The second difficulty which I have to mention as at first sight attaching to the views of Mr. Gulick and myself on the subject of Isolation is, that in an isolated section of a species Mr. Francis Galton's law of regression in the average character of offspring to the typical character of the group through reversion or atavism (Natural Inheritance, p. 97) must have the effect of neutralizing the segregative influence of mere apogamy. That such, however, cannot be the case has been well shown by Mr. Gulick in his paper on Intensive Segregation. Without at all disputing the validity of Mr. Galton's law, he proves that "it can hold in full force only where there is free crossing, otherwise no divergent race could ever be formed by any amount of selection and independent breeding[14]." This is so self-evident that I need not quote his demonstration of the point.
In conclusion, then, and having regard to the principle of isolation as a whole, or in all the many and varied forms in which this principle obtains, I trust that I have redeemed the promise with which I set out—viz. to show that in relation to the theory of descent this principle is of an importance second to no other, not even excepting heredity, variability, and the struggle for existence. This has now been fully shown, inasmuch as we have clearly seen that the importance of the struggle for existence, and consequent survival of the fittest, arises just because survival of the fittest is a form, and a very stringent form, of isolation; while, as regards both heredity and variability, we are now in a position to see that the more fully we recognize their supreme importance as principles concerned in organic evolution, the more must we also recognize that any rational theory of such evolution becomes, in the last resort, a theory of the different modes in which efficient isolation can be secured. For, in whatever degree the process of organic evolution has been dependent upon heredity with variability, in that degree must it also have been dependent upon the means of securing homogamy, whereby alone the force of heredity can be made to expend itself in the innumerable directions of progressive change, instead of continually neutralizing the force of variability by promiscuous intercrossing.
So far we have been concerned with the principle of Isolation in general. We have now to consider that form of isolation which arises in consequence of mutual infertility between the members of any group of organisms and those of all other similarly isolated groups occupying simultaneously the same area.
Against the view that natural selection is a sufficient explanation of the origin of species, there are two fatal difficulties: one, the contrast between natural species and domesticated varieties in respect of cross-sterility; the other, the fact that natural selection cannot possibly give rise to polytypic as distinguished from monotypic evolution. Now it is my belief that the theory of physiological selection fully meets both these difficulties. Indeed I hold this to be undeniable in a formal or logical sense: the only question is as to the evidence which can be adduced for the theory in a practical or biological sense. Therefore in this chapter, where the theory has first of all to be stated, I shall restrict the exposition as much as possible to the former, leaving for subsequent consideration the biological side.
The following is a brief outline sketch of this theory[15].
Of all parts of those variable objects which we call organisms, the most variable is the reproductive system; and the variations may carry with them functional changes, which may be either in the direction of increased or of diminished fertility. Consequently variations in the way of greater or less fertility frequently take place, both in plants and animals; and probably, if we had adequate means of observing this point, we should find that there is no one variation more common. But of course where infertility arises—whether as a result of changed conditions of life, or, as we say, spontaneously—it immediately becomes extinguished, seeing that the individuals which it affects are less able (if able at all) to propagate and to hand on the variation. If, however, the variant, while showing some degree of infertility with the parent form, continues to be as fertile as before when mated with similar variants, under these circumstances there is no reason why such differential fertility should not be perpetuated.
Stated in another form this suggestion enables us to regard many, if not most, species as the records of variations in the reproductive systems of their ancestors. When variations of a non-useful kind occur in any of the other systems or parts of organisms, they are, as a rule, immediately extinguished by intercrossing. But whenever they arise in the reproductive system in the way here suggested, they tend to be preserved as new natural varieties, or incipient species. At first the difference would only be in respect of the reproductive systems; but eventually, on account of independent variation, other differences would supervene, and the variety would take rank as a true species.
Now we must remember that physiological isolation is not like those other forms of isolation (e.g. geographical) which depend for their occurrence on accidents of the environment, and which may therefore take place suddenly in a full degree of completeness throughout a large section of a species. Physiological isolation depends upon distinctive characters belonging to organisms themselves; and it would be opposed to the whole theory of descent with progressive modification to imagine that absolute sterility usually arises, in a single generation between two sections of a perfectly fertile species. Therefore evolutionists must believe that in most, if not in all cases—could we trace the history, say of any two species, which having sprung from a single parent stock on a common area, are now absolutely sterile with one another—we should find that this mutual sterility had been itself a product of gradual evolution. Starting from complete fertility within the limits of a single parent species, the infertility between derivative or divergent species, at whatever stage in their evolution this began to occur, must usually at first have been well-nigh imperceptible, and thenceforth have proceeded to increase stage by stage.
But, if it be true that physiological isolation between genetically allied groups must usually itself have been the product of a gradual evolution; and if, when fully evolved, it constitutes a condition of the first importance to any further differentiation of these groups (by preventing fusion again into one group, more or less resembling the original parent form), do we not perceive at least a strong probability that in the lower stages of its evolution such mutual infertility must have acted as a segregating influence between the diverging types, in a degree proportional to its own development? The importance of mutual sterility as a condition to divergent evolution is not denied, when this sterility is already present in an absolute degree; and we have just seen that, before it can have attained to this absolute degree it must presumably, and as a rule, itself have been the subject of a gradual development. Does it not therefore become, on merely antecedent grounds, in a high degree probable, that from the moment of its inception this isolating agency must have played the part of a segregating cause, in a degree proportional to that of its completeness as a physiological character?
Whoever answers this question in the affirmative will have gone most of the way towards accepting, on merely antecedent grounds, the theory of physiological selection. And therefore it is that I have begun this statement of the theory by introducing it upon these grounds, thereby hoping to show how extremely simple—how almost self-evident—is the theory which it will now be my endeavour to substantiate. I may here add that the theory was foreshadowed by Mr. Belt in 1874[16], clearly enunciated in its main features by Mr. Catchpool in 1884[17], and very fully thought out by Mr. Gulick during a period of about fifteen years, although he did not publish until a year after the appearance of my own paper in 1886[18].
I must next proceed to state some of the leading features of physiological selection in further detail.
It has already been shown that Darwin clearly perceived that the very general occurrence of some degree of infertility between allied species cannot possibly be attributed to the direct agency of natural selection. His explanation was that the slight structural modifications entailed by the transformation of one specific type into another, so react upon the highly delicate reproductive system of the changing type as to render it in some degree infertile with its parent type. Now the theory of physiological selection begins by traversing this view. It does not, however, deny that in some cases the morphological may be the prior change; but it strenuously denies that this must be so in all cases. Indeed, according to my statement in 1886, the theory inclines to the view that, as a rule, the physiological change is prior. At the same time, the theory, as I have always stated it, maintains that it is immaterial whether, "in the majority of instances," the physiological change has been prior to the morphological, or vice versa; since in either case the physiological change will equally make for divergence of character.
To show this clearly the best way will be to consider the two cases separately, taking first that in which the physiological change has priority. In this case our theory regards any morphological changes which afterwards supervene as due to the independent variability which will sooner or later arise under the physiological isolation thus secured. But to whatever causes the subsequent morphological changes may be due, the point to notice is that they are as a general rule, consequent upon the physiological change. For in whatever degree such infertility arises between two sections of a species occupying the same area, in that degree is their interbreeding prevented, and, therefore, opportunity is given for a subsequent divergence of type, whether by the influence of independent variability alone, or also by that of natural selection, as now acting more or less independently on each of the partially separated groups. In short, all that was said in the foregoing chapters with respect to isolation in general, here applies to physiological isolation in particular; and by supposing such isolation to have been the prior change, we can as well understand the subsequent appearance of morphological divergence on continuous areas, as in other forms of isolation we can understand such divergence on discontinuous areas, seeing that even a moderate degree of cross-infertility may be as effectual for purposes of isolation as a high mountain-chain, or a thousand miles of ocean.
Here, then, are two sharply-defined theories to explain the very general fact of there being some greater or less degree of cross-infertility between allied species. The older, and hitherto current theory, supposes the cross-infertility to be but an accident of specific divergence, which, therefore, has nothing to do with causing the divergence. The newer theory, on the other hand, supposes the cross-infertility to have often been a necessary condition to the divergence having begun at all. Let us now consider which theory has most evidence in its favour.
First of all we have to notice the very general occurrence of the fact in question. For when we include the infertility of hybrids, as well as first crosses, the occurrence of some degree of infertility between allied species is so usual that Mr. Wallace recommends experiments to ascertain whether careful observation might not prove, even of species which hybridize, "that such species, when crossed with their near allies, do always produce offspring which are more or less sterile inter se[19]." This seems going too far, but nevertheless it is the testimony of a highly competent naturalist to the very general occurrence of an association between the morphological differentiation of species and the fact of a physiological isolation. Now I regard it as little short of self-evident that this general association between mutual infertility and innumerable secondary, or relatively variable morphological distinctions, is due to the former having been an original and a necessary condition to the occurrence of the latter, in cases where intercrossing has not been otherwise prevented.
The importance of physiological isolation, when once fully developed, cannot be denied, for it is evident that if such isolation could be suddenly destroyed between two allied species occupying a common area, they would sooner or later become fused into a common type—supposing, of course, no other form of isolation to be present. The necessity then for this physiological form of isolation in maintaining a specific differentiation which has been already attained cannot be disputed. Yet it has been regarded as "Darwinian heresy" to suggest that it can have been of any important service during the process of attainment, or while the specific differentiation is being advanced, and this notwithstanding that the physiological change must presumably have developed pari passu with the morphological, and notwithstanding that in countless cases the former is associated with every conceivable variety of the latter.
Again, why should the physiological change be thus associated with every conceivable variety of morphological change? Throughout the length and breadth of both vegetable and animal kingdoms we find this association, in the great majority of cases, where new species arise. Therefore, on the supposition that in all such cases the physiological change has been adventitiously induced by the morphological changes, we have to face an apparently unanswerable question—Why should the reproductive mechanism of all organic beings have been thus arranged, as it were, to change in immediate response to the very slightest alteration in the complex harmony of "somatic" processes, which now more than ever is recognized as exercising so comparatively little influence on the hereditary endowments of this mechanism? Consider the difference between a worm and the bird that is eating it, an oak tree and the gall-insect that is piercing it: are we to suppose that in all cases, no matter how greatly the types differ, they must agree in this, that when any parts of these complex structures change, ever so slightly, the reproductive system is almost certain to be adventitiously affected, yet always thus affected in the same peculiar way?
If it be answered that the reproductive system is known to be very sensitive to slight changes in the external conditions of life, the answer proves too much. For though this is true, yet our opponents must acknowledge that the reproductive system is not so sensitive, in this particular respect, as their interpretation of the origin of specific infertility requires. The proof of this point is overwhelming, for there is the evidence from the entire range of our domesticated productions, both vegetable and animal. Here the amount of structural change, which has been slowly accumulated by artificial selection, is often much greater in amount, and incomparably more rapid, than that which has been induced between allied species by natural selection; and yet there is scarcely any indication of the reproductive system having been affected in the particular way that our opponents' theory requires. There are many instances of its having been affected in sundry other ways (chiefly, however, without any accompanying morphological change); but among all the thousands of our more or less enormously modified artificial types, there is scarcely one instance of such a peculiar sexual relation between the modified descendants of a common type as so usually obtains between allied species in nature. Yet in all other respects evolutionists are bound to believe that the process of modification has been in both cases strictly analogous. Why then this conspicuous difference with respect to the reproductive system?
The answer is simple. It has never been the object of breeders or of horticulturists to select variations in the direction of cross-infertility, for the swamping effects of intercrossing are much more easily and rapidly prevented by artificial isolation. Consequently, although they have been able to modify natural types in so many directions and in such high degrees with regard to morphology, there has been no accompanying physiological modification of the kind required. But in nature there is no such thing as artificial, i.e. intentional, isolation. Consequently, on common areas it must usually happen that those changes of morphology which are associated with cross-infertility are the only ones which can arise. Hence the very remarkable contrast between our domesticated varieties and natural species with regard to cross-infertility is just what the present theory would expect, or, indeed, require. But on any other theory it has hitherto remained inexplicable.
In particular, the contrast in question has constituted one of the main difficulties with which the theory of natural selection has hitherto had to contend, not only in the popular mind, but also in the judgement of naturalists, including the joint-authors of the theory themselves. Thus Darwin says:—
The fertility of varieties is, with reference to my theory, of equal importance with the sterility of species, for it seems to make a broad and clear distinction between varieties and species[20].
And Mr. Wallace says:—
One of the greatest, or perhaps we may say the greatest, of all the difficulties in the way of accepting the theory of natural selection as a complete explanation of the origin of species, has been the remarkable difference between varieties and species in respect of fertility when crossed[21].
Now, in view of this conspicuous contrast, Darwin suggested that species in a state of nature "will have been exposed during long periods of time to more uniform conditions than have domesticated varieties, and [that] this may well make a wide difference in the result." Now we have to remember that species, living and extinct, are numbered by millions, and represent every variety of type, constitution, and habits; is it probable, then, that this one peculiarity of the reproductive system should be due, in so many cases, to some merely incidental effect produced on that system by uniform conditions of life? Again, ex hypothesi, at the time when a variety is first forming, the influence exercised by uniform conditions of life (whatever in different cases this may happen to be) cannot be present as regards that variety: yet this is just the time when its infertility with the parent (or allied) form is most likely to have arisen; for it is just then that the nascent variety would otherwise have been most liable to extinction by free intercrossing—even supposing that in the presence of such intercrossing the variety could ever have come into existence at all.
Mr. Wallace meets the difficulty by arguing that sterility between allied species may have been brought about by the direct influence of natural selection. But, as previously remarked, this view is expressly opposed to that of Darwin, who held that Wallace's contention is erroneous.
It will be seen, then, that both Darwin, and Wallace, fully recognize the necessity of finding some explanation of the infertility of allied species, over and above the mere reaction of morphological differentiation on the physiology of the reproductive system, and they both agree in suggesting additional causes, though they entirely disagree as to what these causes are. Now, the theory of physiological selection likewise suggests an additional cause—or, rather, a new explanation—and one which is surely the most probable. For what is to be explained? The very general association of a certain physiological peculiarity with that amount of morphological change which distinguishes species from species, of whatever kind the change may be, and in whatever family of the animal or vegetable kingdom it may occur. Well, the theory of physiological selection explains this very general association by the simple supposition that, at least in a large number of cases, it was the physiological peculiarity which first of all led to the morphological divergence, by interposing the bar of sterility between two sections of a previously uniform species; and by thus isolating the two sections one from another, started each upon a subsequently independent course of divergent evolution.
Or, to put it in another way, if the occurrence of this physiological peculiarity has been often the only possible means of isolating two sections of a species occupying a common area, and thus giving rise to a divergence of specific type (as obviously must have been the case wherever there was an absence of any other form of isolation), it is nothing less than a necessary consequence that many allied species should now present the physiological peculiarity in question. Thus the association between the physiological peculiarity and the morphological divergence is explained by the simple hypothesis, that the former has acted as a necessary condition to the occurrence of the latter. In the absence of other forms of isolation, the morphological divergence could not have taken place at all, had not the physiological peculiarity arisen; and hence it is that we now meet with so many cases where such divergence is associated with this peculiarity.
So far we have been considering the physiological change as historically the prior one. Here, at first sight, it may seem that the segregative power of physiological selection must end; for it may well seem impossible that the physiological change can ever be necessary for the divergence of morphological varieties into true species in cases where it has not been the prior change, but has only set in after morphological changes have proceeded far enough to have already constituted definite varieties. A little thought, however, will show that physiological selection is quite as potent a condition to the differentiation of species when it occurs after varietal divergence has begun, as it is when it occurs before the divergence—and hence that it really makes no difference to the theory of physiological selection whether, in particular cases, the cross-infertility arises before or after any structural or other modifications with which it is associated.
For the theory does not assert that all varieties have been due to physiological selection. There are doubtless many other causes of the origin of varieties besides cross-infertility with parent forms; but, as a general rule, it does not appear that they are by themselves capable of carrying divergence beyond a merely varietal stage. In order to carry divergence to the stage of producing species, it appears to be a general condition that, sooner or later, cross-infertility should arise—seeing that, when varieties do succeed in becoming species, we almost invariably find that, as a matter of fact, cross-infertility has arisen. Hence, if cross-infertility has thus usually been a necessary condition to a varietal divergence becoming specific, it can make no material difference when the incipient infertility arose.
It may be asked, however, whether I suppose that, when the physiological change is subsequent, it is directly caused by change of structure, size, colour, &c., or that it arises, so to speak, accidentally, from other causes which may have affected the sexual system in the required way. To this question I may briefly reply, that, looking to the absence of any influence exercised on the reproductive systems of our domesticated plants and animals by the great and varied changes which so many of these forms present, it would seem that among natural varieties such closely analogous changes are presumably not the usual causes of the physiological change, even where the latter are subsequent to the former. Nevertheless, I do not deny that in some of these cases changes of structure, size, colour, &c., may be the causes of the physiological change by reacting on the sexual system in the required way. But in such cases free intercrossing will have prevented the perpetuation of any morphological changes, save those which have the power of so reacting on the reproductive system as to produce the physiological change, and thus to protect themselves against the full and adverse power of free intercrossing. We know that slight or initial changes of structure, colour, &c., frequently occur as varieties, and yet that on common areas very few of these varieties become distinct species: free intercrossing prevents any such further divergence of character. But if in the course of many such abortive attempts, as it were, to produce a new species, nature happens to hit upon a structural or a colour variation which is capable of reacting on the sexual system in the particular way required, then this variation will be enabled to protect itself against free intercrossing in proportion to its own development. Or, in other words, the more it develops as a morphological change, the more will it increase the physiological change; while the more the physiological change is thus increased, the more will it in turn promote the morphological. By such action and reaction the development of each furthers the development of the other, till from an almost imperceptible variety, apparently quite fertile with its parent form, there arises a distinct species absolutely sterile with its parent form. In such cases, therefore, it is still the physiological conditions which have selected the particular morphological changes capable of so reacting on the reproductive system as to produce cross-infertility, and thus to protect themselves against the destructive power of free intercrossing. So to speak, free intercrossing is always on the watch to level down any changes which natural selection, or any other cause of varietal divergence, may attempt to produce; and therefore, in order to produce—or to increase—such divergence in the absence of any other form of isolation, natural selection must hit upon such changes of structure, form, or colour, as are so correlated with the reproductive system as to create the physiological isolation that is required.
To show how the principle of selective fertility may be combined with what apparently is the most improbable form of isolation for this purpose—the geographical—I quote the following suggestion made by Professor Lloyd Morgan in his Animal Life and Intelligence:—
Suppose two divergent local varieties were to arise in adjacent areas, and were subsequently (by stress of competition or by geographical changes) driven together into a single area.... If their unions be fertile, the isolation will be annulled by intercrossing—the two varieties will form one mean or average variety. But if the unions be infertile, the isolation will be preserved, and the two varieties will continue separate. Suppose now, and the supposition is by no means an improbable one, that this has taken place again and again in the evolution of species; then it is clear that those varietal forms which had continued to be fertile together would be swamped by intercrossing; while those varietal forms which had become infertile would remain isolated. Hence, in the long run, isolated forms occupying a common area would be infertile, (p. 107.)
If then cross-sterility may thus arise even in association with geographical isolation, may it not also arise in its absence? And may it not thus give rise to the differentiation of varieties on account of this physiological isolation alone?
Only two further points need be mentioned to make this statement of physiological selection as complete as the present résumé of its main principles requires.
The first is, that, as Mr. Wallace remarks, "every species has come into existence coincident both in space and time with a pre-existing and closely allied species." I regard this as important evidence that physiological selection is one of the natural causes concerned. For the general fact implied is that every species has come into existence on an area occupied by its parent type, and therefore under circumstances which render it imperative that intercrossing with that type should be prevented. In the case of monotypic evolution by natural selection alone, intercrossing with the parent type is prevented through the gradual extinction of that type by successive generations of the developing type. But in the case of polytypic evolution, intercrossing with the parent type can only be prevented by some form of isolation other than natural selection; and here it is evident that cross-infertility with the parent type must be as efficient to that end as any other form of isolation that can be imagined. Consequently we might almost have expected beforehand that in a large proportional number of cases cross-infertility should have been the means employed. And the fact that this is actually the case so far corroborates the only theory which is able to explain it.
The second point is this.
It appears to be comparatively rare for any cause of specific divergence to prove effectual on common areas, unless it sooner or later becomes associated with some degree of cross-infertility. But through this association, the segregating influence of both the causes concerned is, as Mr. Gulick has shown, greatly increased. For instance, if the segregating influence of some degree of cross-infertility be associated with that of any other form of isolation, then, not only will the two segregating influences be added, but multiplied together. And thus, by their mutual action and reaction, divergent evolution is promoted at a rapidly increasing rate.
I will now summarize the main points of the theory of physiological isolation in a categorical form.
1. If no other form of isolation be present, specific divergence can only take place when some degree of cross-infertility has previously arisen between two or more sections of a species.
2. When such cross-infertility has arisen it may cause specific divergence, either (a) by allowing independent variability in each of the physiologically isolated groups; (b) by becoming associated with any other cause of differentiation already operating; or (c) by both these means combined.
3. As some degree of cross-infertility generally obtains between allied species, we are justified in concluding that this has been the most frequent—or, at any rate, the most effective—kind of isolation where the origin of species is concerned; and therefore the kind with which, in the case of species-formation, natural selection, or any other cause of specific divergence, has been most usually associated.
4. Where varietal divergence has begun in the absence of cross-infertility, such divergence seems, as a general rule, to have been incapable of attaining to a specific value.
5. Therefore, in the vast majority of such cases, it must have been those varietal changes of structure, size, colour, &c., which happened to have afterwards been assisted by the reproductive change that were on this account selected as successful candidates for specific differentiation.
6. It follows, that it makes no difference to the general theory of physiological selection in what proportion of cases the physiological change has been the initial change; for, whether prior or subsequent to the varietal changes with which it becomes associated, its presence has been equally important as a condition to specific divergence.
7. When physiological isolation becomes associated with natural selection, or any other form of homogamy, the segregative power of both is augmented. Moreover, so great is the augmentation that even very moderate degrees of physiological isolation—themselves capable of effecting little or nothing—become very powerful when associated with moderate degrees of any other kind of homogamy, and vice versa.
8. The theory of physiological selection effectually explains the divergent evolution of specific types and the cross-infertility of such types when evolved.
To prevent, if possible, the continuance of certain misunderstandings with regard to my original statement of the new theory, let me here disclaim some views which have been assigned to me. They are:
1. That the theory of physiological selection is opposed to the theory of natural selection. Far from this being so, it is—at all events in my own opinion—a very important aid to it, in preventing free intercrossing on a common area, and thus allowing divergent evolution to occur within that area.
2. That, in advancing the theory of physiological selection as "an additional suggestion on the origin of species," I wish to represent it as being the originating cause of all species. What I hold is, that all species must have owed their origin to isolation, in some form or other; but that as physiological selection is only one among many other forms of isolation (including natural selection), and as it can only act on common areas, a large number of species must have been formed without its aid.
3. That I imagine physiological varieties always to arise "sporadically," or as merely individual "sports" of the reproductive system. On the contrary, I expressly stated that this is not the way in which I suppose the "physiological variation" to arise, when giving origin to a new species; but that it arises, whenever it is effectual, as a "collective variation" affecting a number of individuals simultaneously, and therefore characterizing "a whole race, or strain."
4. That I suppose physiological selection always to act alone. This I have never supposed. The essential point is, not that the physiological isolation is unassociated with other forms of isolation, but that unless associated with some degree of physiological isolation, no one of the other forms is capable of originating species on common areas with any approach to frequency. This proposition is the essence of the new theory, and I take it to be proved, not only by general deductive reasoning which shows that it must be so, but also by the fact of an otherwise inexplicable association between specific divergence on common areas and some more or less considerable degree of mutual infertility.
I will now give an outline sketch of the evidences in favour of the theory which has been set forth in the preceding chapter, stating first what is the nature of the verification which it requires.
The theory is deduced from a highly general association between distinctive specific characters of any kind and a relatively constant specific character of a particular kind—namely, sexual exclusiveness. For it is from this highly general association that the theory infers that this relatively constant specific character has been at least one of the needful conditions to the development of the other specific characters with which it is found associated. Hence the necessary verification must begin by showing the strength of the theory on these merely deductive, or antecedent, grounds. It may then proceed to show how far the facts of organic nature corroborate the theory in other and independent ways.
First, let it be carefully observed that here we have to do only with the fact of selective fertility, and with its consequences as supposed by the theory: we have nothing to do either with its causes or its degrees. Not with its causes, because in this respect the theory of physiological selection is in just the same position as that of natural selection: it is enough for both if the needful variations are provided, without its being incumbent on either to explain the causes which produce them. Not with its degrees, because, in the first place, it can only be those degrees of variation which in particular cases are supposed adequate to induce specific divergence, that fall within the scope of the theory; and because, in the second place, degrees which are adequate only to induce—or to assist in inducing, varietal divergence, must always tend to increase, or pass into higher degrees.
The antecedent standing or logical basis of the theory has already been in large measure displayed in the preceding chapter; for it was impossible to state the theory without thereby showing in how considerable a degree it is self-evident. A brief recapitulation is therefore all that is here necessary.
It has been shown that divergent or polytypic evolution on common areas is inexplicable by natural selection alone. Hence the question arises: What form of isolation has, under such circumstances, rendered possible divergent evolution? In answer to this question the theory of physiological selection suggests that variations in the reproductive function occur in such a way as to isolate more or less perfectly from each other different sections of a species. While cross-fertility remains unimpaired among the members of each section, there is more or less cross-infertility when members of either section mate with those of the other. Thus a physiological barrier is interposed between the two sections; and any divergences of structure, colouring, or instinct arising in the members of either section will not in any way be affected by such divergences as arise among the members of the other.
In support of this suggestion, it has been shown in the preceding chapter that the very general association of cross-infertility with specific differentiation points most strongly to the inference that the former has usually been an indispensable condition to the occurrence of the latter. It cannot be denied that in many cases the specific distinction is now maintained by means of that sexual isolation which cross-infertility confers: it is therefore probable that such isolation has been instrumental in securing its initial attainment.
This probability is strengthened by the observed fact that the general association in question is conspicuously absent in the case of domesticated varieties, notwithstanding that their multitudinous and diverse varietal characters usually equal, and frequently surpass, specific characters in their degrees of divergence.
Since, then, it would seem to be impossible for divergent evolution on common areas to take place in the absence of some mode of isolation; since cross-infertility appears to be the only possible mode under the given circumstances; and since among domesticated varieties, where isolation is otherwise secured by artificial means, cross-infertility is usually absent, the logical foundations of the theory of physiological selection would seem to be securely laid.
We may therefore pass to more special lines of evidence.
Darwin has adduced very good evidence to show that large areas, notwithstanding the disadvantages which (on his theory) must arise from free intercrossing, are what he terms better manufactories of species than smaller areas, such as oceanic islands. On the other hand, as a matter of fact, oceanic islands are comparatively rich in peculiar species. These two statements, however, are not incompatible. Smaller areas are, as a rule, rich in peculiar species relatively to the number of their inhabitants; but it does not follow that they are rich in species as contrasted with larger areas containing very many more inhabitants. Therefore, the rules are that large areas turn out an absolutely greater number of specific types than small areas; although, relatively to the number of individuals or amount of population, the small areas turn out a larger number of species than the large areas.
Now, these two complementary rules admit of being explained as Darwin explains them. Small and isolated areas are rich in species relatively to the amount of population, because, as we have before seen, this population has been permitted to develop an independent history of its own, shielded from intercrossing with parent forms, and from competition with exotic forms; while, at the same time, the homogamy thus secured, combined with change of environment, will give natural selection an improved chance of finding new points of departure for its operation. On the other hand, large and continuous areas are favourable to the production of numerous species, first, because they contain a large population, thus favouring the occurrence of numerous variations; and, secondly, because the large area furnishes a diversity of conditions in its different parts, as to food, climate, attitude, &c., and thus so many different opportunities for the occurrence of sundry forms of homogamy. Now, it is obvious that of all these sundry forms of homogamy, physiological selection must have what may be termed a first-rate opportunity of assisting in the manufacture of species on large areas. For not only is it upon large and continuous areas that the antagonistic effects of intercrossing are most pronounced (and, therefore, that the influence of physiological selection must be most useful in the work of species-making); but here also the diversity in the external conditions of life, which the large area supplies to different parts of the extensive population, cannot fail to furnish physiological selection with a greater abundance of that particular variation in the reproductive system on which its action depends. Again, and of still more importance, on large areas there are a greater number of species already differentiated from one another as such; thus a greater number of already sexually differentiated forms are presented for further differentiation at the hands of physiological selection. For all these reasons, therefore, we might have expected, upon the new theory, that large and continuous areas would be good manufactories of species.
Again, Darwin has shown that not only large areas, but likewise "dominant" genera within those areas, are rich in species. By dominant genera he meant those which are represented by numerous individuals, as compared with other genera inhabiting the same area. This general rule he explains by the consideration that the qualities which first led to the form being dominant must have been useful; that these would be transmitted to the otherwise varying offspring; and, therefore, that when these offspring had varied sufficiently to become new species, they would still enjoy their ancestral advantages in the struggle for existence. And this, doubtless, is in part a true explanation; but I also think that the reason why dominant genera are rich in species, is chiefly because they everywhere present a great number of individuals exposed to relatively great differences in their conditions of life: or, in other words, that they furnish the best raw material for the manufacture of species by physiological selection, as explained in the last paragraph. For, if the fact of dominant genera being rich in species is to be explained only by natural selection, it appears to me that the useful qualities which have already led to the dominance of the ancestral type ought rather to have proved inimical to its splitting up into a number of subordinate types. If already so far "in harmony with its environment" as to have become for this reason dominant, one would suppose that there is all the more reason for its not undergoing change by the process of natural selection. Or, at least, I do not see why the fact of its being in an unusual degree of harmony with its environment should in itself constitute any unusual reason for its modification by survival of the fittest. On the other hand, as just observed, I do very plainly see why such a reason is furnished for the modifying influence of physiological selection.
Let us next turn to another of Darwin's general rules with reference to distribution. He took a great deal of trouble to collect evidence of the two following facts, namely, (1) that "species of the larger genera in each country vary more frequently than the species of the smaller genera"; and (2) that "many of the species included within the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges[22]." By larger genera he means genera containing many species; and he accounts for these general facts by the principle, "that where many species of a genus have been formed, on an average many are still forming." But how forming? If we say by natural selection alone, we should expect to find the multitudinous species differing from one another in respect of features presenting well-marked adaptive meanings; yet this is precisely what we do not find. For Darwin's argument here is that "in large genera the amount of difference between the species is often exceedingly small, so that in this respect the species of the larger genera resemble varieties more than do the species of the smaller genera." Therefore the argument, while undoubtedly a very forcible one in favour of the fact of evolution, appears to me scarcely consistent with the view of this evolution being due solely to natural selection. On the other hand, the argument tells strongly (though unconsciously) in favour of physiological selection. For the larger a genus, or the greater the number of its species, the greater must be the opportunity for the occurrence of that particular kind of variation on which the principle of physiological selection depends. The species of a genus may be regarded as so many varieties which have already been separated from one another physiologically; therefore each of them may now constitute a new starting-point for a further and similar separation—particularly as, in virtue of their previous segregation, many are now exposed to different conditions of life. Thus, it seems to me, we can well understand why it is that genera already rich in species tend to grow richer; while such is not the case in so great a degree with genera that are poor in species. Moreover, we can well understand that, multiplication of species being as a rule, and in the first instance, determined by changes in the reproductive system, wherever a large number of new species are being turned out, the secondary differences between them should be "often exceedingly small"—a general correlation which, so far as I can see, we are not able to understand on the theory of natural selection.
The two subsidiary facts, that very closely allied species have restricted ranges, and that dominant species are rich in varieties, both seem to tell more in favour of physiological than of natural selection. For "very closely allied species" is but another name for species which scarcely differ from one another at all except in their reproductive systems; and, therefore, the more restricted their ranges, the more certainly would they have become fused by intercrossing with one another, had it not been for the barrier of sterility imposed by the primary distinction. Or rather, I should say, had it not been for the original occurrence of this barrier, these now closely-allied species could never have become species. Again, that dominant species should be rich in varieties is what might have been expected; for the greater the number of individuals in a species, the greater is the chance of variations taking place in all parts of the organic type, and particularly in the reproductive system, seeing that this system is the most sensitive to small changes in the conditions of life, and that the greater the number of individuals composing a specific type, the more certainty there is of some of them encountering such changes. Hence, the richness of dominant species in varieties is, I believe, mainly due to the greater opportunity which such species afford of some degree of cross-infertility arising between their constituent members.
Here is another general fact, also first noticed by Darwin, and one which he experiences some difficulty an explaining on the theory of natural selection. He says:—
In travelling from north to south over a continent, we generally meet at successive intervals with closely-allied or representative species, evidently filling the same place in the economy of the land. These representative species often meet and interlock, and as one becomes rarer and rarer, the other becomes more and more frequent, till the one replaces the other. But if we compare these species where they intermingle, they are generally as absolutely distinct from each other in every detail of structure as are specimens taken from the metropolis of each.... In the intermediate region, having intermediate conditions of life, why do we not now find closely-linking intermediate varieties? This difficulty for a long time quite confounded me. But I think it can in large part be explained[23].