[51] _Animal Life and Intelligence_, pp. 98, 99 (1890-1891).

Similarly, and still more recently, Professor Le Conte writes:--

It is evident, then, as Romanes claims, that natural selection alone tends to _monotypic_ evolution. Isolation of some sort seems necessary to _polytypic_ evolution. The tree of evolution under the influence of natural selection alone grows palm-like from its terminal bud. Isolation was necessary to the starting of lateral buds, and thus for the profuse ramification which is its most conspicuous character[52].

[52] _The Factors of Evolution_ (1891).

In order to complete this historical review, it only remains to consider Mr. Wallace's utterances upon the subject.

It is needless to say that he stoutly resists the view of Weismann, Delbuf, Gulick, and myself, that specific divergence can ever be due--or, as I understand him, even so much as a.s.sisted--by this principle of indiscriminate isolation (apogamy). It will be remembered, however, that Mr. Gulick has adduced certain general principles and certain special facts of geographical distribution, in order to prove that apogamy eventually leads to divergence of character, provided that the isolated section of the species does not contain any very large number of individuals. Now, Mr. Wallace, without making any reference to this argument of Mr. Gulick, simply states the reverse--namely, that, as a matter of fact, indiscriminate isolation is not found to be a.s.sociated with divergence of character. For, he says, "there is an entire absence of change, where, if this were a _vera causa_, we should expect to find it[53]." But the only case which he gives is that of Ireland.

[53] _Darwinism_, p. 151.

This, he says, furnishes "an excellent test case, for we know that it [Ireland] has been separated from Britain since the end of the glacial epoch: ... yet hardly one of its mammals, reptiles, or land molluscs has undergone the slightest change[54]." Here, however, Mr. Wallace shows that he has failed to understand "the views of those who, like Mr.

Gulick, believe isolation itself to be a cause of modification of species"; for it belongs to the very essence of these views that the efficiency of indiscriminate isolation as a "_vera causa_" of organic evolution varies inversely with the number of individuals (i. e. the size of the species-section) exposed to its influence. Therefore, far from being "an excellent test case," the case of Ireland is unsatisfactory. If we are in search of excellent test cases, in the sense intended by Mr. Wallace, we ought not to choose a large island, which from the time of its isolation must have contained large bulks of each of the geographically separated species concerned: we ought to choose cases where as small a number as possible of the representatives of each species were in the first instance concerned. And, when we do this, the answer yielded by any really "excellent test case" is unequivocal.

[54] _Ibid._

No better test case of this kind has ever been furnished than that of Mr. Gulick's land-sh.e.l.ls, which Mr. Wallace is specially considering in the part of his book where the sentence above quoted occurs. How, then, does he meet this case? He meets it by a.s.suming that in all the numerous adjacent valleys of a small island there must be as many differences of environment, each of which is competent to induce slight varietal changes on the part of its occupants by way of natural selection, although in no one case can the utility of these slight changes be surmised. Now, against this explanation there are three overwhelming considerations. In the first place, it is purely gratuitous, or offered merely in order to save the hypothesis that there _can_ be no other cause of even the most trivial change in species than that which is furnished by natural selection. In the second place, as Mr. Gulick writes to me in a private letter, "if the divergence of Sandwich Island land molluscs is wholly due to exposure to different environments, as Mr. Wallace argues on pages 147-150, then there must be completely occult influences in the environment that vary progressively with each successive mile. This is so violent an a.s.sumption that it throws doubt on any theory that requires such support." In the third place, the a.s.sumption that the changes in question must have been due to natural selection, is wholly incompatible with the facts of isolation elsewhere--namely, in those cases where (as in that of Ireland) a large section of species, instead of a small section, has been indiscriminately isolated. Mr. Wallace, as we have seen, inadvertently alludes to these "many other cases of isolation" as evidence against apogamy being _per se_ a cause of specific change. But although, for the reason above stated, they are without relevancy in this respect, they appear to me fatal to the explanation which he gives of specific changes under apogamy where only small sections of species are concerned. For example, can it be rationally maintained that there are more differences of environment between every two of the many contiguous valleys of a small island, such as Mr. Gulick describes, than there are in the incomparably larger area of the whole of Ireland? But, if not, and if natural selection is able to work such "occult" wonders in each successive mile on the Sandwich Islands, why has it so entirely lost this magic power in the case of Ireland--or in the "many other cases of isolation" to which Mr. Wallace refers? On his theory there is no coherent answer to be given to this question, while on our theory the answer is given in the very terms of the theory itself. The facts are plainly just what the theory requires that they should be; and therefore, if they were not as they are, the theory would be deprived of that confirmation which it now derives from them.

Thus, in truth, though in an opposite way, the case of Ireland is, as Mr. Wallace says, "an excellent test case," when once the theory of apogamy as a "_vera causa_" of specific change is understood; and the effect of applying the test is fully to corroborate this theory, while at the same time it as fully negatives the other. For the consideration whereby Mr. Wallace seeks to explain the inactivity of natural selection in the case of Ireland is not "coherent." What he says is, "That changes have not occurred through natural selection, is perhaps due to the less severe struggle for existence, owing to the smaller number of competing species[55]." But even with regard to molluscs alone, there is a greatly larger number of species in Ireland than occurs in any one valley of the Sandwich Islands; while if we have regard to all the other cla.s.ses of animal life, comparison entirely fails.

[55] _Loc. cit._, p. 151.

Much more to the point are certain cases which were adduced long ago by Weismann in his essay previously considered. Nevertheless, although this essay was published as far back as 1872, and, although it expressly deals with the question of divergence of character through the mere prevention of intercrossing (Amixia), Mr. Wallace nowhere alludes to these cases _per contra_, which are so much more weighty than his own "test case" of Ireland. Of such are four species of b.u.t.terflies, belonging to three genera[56], which are identical in the polar regions and in the Alps, notwithstanding that the spa.r.s.e Alpine populations have been presumably separated from their parent stocks since the glacial period; or of certain species of fresh water crustaceans (_Apus_), the representatives of which are compelled habitually to form small isolated colonies in widely separated ponds, and nevertheless exhibit no divergence of character, although apogamy has probably lasted for centuries. These cases are unquestionably of a very cogent nature, and appear of themselves to prove that apogamy alone is not invariably capable of inducing divergence--at any rate, so rapidly as we might expect. There appears, however, to be another factor, the presence or absence of which makes a great difference. This as stated in the text, is the degree in which a specific type is stable or unstable--liable or not liable to vary. Thus, for example, the Goose is what Darwin calls an "inflexible" type as compared with most other domesticated birds.

Therefore, if a lot of geese were to be indiscriminately isolated from the rest of their species, the probability is that in a given time their descendants would not have diverged from the parent type to such an extent as would a similar lot of ducks under similar circ.u.mstances: the more stable specific type would require a longer time to change under the influence of apogamy alone. Now, the b.u.t.terflies and crustaceans quoted by Weismann may be of a highly stable type, presenting but a small range of individual variability; and, if so, they would naturally require a long time to exhibit any change of type under the influence of apogamy alone. But, be this as it may, Weismann himself adduces these cases merely for the sake of showing that there are cases which seem to tell against the general principle of modification as due to apogamy alone--i.e. the general principle which, under the name amixia, he is engaged in defending. And the conclusion at which he himself arrives is, that while it would be wrong to affirm that apogamy _must_ in all cases produce divergence, we are amply justified in affirming that in many cases it _may_ have done so; while there is good evidence to prove that in not a few cases it _has_ done so, and therefore should be accepted as one of the factors of organic evolution[57].

[56] Namely, _Lycaena denzelii_, _L. pheretes_, _Argynnis pales_, _Erebia mante_.

[57] Since the above was written, I have heard of some cases which seem to present greater difficulties to our theory than those above quoted. These refer to some of the numerous species of land mollusca which inhabit the isolated rocks near Madeira (Dezertas). My informant is Dr. Grabham, who has himself investigated the matter, and reports as follows:--

"It is no uncommon thing to meet with examples of the same species, sub-fossil, recent, and living upon one spot, and presenting no variation in the long record of descent." Then, after naming these examples, he adds, "All seem to vary immediately on attaining new ground, a.s.suming many aspects in different districts."

Unquestionably these statements support, in a very absolute manner, Mr. Wallace's opinion, while making directly against my own. It is but fair, however, to add that the cases are not numerous (some half-dozen at the most, and all within the limits of a single genus), and that, even in the opinion of my informant himself, the facts have not hitherto been sufficiently investigated for any decisive judgement to be formed upon them.

My view from the very first has been that variations in the way of cross-infertility are of frequent occurrence (how, indeed, can they be otherwise, looking to the complex conditions that have to be satisfied in every case of full fertility?); and, therefore, however many of such variations are destined to die out, whenever one arises, "under suitable conditions," "it must inevitably tend to be preserved as a new natural variety, or incipient species." Among the higher animals--which are "comparatively few in number"--I think it probable that some slight change of form, colour, habit, &c., must be usually needed either to "superinduce," or, which is quite a different thing, to _coincide_ with the physiological change But in the case of plants and the lower invertebrata. I see no reason for any frequent concomitance of this kind; and therefore believe the physiological change to be, "as a general rule," the primordial change. At the same time, I have always been careful to insist that this opinion had nothing to do with "the essence of physiological selection"; seeing that "it was of no consequence" to the theory in what proportional number of cases the cross-sterility had begun _per se_, had been superinduced by morphological changes, or only enabled to survive by happening to coincide with any other form of h.o.m.ogamy. In short, "the essence of physiological selection" consists in _all_ cases of the diversifying _effect_ of cross-infertility, whensoever and howsoever it may happen in particular cases to have been _caused_.

Thus I emphatically reaffirm that "from the first I have always maintained that it makes no essential difference to the theory _in what proportional number of cases_ they [the physiological variations] have arisen 'alone in an otherwise undifferentiated species'"; therefore, "even if I am wrong in supposing that physiological selection can _ever_ act alone, the _principle_ of physiological selection, as I have stated it, is not thereby affected. And this principle is, as Mr. Wallace has re-stated it, 'that some amount of infertility characterizes the distinct varieties which are in process of differentiation into species'--infertility whose absence, 'to obviate the effects of intercrossing, may be one of the _usual_ causes of their failure to become developed into distinct species.'"

These last sentences are quoted from the correspondence in _Nature_[58], and to them Mr. Wallace replied by saying, "if this is not an absolute change of front, words have no meaning"; that "if this is 'the whole essence of physiological selection,' then physiological selection is but a re-statement and amplification of Darwin's views"; that such a "change of front" is incompatible, not only with my term "physiological selection," but also with my having "acknowledged that Mr. Catchpool had 'very clearly put forward the theory of physiological selection'"; and much more to the same effect.

[58] Vol. xliii. p. 127.

Now, to begin with, it is due to Mr. Catchpool to state that his only publication upon this subject is much too brief to justify Mr.

Wallace's, inference, that he supposes variations in the way of cross-infertility always to arise "alone in an otherwise undifferentiated species." What Mr. Catchpool's opinion on this point may be, I have no knowledge; but, whatever it is, he was unquestionably the first writer who "clearly stated the leading principles" of physiological selection, and this fact I am very glad to have "acknowledged." In my correspondence with Mr. Wallace, however, I not only named Mr. Catchpool: I also named--and much more prominently--Mr.

Gulick. For even if I were to grant (which I am far indeed from doing) that there was any want of clearness in my own paper touching the point in question, I have now repeatedly shown that it is simply impossible for any reader of Mr. Gulick's papers to misunderstand _his_ views with regard to it. Accordingly, I replied to Mr. Wallace in _Nature_ by saying:--

Not only have I thus from the first fully recognized the sundry other causes of specific change with which the physiological variations may be a.s.sociated; but Mr. Gulick has gone into this side of our common theory much more fully, and elaborately calculated out the high ratio in which the differentiating agency of any of these other causes must be increased when a.s.sisted by--i.

e. a.s.sociated with--even a moderate degree of the selective fertility, and vice versa. Therefore, it is simply impossible for Mr. Wallace to show that "our theory" differs from his in this respect. Yet it is the only respect in which his reply alleges any difference. (Vol. xliii. p. 127.)

I think it is to be regretted that, in his answer to this, Mr. Wallace alludes only to Mr. Catchpool, and entirely ignores Mr. Gulick--whose elaborate calculations above alluded to were communicated to the Linnaean Society by Mr. Wallace himself in 1887.

The time has now come to prove, by means of quotations, that I have from the first represented the "principle," or "essence," of physiological selection to consist in selective fertility furnishing a needful condition to specific differentiation, in at least a large proportional number of allied species which afterwards present the reciprocal character of cross-sterility; that I have never represented variations in the way of this selective fertility as necessarily const.i.tuting the initial variations, or as always arising "alone, in an otherwise undifferentiated species"; and that, although I have uniformly given it as my opinion that these variations do _in some cases_ thus arise (especially among plants and lower invertebrata), I have as uniformly stated "that it makes no difference to the theory in what proportional number of cases they have done so"--or even if, as Mr. Wallace supposes, they have never done so in any case at all[59]. These statements (all of which are contradictory of the only points of difference alleged) have already been published in my article in the _Monist_ of October, 1890.

And although Mr. Wallace, in his reply to that article, ignores my references to the "original paper," it is scarcely necessary to quote the actual words of the paper itself, since the reader who is further interested in this controversy can readily refer to it in the _Journal of the Linnaean Society_ (vol. xix. pp. 337-411).

[59] This refers to what I understand Mr. Wallace to say in the _Nature_ correspondence is the supposition on which his own theory of the origin of species by cross-infertility is founded. But in the original statement of that theory itself, it is everywhere "supposed" that when species are originated by cross-infertility, the _initial_ change _is_ the physiological change. In his original statement of that theory, therefore, he literally went further than I had gone in my "original paper," with reference to supposing the physiological change to be the initial change. I do not doubt that this is due to some oversight of expression; but it is curious that, having made it, he should still continue his endeavour to fix exactly the same oversight upon me.

Having arrived at these results with regard to the theory of Isolation in general and of Physiological Isolation in particular, I arrive also at the end of this work. And if, while dealing with the post-Darwinian period, I have imparted to any general reader the impression that there is still a great diversity of expert opinion; I must ask him to note that points with reference to which disagreement still exists are but very subordinate to those with regard to which complete agreement now prevails. The noise of wrangling disputations which has so filled the camp of evolutionists since the death of their captain, is apt to hide from the outside world the solid unanimity that prevails with regard to all the larger and more fundamental questions, which were similarly the subjects of warfare in the past generation. Indeed, if we take a fair and general view of the whole history of Darwinism, what must strike us as the really significant fact is the astonishing unanimity which has been so rapidly attained with regard to matters of such immeasurable importance. It is now but little more than thirty years since the publication of the _Origin of Species_; and in that period not only have all naturalists unequivocally embraced the doctrine of descent considered as a fact; but, in one degree or another, they have all as unequivocally embraced the theory of natural selection considered as a method. The only points with regard to which any difference of opinion still exist, have reference to the precise causation of that mighty stream of events which, under the name of organic evolution, we have now all learnt to accept as scientifically demonstrated. But it belongs to the very nature of scientific demonstration that, where matters of great intricacy as well as of high generality are concerned, the process of demonstration must be gradual, even if it be not always slow. It is only by the labours of many minds working in many directions that, in such cases, truth admits of being eventually displayed. Line upon line, precept upon precept, here a little and there a little--such is the course of a scientific revelation; and the larger the subject-matter, the more subtle and the more complex the causes, the greater must be the room for individual differences in our reading of the book of Nature.

Now, if all this be true, must we not feel that in the matter of organic evolution the measure of agreement which has been attained is out of all proportion to the differences which still remain--differences which, although of importance in themselves, are insignificant when compared with those which once divided the opinions of not a few still living men? And if we are bound to feel this, are we not bound further to feel that the very intensity of our disputations over these residual matters of comparative detail, is really the best earnest that can be given of the determination of our quest--determination which, like that of our fathers, cannot fail to be speedily rewarded by the discovery of truth?

Nevertheless, so long as this noise of conflict is in the Senate, we cannot wonder if the people are perplexed. Therefore, in conclusion, I may ask it to be remembered exactly what are the questions--and the only questions--which still divide the parties.

Having unanimously agreed that organic evolution is a fact and that natural selection is a cause, or a factor in the process, the primary question in debate is whether natural selection is the only cause, or whether it has been a.s.sisted by the co-operation of other causes. The school of Weismann maintain that it is the only cause; and therefore deem it worse than useless to search for further causes. With this doctrine Wallace in effect agrees, excepting as regards the particular case of the human mind. The school of Darwin, on the other hand--to which I myself claim to belong--believe that natural selection has been to a considerable extent supplemented by other factors; and, therefore, although we further believe that it has been the "main" factor, we agree with Darwin himself in strongly reprobating all attempts to bar _a priori_ the progress of scientific investigation touching what, if any, these other factors may be. Lastly, there are several more or less struggling schools, chiefly composed of individual members who agree with each other only to the extent of holding that the causal agency of natural selection is not so great as Darwin supposed. The Duke of Argyll, Mr. Mivart and Mr. Geddes may be named in this connexion; together with the self-styled neo-Lamarckians, who seek to magnify the Lamarckian principles at the expense of the distinctively Darwinian.

This primary difference of opinion leads deductively to certain secondary differences. For if a man starts with the premiss that natural selection must necessarily be the "exclusive" cause of organic evolution, he is likely to draw conclusions which another man would not draw who starts with the premiss that natural selection is but the "main" cause. Of these subordinate differences the most important are those which relate to the possible transmission of acquired characters, to the necessary (or only general) utility of specific characters, and to the problem touching the inter-sterility of allied species. But we may well hope that before another ten years shall have pa.s.sed, even these still outstanding questions will have been finally settled; and thus that within the limits of an ordinary lifetime the theory of organic evolution will have been founded and completed in all its parts, to stand for ever in the world of men as at once the greatest achievement in the history of science, and the most splendid monument of the nineteenth century.

In the later chapters of the foregoing treatise I have sought to indicate certain matters of general principle, which many years of study specially devoted to this great movement of contemporary thought have led me to regard as almost certainly sound in themselves, and no less certainly requisite as complements of the Darwinian theory. I will now conclude by briefly summarizing these matters of general principle in the form of twelve sequent propositions. And, in doing so, I may ask it to be noticed that the system which these propositions serve to express may now claim, at the least, to be a strictly logical system. For the fact that, not merely in its main outlines, but likewise in its details, it has been independently constructed by Mr. Gulick, proves at any rate this much; seeing that, where matters of such intricacy are concerned, nothing but accurate reasoning from a common foundation of _data_ could possibly have yielded so exact an agreement. The only difference between us is, that Mr. Gulick has gone into much further detail than I have ever attempted in the way of cla.s.sifying the many and varied forms of isolation; while I have laid more special stress upon the physiological form, and found in it what appears to me a satisfactory solution of "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"--namely, "the remarkable difference between varieties and species when crossed."

GENERAL CONCLUSIONS.

1. NATURAL SELECTION IS PRIMARILY A THEORY OF THE c.u.mULATIVE DEVELOPMENT OF ADAPTATIONS WHEREVER THESE OCCUR; AND THEREFORE IS ONLY INCIDENTALLY, OR LIKEWISE, A THEORY OF THE ORIGIN OF SPECIES IN CASES WHERE ALLIED SPECIES DIFFER FROM ONE ANOTHER IN RESPECT OF PECULIAR CHARACTERS, WHICH ARE ALSO ADAPTIVE CHARACTERS.

2. HENCE, IT DOES NOT FOLLOW FROM THE THEORY OF NATURAL SELECTION THAT ALL SPECIES--MUCH LESS ALL SPECIFIC CHARACTERS--MUST NECESSARILY HAVE OWED THEIR ORIGIN TO NATURAL SELECTION; SINCE IT CANNOT BE PROVED DEDUCTIVELY FROM THE THEORY THAT NO "MEANS OF MODIFICATION" OTHER THAN NATURAL SELECTION IS COMPETENT TO PRODUCE SUCH SLIGHT DEGREES OF MODIFICATION AS GO TO CONSt.i.tUTE DIAGNOSTIC DISTINCTIONS BETWEEN CLOSELY ALLIED SPECIES; WHILE, ON THE OTHER HAND, THERE IS AN OVERWHELMING Ma.s.s OF EVIDENCE TO PROVE THE ORIGIN OF "A LARGE PROPORTIONAL NUMBER OF SPECIFIC CHARACTERS" BY CAUSES OF MODIFICATION OTHER THAN NATURAL SELECTION.

3. THEREFORE, AND UPON THE WHOLE, AS DARWIN SO EMPHATICALLY HELD, "NATURAL SELECTION HAS BEEN THE MAIN, BUT NOT THE EXCLUSIVE MEANS OF MODIFICATION."

4. EVEN IF IT WERE TRUE THAT ALL SPECIES AND ALL SPECIFIC CHARACTERS MUST NECESSARILY OWE THEIR ORIGIN TO NATURAL SELECTION, IT WOULD STILL REMAIN ILLOGICAL TO DEFINE THE THEORY OF NATURAL SELECTION AS INDIFFERENTLY A THEORY OF SPECIES OR A THEORY OF ADAPTATIONS; FOR, EVEN UPON THIS ERRONEOUS SUPPOSITION, SPECIFIC CHARACTERS AND ADAPTIVE CHARACTERS WOULD REMAIN VERY FAR INDEED FROM BEING CONTERMINOUS--MOST OF THE MORE IMPORTANT ADAPTATIONS WHICH OCCUR IN ORGANIC NATURE BEING THE COMMON PROPERTY OF MANY SPECIES.

5. IN NO CASE CAN NATURAL SELECTION HAVE BEEN THE CAUSE OF MUTUAL INFERTILITY BETWEEN ALLIED, OR ANY OTHER, SPECIES--_I.E._ OF THE MOST GENERAL OF ALL "SPECIFIC CHARACTERS."

6. WITHOUT ISOLATION, OR THE PREVENTION OF FREE INTERCROSSING, ORGANIC EVOLUTION IS IN NO CASE POSSIBLE. THEREFORE, IT IS ISOLATION THAT _HAS_ BEEN "THE EXCLUSIVE MEANS OF MODIFICATION," OR, MORE CORRECTLY, THE UNIVERSAL CONDITION TO IT. THEREFORE, ALSO, HEREDITY AND VARIABILITY BEING GIVEN, THE WHOLE THEORY OF ORGANIC EVOLUTION BECOMES A THEORY OF THE CAUSES AND CONDITIONS WHICH LEAD TO ISOLATION.

7. ISOLATION MAY BE EITHER DISCRIMINATE OR INDISCRIMINATE. WHEN DISCRIMINATE, IT HAS REFERENCE TO RESEMBLANCES BETWEEN INDIVIDUALS CONSt.i.tUTING THE ISOLATED COLONY OR GROUP; WHEN INDISCRIMINATE, IT HAS NO SUCH REFERENCE. IN THE FORMER CASE THERE ARISES h.o.m.oGAMY, AND IN THE LATTER CASE THERE ARISES APOGAMY.

8. EXCEPT WHERE VERY LARGE POPULATIONS ARE CONCERNED, INDISCRIMINATE ISOLATION ALWAYS TENDS TO BECOME INCREASINGLY DISCRIMINATE; AND, IN THE MEASURE THAT IT DOES SO, APOGAMY Pa.s.sES INTO h.o.m.oGAMY, BY VIRTUE OF INDEPENDENT VARIABILITY.

9. NATURAL SELECTION IS ONE AMONG MANY OTHER FORMS OF DISCRIMINATE ISOLATION, AND PRESENTS IN THIS RELATION THE FOLLOWING PECULIARITIES:-- (_A_) THE ISOLATION IS WITH REFERENCE TO SUPERIORITY OF FITNESS; (_B_) IS EFFECTED BY DEATH OF THE EXCLUDED INDIVIDUALS; AND (_C_) UNLESS a.s.sISTED BY SOME OTHER FORM OF ISOLATION, CAN ONLY EFFECT MONOTYPIC AS DISTINGUISHED FROM POLYTYPIC EVOLUTION.

10. IT IS A GENERAL LAW OF ORGANIC EVOLUTION THAT THE NUMBER OF POSSIBLE DIRECTIONS IN WHICH DIVERGENCE MAY OCCUR CAN NEVER BE MORE THAN EQUAL TO THE NUMBER OF CASES OF EFFICIENT ISOLATION; BUT, EXCEPTING NATURAL SELECTION, ANY ONE FORM OF ISOLATION NEED NOT NECESSARILY REQUIRE THE CO-OPERATION OF ANOTHER FORM IN ORDER TO CREATE AN ADDITIONAL CASE OF ISOLATION, OR TO CAUSE POLYTYPIC AS DISTINGUISHED FROM MONOTYPIC EVOLUTION.

11. WHERE COMMON AREAS AND POLYTYPIC EVOLUTION ARE CONCERNED, THE MOST GENERAL AND MOST EFFICIENT FORM OF ISOLATION HAS BEEN THE PHYSIOLOGICAL, AND THIS WHETHER THE MUTUAL INFERTILITY HAS BEEN THE ANTECEDENT OR THE CONSEQUENT OF MORPHOLOGICAL CHANGES ON THE PART OF THE ORGANISMS CONCERNED, AND WHETHER OR NOT THESE CHANGES ARE OF AN ADAPTIVE CHARACTER.

12. THIS FORM OF ISOLATION--WHICH, IN REGARD TO INCIPIENT SPECIES, I HAVE CALLED PHYSIOLOGICAL SELECTION--MAY ACT EITHER ALONE OR IN CONJUNCTION WITH OTHER FORMS OF ISOLATION ON COMMON AREAS: IN THE FORMER CASE ITS AGENCY IS OF MOST IMPORTANCE AMONG PLANTS AND THE LOWER CLa.s.sES OF ANIMALS; IN THE LATTER CASE ITS IMPORTANCE CONSISTS IN ITS GREATLY INTENSIFYING THE SEGREGATIVE POWER OF WHATEVER OTHER FORM OF ISOLATION IT MAY BE WITH WHICH IT IS a.s.sOCIATED.