THE MUTATION THEORY 505 



of the stars, to the use of the test tube and the balance by the chemist, 

 and of the galvanometer by the physicist. All these are unnatural 

 instruments used to torture Nature's secrets from her. I venture to 

 think that the real antithesis is not between unnatural and natural 

 treatment of Nature, but rather between controlled or verifiable data 

 on the one hand, and unrestrained generalization on the other. 



If a systematist were asked whether these new races of Drosophila 

 are comparable to wild species, he would not hesitate for a moment. 

 He would call them all one species. If he were asked why, he would 

 say, I think, "These races differ only in one or two striking points, while 

 in a hundred other respects they are identical even to the minutest 

 details. " He would add, that as large a group of wild flies would show 

 on the whole the reverse relations, viz., they would differ in nearly 

 every detail and be identical in only a few points. In all this I en- 

 tirely agree with the systematist, for I do not think such a group of 

 types differing by one character each, is comparable to most wild 

 groups of species because the difference between wild species is due to 

 a large number of such single differences. The characters that have 

 been accumulated in wUd species are of significance in the maintenance 

 of the species, or at least we are led to infer that even though the visible 

 character 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 habitat 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 con- 

 stitute the Mendelian pair. 



And, finally, I want to urge upon your attention another question. 

 Evolution of wild species appears to have taken place by modifying 

 and improving bit by bit the structures and habits that the animal or 

 plant already possessed. We have seen that there are thirty mutant 

 factors 3 1 least that have an influence on eye color, and it is probable 

 that there are at least as many normal factors that are involved in the 

 production of the red eye of the wild fly. 



