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 classes of animal life, comparison entirely fails.
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 butterflies, belonging to three genera[56], which are identical in the polar regions and in the Alps, notwithstanding that the sparse 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 circumstances: the more stable specific type would require a longer time to change under the influence of apogamy alone. Now, the butterflies 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].