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].

[20] _Origin of Species_, p. 136.

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].

[21] _Darwinism_, p. 152.

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, const.i.tution, 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 a.s.sociation 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 a.s.sociation 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 a.s.sociation 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 a.s.sociated 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 const.i.tuted 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 a.s.sociated.

For the theory does not a.s.sert 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 s.e.xual 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 a.n.a.logous 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 s.e.xual 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 s.e.xual 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 compet.i.tion 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 a.s.sociation 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 _resume_ of its main principles requires.

The first is, that, as Mr. Wallace remarks, "every species has come into existence coincident both in s.p.a.ce 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 circ.u.mstances 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 a.s.sociated with some degree of cross-infertility. But through this a.s.sociation, 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 a.s.sociated 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 a.s.sociated 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 a.s.sociated.

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 a.s.sisted 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 a.s.sociated, its presence has been equally important as a condition to specific divergence.

7. When physiological isolation becomes a.s.sociated with natural selection, or any other form of h.o.m.ogamy, 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 a.s.sociated with moderate degrees of any other kind of h.o.m.ogamy, 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 a.s.signed 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 una.s.sociated with other forms of isolation, but that unless a.s.sociated 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 a.s.sociation between specific divergence on common areas and some more or less considerable degree of mutual infertility.

CHAPTER IV.

EVIDENCES OF PHYSIOLOGICAL SELECTION.

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 a.s.sociation between distinctive specific characters of _any_ kind and a relatively constant specific character of a _particular_ kind--namely, s.e.xual exclusiveness.

For it is from this highly general a.s.sociation 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 a.s.sociated. 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 inc.u.mbent 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 a.s.sist in inducing, _varietal_ divergence, must always tend to increase, or pa.s.s into higher degrees.

_Antecedent Standing of the Theory._

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 circ.u.mstances, 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 a.s.sociation 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 s.e.xual 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 a.s.sociation in question is conspicuously absent in the case of domesticated varieties, notwithstanding that their mult.i.tudinous and diverse varietal characters usually equal, and frequently surpa.s.s, specific characters in their degrees of divergence.