In considering any "supplementary theory" of the origin of species, it is obviously absurd to disregard the realm of organic nature as a whole, and to fasten attention exclusively upon the part of it where a particular difficulty against the theory may be supposed to lie. As will presently be shown, Mr. Wallace is entirely mistaken in supposing that his particular difficulty does lie against the theory in any part of organic nature; but, even if this could not have been shown, it would not have followed that the theory of physiological selection is inapplicable to _all_ the classes of the animal and vegetable kingdoms, because it is taken to be inapplicable to _some_. One might just as well argue against Mr. Darwin's theory of sexual selection on the ground that it cannot be held to apply to the coloration and the sculpture of shells. If either sexual selection or physiological selection were put forward as an _exclusive_ theory of the origin of _all_ species, this kind of argument would, of course, have been valid; but as the matter actually stands, it is largely irrelevant.
I say _largely_ irrelevant, because I do not dispute that there is this much force in it. If the theory of physiological selection can be proved inapplicable to Birds and Mammals (which are the only classes that Mr. Wallace considers in connection with it), its applicability to all other divisions, both of the animal and vegetable kingdoms, would be rendered doubtful; seeing that the process of species-formation appears to have been _everywhere_ more or less associated with the occurrence of "selective sterility"; and hence, if in any division of organic nature it could be shown that selective sterility cannot possibly have been a cause of specific differentiation, we might well doubt whether it has been such a cause elsewhere—just as we may doubt whether sexual selection has been a cause of the brilliant colors of birds and butterflies, because we know, that it cannot have been a cause of the equally brilliant colors of corals and flowers. But, as far as physiological selection is concerned, no such question can arise, as I will presently proceed to show.
First of all, however, it is desirable briefly to indicate the strength of this theory in the parts of organic nature where Mr. Wallace's sole criticism cannot possibly be held to apply—viz., the larger part of the vegetable kingdom, where ovules are fertilised either by insects or by the wind. Here the phenomena of "prepotency" are highly suggestive—not to say, in my opinion, virtually demonstrative—of physiological selection; seeing that, as Mr. Darwin remarks in another connexion: