that have been accumulated in wild species are of significance in the maintenance of the species, or at least we are led to infer that even though the visible character that we attend to may not itself be important, one at least of the other effects of the factors that represent these characters is significant. It is, of course, hardly to be expected that any random change in as complex a mechanism as an insect would improve the mechanism, and as a matter of fact it is doubtful whether any of the mutant types so far discovered are better adapted to those conditions to which a fly of this structure and habits is already adjusted. But this is beside the mark, for modern genetics shows very positively that adaptive characters are inherited in exactly the same way as are those that are not adaptive; and I have already pointed out that we cannot study a single mutant factor without at the same time studying one of the factors responsible for normal characters, for the two together constitute the Mendelian pair.
And, finally, I want to urge on your attention a question that we are to consider in more detail in the last lecture. Evolution of wild
