is perfect mutual fertility between, for instance, many of the species of _Achatinella_ on the Sandwich Islands or of _Nanina_ on Celebes, or between the species of thrushes on the different islands of the Galapagos Archipelago, or between these and the ancestral species on the adjacent continent, if that species is still in existence. For there was no reason why sexual antipathy to the parent form should have developed in any of these adaptation forms which have arisen in isolation, and therefore it has probably not been evolved.

That our view of the mutual sterility between species, as an adaptation to the utility of precise species-limitation, is the correct one is evidenced not only by our domesticated races, but even more clearly by plants, in regard to which it is particularly plain that the sexual relations between two species are adaptational. We have already seen in what a striking way the sensitiveness of the stigma of a flower is regulated in reference to pollen from the same plant, that some species are not fertilizable by their own pollen at all, that others yield very little seed when self-fertilization is effected, and that others again are quite fertile--as much so as with the pollen of another plant of the same species. We regarded these gradations of sexual sensitiveness as adaptations to the perfectly or only moderately well-assured visits of insects, or to their entire absence. I wish to cite these cases as well as the heterostylism of some flowers as evidence in support of the conception of the mutual sterility of species which I have just outlined. But this only in passing. The point to which I chiefly wish to direct attention is the mutual fertility of many plant-species. In lower as well as in higher plants fertile hybrids occur not infrequently under natural conditions, and cultivated hybrids, such as the new _Medicago media_, a form made by mingling two species of clover, may go on reproducing with its own kind for a considerable time. A number of Phanerogams yield fertile hybrids, and in Orchids even species of different genera have been crossed and have yielded offspring which was in some cases successfully crossed with a third genus.

If these facts prove anything it is that the factors which determine the mutual sterility of species are quite distinct from their morphological differences, in other words, from the diagnostic characters of the specific type. For a long time the verdict on this matter was too entirely based on observations made on animals, among which mutual sterility arises relatively easily, even where it was not intended (_sit venia verbo!_). Even the pairing, but still more the period of maturity, the relations of maturity in ovum and sperm, and even the most minute details in the structure of the sperm-cell, the egg-shell, envelope, &c., have to be taken into account, and these may bring about mutual or, as Born has shown, one-sided sterility. We know, through the researches of Strasburger, that a great many Phanerogams, when pollinated artificially from widely separated species of different genera and families, will at least allow the pollen-tube to penetrate down to the ovule, and that in many cases amphimixis actually results. It follows that we must not lay too great stress upon the mutual sterility which occurs almost without exception among the higher animals, but must turn to the plants with greater confidence.

Among plants there is very widely distributed mutual fertility between species. I doubt, however, whether the observations on this point are sufficient to warrant any certain conclusion in regard to the importance of the phenomena in the formation of species. At least it is not easy to see why the mutual sterility of many species of plants should not have been necessary or useful in separating species, and why it was not therefore evolved. We may point to the fact that animals can move from place to place as the chief reason, and this factor does undoubtedly play a part, but the widespread crossing of plants by insects makes up to some extent, as far as sexual intermingling is concerned, for their inability to move from place to place. I do not know whether the species of orchid which are fertile with one another belong to different countries, so that we may assume that they originated in isolation, or whether fertile orchids from the same area are fertilized by different insects and are thus sexually isolated. This and many other things must be taken into consideration. Probably these relations have not yet been adequately investigated; probably what is known by some experts has not yet been made available to all. Future investigations and studies must throw more light upon the problem.

In any case, however, we can see from the frequency of mutual fertility among plants that mutual sterility is not a _conditio sine qua non_ of the splitting up of species, and we must beware of laying too great stress upon it even among animals. Germinal selection is a process which not only forms the basis of all personal selection, but which is also able to give rise of itself, without the usual aid of sexual intermingling, to a new specific type. And we cannot with any confidence dispute that, even without amphigony, a certain degree of personal selection may not ensue solely on the basis of the favourable variational departures originating in the germ-plasm. It would be premature to express any definite views on this point as yet, but the diverse cases of purely asexual or parthenogenetic reproduction in groups of plants rich in species make this hypothesis seem probable.

The most remarkable example of this is probably to be found in the Lichens, the symbiotic nature of which we have already discussed, in which--now at least--neither the Fungus nor the Alga associated with it is known to exhibit sexual reproduction. If this is really the case, then the existence of numerous and well-marked species of lichens leads us to the hypothesis just expressed, and we must suppose that the unity of the specific type is attained in this case solely by a continual sifting of the useful from the useless variations of the determinants, and through purely germinal intensification of the surviving variational tendencies.

Of course it is possible that the mutual adaptation of the Algæ and Fungi in the evolution of species of Lichens took place very long ago, at a time when sexual reproduction still existed, at least in one of the associated organisms, the Fungus. The Ascomycetes, to which most of the Lichens belong, do not at present usually exhibit the process of amphimixis, as I have already noted; but it may perhaps be still possible to decide whether they must have exhibited it, or at least could have exhibited it at an earlier stage in their evolution. As the group of Thallophytes is a very ancient one, it is not inconceivable that the modern species of Lichens have existed for a long time, and that they had their origin in the remote past with the assistance of amphimixis.

Nor need it be objected to this supposition that it has been found possible to _make new Lichens_ by bringing together Fungi and Algæ which had not previously been associated with one another; for in the first place both were already adapted to partnership with other species, and, moreover, so far no one has succeeded in rearing these artificial lichens for any length of time, still less in seeing them evolve into specific forms persistent in natural conditions.

But if this supposition should prove to be not only improbable, but actually erroneous, then the existence of Lichens would afford a clear proof that the 'type' of the species does not depend essentially upon the constant intermingling of individuals, but upon a process which we may best designate _uniformity of adaptation_. We have simply to suppose that under similar external influences similar variational tendencies were started by germinal selection in all the individuals of the two parent species of a lichen, and set a-going by germinal selection, just as a warmer climate gives rise to a black variety in the butterfly _Polyommatus phlæas_, because similar determinants of the germ-plasm of all the individuals were impelled to vary in the same manner and direction. This would then give rise to quite definite variations, and since only the suitable variational tendencies could survive, primitive though never complicated adaptations would arise. But we cannot assume that the lichens are not adapted to the conditions of their life as well as all other organisms. We cannot judge how far even their shape is to be regarded as an adaptation, whether the formation of encrusting growths, of tree-like forms, of cup or bush-lichens, may not be regarded as adaptations towards a full utilization of the conditions of their life--but even if this is not the case, the formation of soredia remains an undoubted adaptation to the symbiosis of those lichens which exhibit them. The soredia cannot depend upon the direct effect of the conditions of life, for they are reproductive bodies which did not exist before the existence of the lichen, and only originated to facilitate their distribution.

Thus there is still a great deal that is doubtful in our theories as to the transformations of organisms, and much remains still to be done. But even though we may doubt whether adaptations could come about in multicellular organisms without amphigony, we may be quite certain of the converse, that is, that the specific type can be changed in every individual feature by natural selection on the basis of amphigony, even as regards invisible features which only express themselves in altered periods of growth. Even when there is no isolation whatever and no mutual sterility, and when a mobile species is uniformly distributed over a large area, a splitting up into races in regard to one particular character may occur, _simply through adaptation_ to the spatially different climatic conditions of the area inhabited.

Early in these lectures we discussed the twofold protective value of the coloration of the 'variable hare' (_Lepus variabilis_), which is distributed over the Arctic zone of the Old and New World, and also occurs in the higher regions of the Alps. Wherever there is a sharp contrast between winter and summer the variable hare exhibits the same specific type, being brown in summer and white in winter, but in regard to this very character of colour-change it forms races to some extent, for it is white for a longer or shorter time according to the length of the winter--in Greenland for the whole twelve months of the year, in Northern Norway only for eight or nine months, in the Alps for six or seven months, but in the south of Sweden and in Ireland not at all. There it remains brown in winter like our common hare (_Lepus timidus_). This is not a question of the direct effect of cold; if it were the species would become white in Southern Sweden also, for there is no lack of severe cold there, but the ground is not so uninterruptedly covered with snow, and so the white colour of the hare would be as often, probably oftener, a danger than a safeguard, and the more primitive double coloration has therefore been done away with by natural selection. The change of colour is thus hereditarily fixed, as is proved by the fact that the Alpine hare, if caught and kept in the valleys below, puts on a white dress at the usual time, which the common hare never does.

As in Southern Sweden the winter coloration has been wholly eliminated, so, conversely, from there to the Arctic zone the summer colouring has been more and more crowded out, and in the Farthest North it has totally disappeared from the characters of the species. We thus see that wherever the species lives the double colouring is regulated, as regards the duration of the winter coat, in exact harmony with the external conditions. There is a pure white, a pure brown, and a colour-changing race, and the latter is subdivided into two--one wearing the winter dress for six, the other for eight months. Probably these could be still further subdivided, if the different regions of the Scandinavian Peninsula were investigated individually from south to north. That the duration of the winter dress has its roots in the germ-plasm, and does not depend solely on the earlier or later period at which the cold sets in, is made clear by the two extreme forms, the white and the brown _Lepus variabilis_, as well as by the behaviour of captive animals. The familiar case of Ross's lemming, which remained brown in the warm cabin, and then suddenly became white when it was exposed to the cold of winter, only shows that the cold acts as a liberating stimulus. The preparatory changes in the pellage are already present, and the stimulus of cold brings them rapidly to a climax. Here, therefore, the necessary variations of the relevant germinal parts must have continually presented themselves for selection, which is intelligible enough, since it is merely a question of plus- or minus-variations. The fact that the six-months' dress can be transformed into an eight-months' dress must have its cause in some minute biological units of the germ-plasm; the determinants of the fur must be able to vary in such a way that a longer or shorter duration of the winter's coat is the result. The possibility of the whole variation depends upon the continual fluctuations of all determinants, now towards plus, now towards minus, and the necessity and inevitableness of each adaptation to the duration of the winter lies in the unceasing personal selection--the inexorable preferring of the better adapted.

LECTURE XXXV

THE ORIGIN AND THE EXTINCTION OF SPECIES

Adaptation does not depend upon chance--The case of eyes--Of leaf-mimicry--All persistent change depends ultimately on selection--Mutual sterility without great significance--Relative isolation (_Lepus variabilis_)--Influence of hybridization--Decadence of species--Differences in the duration of decadence--Natural death of individuals--Extinction due to excessive variability (Emery)?--_Machairodus_ as interpreted by Brandes--Lower types more capable of adaptation than higher--Flightless birds--Disturbance of insular fauna and flora by cultivation--The big game of Central Europe.

In the polar hare we have a case in which the adaptations to the life conditions both of time and space are recognizable as the effect of definite causes, and thus as a necessity; but the same must be true everywhere even in regard to the most complex adaptations which seem to depend entirely upon chance; everywhere adaptation results of necessity--if it is possible at all with the given organization of the species--as certainly as the adaptation dress of the hare depends on the length of the winter, and in point of fact not less certainly than the blue colour of starch on the addition of iodine. The most delicate adaptations of the vertebrate eye to the task set for it by life in various groups have been gradually brought about as the necessary results of definite causes, just in the same way as the complex protective markings and colouring on the wing of the _Kallima_ and other leaf-mimicking butterflies.

That adaptations can be regarded as mechanically necessitated is due to the fact that in every process of adaptation the same direction of variation on the part of the determinants concerned is guaranteed, since personal selection eliminates those which vary in a wrong direction, so that only those varying in a suitable direction survive, and they then continue to vary in the same direction. But the greatest difference between our conception of natural selection and that of Darwin lies in this: that Darwin regarded its intervention as dependent upon chance, while we consider it as necessary and conditioned by the upward and downward intra-germinal fluctuation of the determinants. Appropriate variational tendencies not only _may_ present themselves, they _must_ do so, if the germ-plasm contains determinants at all by whose fluctuations in a plus or minus direction the appropriate variation is attainable.

That a horse should grow wings is beyond the limits of the possibilities of equine variation--there are no determinants which could present variations directed towards this goal; but that any multicellular animal which lives in the light should develop eyes lies within the variational possibilities of its ectoderm determinants, and in point of fact almost all such animals do possess eyes, and eyes, too, whose functional capacity may be increased in any direction, and which are adaptable and modifiable in any manner in accordance with the requirements of the case. As soon as the determinants of the most primitive eye came into existence, they formed the fundamental material by whose plus- or minus-variations all the marvellous eye structures might be brought about, which we find in the different groups of the Metazoa, from a mere spot sensitive to light to a shadowy perception of a moving body, and from that again to the distinct recognition of a clear image, which we are aware of in our own eyes. And what wonderful special adaptations of the eye to near and to distant vision, to vision in the dusk and at night, or in the great ocean-depths, to recognition of mere movement or the focussing of a clear image, have been interpolated in the course of this evolution!

All such adaptations are possible, because they can proceed from variations of determinants which are in existence; and in the same way it is possible, at every stage of the evolution of organisms, for eyes to degenerate again, whether they have been high up or low down in the scale of gradations of this perhaps the most delicate of all our sense-organs. As soon as a species migrated permanently from the light into perfect darkness its eyes began to degenerate. We know blind flat worms, blind water-fleas and Isopods, also blind insects and higher Crustaceans, and even blind fishes and amphibians, the eyes of which are now to be found at very different levels of degeneration, as Eigenmann has recently shown in regard to several species of cave-dwelling salamanders of the State of Ohio. In all these cases it is only necessary for the determinants of the eye to continue to vary in the minus direction, and the disappearance of the eye must be gradually brought about.

We must picture upward development in quite a similar way. The forest butterflies of the Tropics could not possibly all have their under surfaces coloured like a leaf if the protective pattern depended solely upon the chance of a useful variation presenting itself. It always presented itself through the fluctuations of the determinants, and thus the appropriate colourings were not merely able to develop, but of necessity did so in gradually increasing perfection. If chance played any part in the matter, it would be quite unintelligible why the protective colouring should occur only where it acts as a protection, and why, for instance, it should not appear sometimes upon the upper surface of the butterfly wing, or upon the posterior wings which are covered when the butterfly is at rest. We have already studied in detail the precision with which the coloration is localized on minute points and corners of the wing: this can only be understood if natural selection works with the certainty of a perfect mechanism. Chance only comes into the matter in so far as it depends upon chance whether the relevant determinants in one id or another are to vary in the direction of plus or minus; but as the germ-plasm contains many ids, and chance may decide it differently in each of these, the presence of a majority of determinants varying in a desirable direction does not depend upon chance, for if they are not contained in one individual they are in another. It is only necessary that they should be present in some, and that these should be selected for reproduction.

We must therefore regard natural selection, that is to say, _personal selection_, as a mechanical process of development, which begins with the same certainty and works 'in a straight line' towards its 'goal,' just as any principle of development might be supposed to do. Fundamentally it is after all a purely internal force which gives rise to evolution, the power of the most minute vital units to vary under changing influences, and it is only the guidance of evolution along particular paths that is essentially left to personal selection, which brings together what is useful and thus determines the direction of further evolution. If we bear in mind that even the minutest variations of the biophors and determinants express nothing more or less than reactions to changed external conditions in the direction of adaptation, and that the same is true of each of the higher categories of vital units, whether they be called cell, tissue, organ, person, or corm, we see that the whole evolution of the forms of life upon the earth depends upon adaptations following each other in unbroken succession, and fitting into each other in the most complex way. The whole evolution is made possible by the power of variation of the living units of every grade, and called forth and directed by the ceaseless changes of the external influences. I said years ago that _everything_ in organic evolution depended upon selection, for every lasting change in a vital unit means adaptation to changed external influences, and implies a preference in favour of the parts of the unit concerned, which are thereby more fitly disposed.

In this sense we can also say that the species is a complex of adaptations, for we have seen that it depends upon the co-operation of different grades of selective processes, that in many cases it is produced solely by germinal selection, but that in very many more personal selection plays the chief part, whether in bringing about sexual adaptations, or adaptations to the conditions of existence.

When we have thus recognized that the origin of a variation in a definite direction results as inevitably when it is called forth by the indirect influence of conditions, that is, through the need for a new adaptation, as when it is induced in the germ-plasm by direct causes such as those of climate, we shall not be disposed to estimate very highly the part played by mutual sterility in the origin of species. We shall rather be inclined to assign it a rôle at a later stage, after the separation of the forms has taken place, and this view is supported by the fact of the mutual sterility of most nearly related species, and by the theoretical consideration that the frequency of hybrids, even if these are always eliminated in the struggle for existence, must signify a loss for both the parent species. But no certain conclusion can be based upon either of these arguments--not upon the theoretical one, because here again we are unable to estimate the extent of this loss; and not upon the argument from fact, because the results of experiments in crossing animals have generally been overestimated, since we are apt to regard the most nearly related animals that are at our disposal as being very closely related. Thus, for instance, horse and ass, horse and zebra are undoubtedly rightly included within the same genus, but the fact that there are several species of zebra in Africa gives us an idea of the number of transition stages that may have existed between the horse and the zebra. Entomologists have sometimes reared hybrids between the most nearly related indigenous species of hawk-moth of the genus _Smerinthus_--hybrids of _Smerinthus ocellata_, the eyed hawk-moth, and _Smerinthus populi_, the poplar hawk-moth. I have myself made many experiments of this kind, and have often succeeded in getting the two species to pair and even to deposit eggs, but I have never seen a caterpillar emerge from them. The hybrids do occur, however, and they have been repeatedly obtained by Standfuss. In external appearance they are intermediate between the parent forms, but with marked divergences, thus, for instance, the beautiful blue eye on the posterior wing of _S. ocellata_ (Fig. 5, vol. i. p. 69) may have almost disappeared or be only indicated. They are sterile. But we know three species of _Smerinthus_ in North America, which are all much nearer to _S. ocellata_ than _S. populi_ is, for they all possess the eye-spot referred to, although it is less well developed. The proof that the most nearly related species do not yield fertile descendants should be sought for by crossing _Smerinthus ocellata_ with one of these American species if it is to have any decisive value.

Experiments of the same kind have been made by Standfuss with different species of indigenous _Saturnia_, and these have shown not only that crossing is possible, but that the hybrids are fertile in their turn. These results are to be valued the more highly because it is well known that Lepidoptera, and even the usually prolific silk-moths, do not readily reproduce in captivity, even within the same species. We have in _Saturnia pyri_, _spini_, and _carpini_ three well-marked distinct species with no intermediate forms in nature, and with quite different colouring in the caterpillars. That these should have been successfully combined in a triple hybrid proves at least that sexual alienation cannot have advanced far in this case.

We must beware, however, of attributing too much to the constant mutual crossing which occurs in a species living on a connected area and of regarding its influence as irresistible. Undoubtedly it must go far towards securing the uniformity of individuals, but not only is it unable to achieve this, but it cannot successfully resist the stronger influences making for variation which may be exerted upon a part of the area of the species. We have already seen that it is quite erroneous to suppose that every new adaptation must be lost sight of again because of the continual crossing with other members of the species upon the same area. Other things being equal, this depends entirely upon the importance of the adaptation in question. Just as climatic influences may be so strong that they entirely overcome the influence of crossing, and give rise to a local race notwithstanding imperfect geographical isolation, so the same may happen in the case of adaptations. It is quite conceivable that the polar hare of Scandinavia may have evolved a whole series of races, each of which is adapted to the duration of the snow in its geographical range, although a crossing of these quick-footed animals must frequently occur in the course of time, even as regards forms from widely separated areas, and although the whole region is inhabited without a break by the species, so that a 'mingling' of the hares of all regions from south to north, and conversely, may take place, and indeed must be continually taking place, though of course very slowly.

It is precisely this extreme slowness with which the intermingling of racial characters take place that seems to me essential for the production of local or, as in this case, regional races. It is not difficult to calculate the rate of 'blood-distribution' if we assume that the conditions for a rapid dissemination are as favourable as possible. Let us assume that it takes place along a certain line--in this case from south to north--and that the numerical strength of the species remains constant, each pair of hares yielding a pair of surviving offspring, which will attain to reproduction. Let us suppose that one of these hares moves his home northwards to the extent of his range, that is, as far as a hare is accustomed to range from his head quarters, and that he pairs with one of the descendants on the next stretch.

Let us further suppose that this stretch is ten kilometres in extent, and that the change of quarters take place once in each year, then the blood of a South Scandinavian hare would have extended ten kilometres further north in ten years, and in a hundred years 100 kilometres; it would not, however, be quite pure, but mixed and thinned by crossing with a hundred mates of different individual bloods, that is, thinned to the extent of 2 to the 100^{th} power, that is, to less than a millionth part. Thus even with these much too favourable assumptions the influence of a region of hares 100 kilometres distant would be actually _nil_ upon the inhabitants of a region which was in process of new adaptation. That the assumptions are too favourable is quite obvious, since every surviving hare would not be likely to move his home, and probably the majority would remain in the old quarters and find mates there. The blood-mingling would therefore take place much more rarely, perhaps only once in ten years, and the wandering descendants of the second generation might move southward, and so neutralize the previous blood-mingling, and so on. But let us keep to our favourable assumptions, and attempt to determine how strong the assimilating influence of the blood-mingling from south to north would be upon a point _A_. The blood of the nearest stretch diluted to a half would affect the inhabitants of _A_ once in each year; the second stretch would only contribute blood of 1/4 strength, the third of 1/8, the fourth of 1/16, and the blood of the tenth would be diluted to 1/1024. A region _B_, extending over twenty such stretches, or 200 kilometres, would thus shelter within it a hare population of which the centre would only be influenced from the periphery in vanishing proportions. If the winter were of equal length over the whole area of _B_, all the inhabitants would be tending to vary the period for which the winter dress was worn in correspondence with the length of the winter, and the centre of the region would be the less impeded in this process because the more peripheral areas would also be approximating to the same adaptation. But since even the admixture of 1/32 of strange blood could have no hindering influence upon a variation, there would remain a region of 2 × 5 = 10 stretches upon which the influence of the non-varying regions would be without effect. There would therefore arise a new race in relation to the duration of the winter dress, and this would not cease abruptly, but would gradually pass over into the neighbouring regions, which however would be pure at their centre, just as is probably the case in reality, if we regard _B_ as any point in the line of distribution from south to north.

The harmony of the individuals within a species will therefore depend in part upon the mingling of hereditary primary constituents associated with reproduction, but in greater part upon adaptation to the same conditions; it is _a similarity of adaptation_, and the strongest influence which sexual reproduction exerts lies not in the mingling of these hereditary constituents alone, but above all in the reduction in the germ-plasm of the two parental hereditary contributions--a reduction which results from and through the sexual intermingling. It is only this that prevents these primary constituents from varying at too unequal a rate in the transformations of species, and causes them ultimately to resemble each other closely again.

But while mutual sterility is not an absolutely necessary condition in the separation of species, it would be going too far in the opposite direction to regard mutual fertility as something general, or to attribute to it a rôle in the origination of new species.

Certain botanists, like Kerner von Marilaun, regard the mingling of species as a means of forming new species with better adaptations; they suppose that fertile hybrids may, in certain circumstances, crowd out the parent species, and themselves become new species. It will be admitted that such cases do occur, that, for instance, in the north of Europe the hybrid between the large and the small water-lily, _Nuphar luteum_ and _Nuphar pumilum_, to which the name _Nuphar intermedium_ has been given, has driven both the parent species from the field, because its seeds mature earlier, and it is therefore better adapted to the short vegetative period of the north, but nevertheless we must maintain that the evolution of species on the whole does not take place through hybridization. Such cases are probably nothing more than rare exceptions. This is corroborated by the entire insignificance of hybridization in animals, among which species appear in the same way as they do in plants, and where the mingling of two species occurs only sporadically and in a few species, never to any very great extent.

If species are complexes of adaptations, based in each case on the given physical constitution of the parent species, then we can readily understand the fact that they are in our experience not fixed or eternal, but that they change in the course of the earth's history. The numerous fossil remains in the various strata of the earth's crust prove that this is true in a high degree, that in almost every one of the more important geological strata new species occur, and that not only species and genera, but families, orders, indeed whole classes of animals, which lived at one time, have now completely disappeared from the face of the earth. We can understand this phenomenon when we reflect that the conditions of life have also been slowly changing through the course of the earth's history, so that the old species had only the alternative of dying out, or of becoming transformed into new species.

But simple as this conclusion is, it can hardly be deduced with certainty from the occurrence and succession of the fossil species alone. For instance, we should strive in vain to recognize the cause which led one of those regularly arranged snail-species of the Steinheim lake basin to become transformed into one or two new species at a particular time, or to find the cause which moved those curious tripartite Crustaceans of primitive times, the Trilobites, which peopled the Silurian seas with such a wealth of forms, to become suddenly scarce towards the end of the Silurian period, and to disappear altogether in the succeeding period, the Devonian. The famous geologist Neumayr sought to refer this striking phenomenon to the fact that just at that time the Cephalopods, 'the most formidable and savage marauders among the invertebrate marine fauna,' gained the ascendancy, and it is quite possible that he was right in his surmise, but who is to prove it? Can we decide even in the case of animals now living whether the losses inflicted on a much persecuted species by an abundant and greedy persecutor exceed the numbers of progeny, and are therefore driving the species gradually towards extermination? Probable as such a supposition appears, it cannot be accepted as proven.

Since in many cases of the extinction of great animal-groups we cannot even prove that there was a simultaneous ascendancy of powerful enemies, other factors must be discovered to which the apparently sudden disappearance may be attributed. Many naturalists have tried to guess at internal reasons for extinction, and have adopted the theory--associated with the tendency to assume mystical principles of evolution--that species in dying out are obeying an internal necessity, as if their birth and death were predestined, as it is in the case of multicellular individuals, as if there were a _physiological death of the species_ as there is of the multicellular individual.

Neumayr showed, however, that the facts of palæontology afford no support for this view. I need not repeat his arguments, but will simply refer to his clear and concise exposition of the problem. It is obvious that our theory of the extinction of species as due to external causes cannot be rejected on the ground that our knowledge of the struggle that species had to maintain for their existence in past times is even mere imperfect than our knowledge of the struggle nowadays, and that we are frequently unable to judge of it at all. But the facts of geology are of value in another, quite different way. They reveal such an extraordinary dissimilarity in the duration of species, and also of the great groups of organisms, that the dissimilarity of itself is sufficient to prevent our regarding the extinction of species as regulated by _internal_ causes. Certain genera of Echinoderms, such as starfish (_Astropecten_), lived in the Silurian times, and they are represented nowadays in our seas by a number of species: and in the same way the Cephalopod genus _Nautilus_ has maintained itself among the living all through the enormous period from the Silurian sea to our own day. Formerly the Nautilids formed a predatory horde that peopled the seas, and, as we have seen, we may perhaps attribute to their dominance the disappearance of an order of Crustaceans, the Trilobites, which were equally abundant at that period. Now only a single species of nautilus (_Nautilus pompilius_) lives on the coral reefs of the southern seas. Similarly, the genus _Lingula_ of the nearly extinct class of Brachiopods, somewhat mussel-like sessile marine animals, has been preserved from the grey dawn of primitive times, with its records in the oldest deposits, and is represented in the living world of to-day by the so-called 'barnacle-goose' mussel, _Lingula anatina_.

On the other hand, we know of numerous species which lasted for quite a short time, such as, for instance, the individual members of the series of Steinheim _Planorbis_ species, or of the Slavonic _Paludina_ species. Not infrequently, too, genera make their appearance and disappear again within the period of one and the same geological stratum.