502 EVOLUTION, GENETICS, AND EUGENICS 



processes that are essential for the effectiveness of inbreeding and selec- 

 tion were prevented: in these lines there was found an accumulation 

 of lethal genes so rapid that it would have taken but a few decades to 

 have brought about the presence of a lethal gene in practically every 

 chromosome of every fly. Although the same general thesis un- 

 doubtedly applies also to mankind we do not yet know the speed of the 

 process here. Its speed depends upon the actual frequency of muta- 

 tions, which it will be very unportant — and extremely difficult — to de- 

 termine in the case of mankind. Meanwhile, no matter what this rate 

 may be, the process remams a real one, which must eventually be 

 reckoned with, and either grappled in tune, and conquered, or else 

 yielded to. 



I have dwelt at length upon this particular appUcation to eugenics, 

 of some of the mutation studies. I beUeve, however, that this is but 

 one example of such appHcations, and that from an increasing knowl- 

 edge of our theoretical science there will inevitably flow an increasingly 

 adequate technique for coping with our refractory human material. 

 Meanwhile, the crying need is for more of the theoretical knowledge — 

 and for the support of pure science, in its investigation of the processes 

 lying at the root of the germ plasm. 



THE CAUSES OF MUTATIONS 



In attempting to determine the causes of the appearance of new 



hereditary characters, we must first of all learn which of the categories 

 of variation we are dealing with. If we find that a so-called mutant 

 has a different number or arrangement of chromosomes, we may say 

 that this change in the chromosomes is the cause of the somatic differ- 

 ences seen in the mutant, for it would be strange if a relatively large 

 change in the hereditary material did not affect the somatic expression 

 of specific characters. As has already been suggested by Gates, the 

 cause of chromosomal aberrations may be environmental, as for ex- 

 ample sudden lowering of temperature durmg critical periods of the 

 germ-cell cycle. We have, however, no controlled experiments that 

 prove this to be the case. Again, there is a tendency to account for 

 non-disjunction and other types of chromosomal aberration through 

 purely internal causes, such as weakness of attraction between homolo- 

 gous chromosomes resulting in a failure of synapsis. In general it may 

 be said that, apart from being able to note a definite correlation 

 between a changed somatic condition and a changed chromosomal 



