338 EVOLUTION, GENETICS, AND EUGENICS 



are more than a dozen mutations, in different loci, which reduce the 

 size of the wing in Drosophila so slightly as to leave it more than half 

 its original length, whereas only four reduce it to less than half-length. 

 Mutant genes with effects so slight as to be visible only by the aid of 

 specific co-genes seem to arise still more frequently. It is reasonable 

 to conclude that the mutations with slighter effects would more often 

 take part in evolution, because they should usually be less deleterious, 

 and this conclusion is born out by observations on the multiplicity 

 with which such factor-differences with relatively slight effects are 

 found in species crosses. 



14. The range of those mutations which are of appropriate magni- 

 tude to be visible is probably very small, in comparison with the entire 

 "spectrum" of mutations, so that there are many more lethals than 

 visible mutations, and probably more subliminal than visible. 



The above empirical and semi-empirical principles must be re- 

 garded as a mere preliminary scaffolding, for the erection of a later 

 more substantial, theory of mutation. Time does not permit me here 

 to discuss which directions of research, and what methods, seem the 

 most promising for future results. Suffice it to say that it is especially 

 important to obtain accurate data concerning the effect of various 

 conditions upon the rate of mutation. This seems one of the logical 

 routes by which to work towards the artificial production of mutation 

 and consequent more perfect control of evolution. At the same time 

 such results should also give a further insight into the structure of the 

 gene. The way is now open, for the first time, to such studies on 

 mutation rate, first through the finding, by Emerson, Baur, Maryatt, 

 Zeleny, and Blakeslee, of a number of specially mutable factors in 

 different organisms, and second, through certain special genetic meth- 

 ods which I have elaborated in Drosophila, for the detection of lethal 

 and other mutations there. 



It has now become recognized that advances in theoretical or 

 "pure" science eventually carry in their train changes in practice of the 

 most far reaching nature — changes which are usually far more radical 

 than those caused by progress in the applied science directly concerned. 

 It may therefore be asked at this point by eugenists: "Are there any 

 applications of the knowledge which has already been gained about 

 mutation in general, to eugenics and to the principles which should 

 govern us in guiding human reproduction?" I think that one such 

 application is already clearly indicated. 



In order to understand the nature of this application it will be nee- 



