338 THE EVOLUTION THEORY 



logical selection.' This view depends — it seems to me — upon an 

 underestimate of the power of natural selection. Romanes believed 

 that when a species began to split up, even the adaptive variations 

 would always disappear again because of the continual crossing, 

 and that only geographical isolation or sexual alienation, that is, 

 physiological selection, would be able to prevent this. But even 

 the fact that there are dimorphic and polymorphic species proves 

 sufficiently that adaptation to two or even several sets of conditions 

 can go on on the same area. In many ants we find many kinds 

 of individuals — the two sexual forms, the workers, and the soldiers, 

 and these last are undoubtedly distinguished by adaptive characters 

 which must be referred to selection. The same is true of the 

 caterpillars, whose coloration is adapted to their surroundings 

 in two different ways. If the individuals of one and the same 

 species can be broken up into two or more different forms and 

 combinations of adaptations, while they are mingling uninterruptedly 

 with one another, natural selection must undoubtedly be able, 

 notwithstanding the continual intermingling of divergent types, to 

 discriminate between them and to separate them sharply from one 

 another. Assuredly then a species can not only exhibit uniform 

 variation on a single area, but may also split up into two without 

 the aid of physiological selection.. Theoretically it is indisputable 

 that of two varieties which are both equally well suited to the 

 struggle for existence, a mixed form arising through crossing may 

 not be able to survive. Let us recall, for instance, the caterpillars, 

 of which some individuals are green and some brown, and let us 

 assume that the brown colour is as effective a protection as the 

 green, then the two forms would occur with equal frequency ; but 

 though a mixed hybrid form which was adapted neither to the green 

 leaves nor to the brown might occasionally crop up, it would always 

 be eliminated. It would occur because the butterflies themselves are 

 alike, whether they owe their origin to green caterpillars or to brown, 

 and thus at first, at least, all sexual combinations would be equally 

 probable. 



I do not believe therefore in a ' physiological selection,' in 

 Romanes's sense, as an indispensable preliminary condition to the 

 splitting of species, but it is a different question whether the mutual 

 sterility so frequently observable between species has not conversely 

 been produced by natural selection in order to facilitate the separation 

 of incipient species. For there can be no doubt that the process of 

 separating two new forms, or even of separating one new form from 

 an old one, would be rendered materially easier if sexual antipathy or 



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