GENETICS 93 



thought to a surprising degree, and makes it nec- 

 essary to consider the possibihty of a reciprocal 

 effect of the body upon the germ cells in connection 

 with any analysis of evolutionary processes. Here 

 the parallel with cytoplasm and nucleus is of in- 

 terest, for effects of cytoplasm on chromosomes 

 must be similar to that of body on germ cells, or 

 rather germinal tissue. In both cases a modifica- 

 tion of fundamental hereditary units is brought 

 into question, and in both the internal environ- 

 ment is the source of stimulus. There is still a 

 difference to be considered, for a somatic condition 

 reacts directly only on the cytoplasm of the germ 

 cells; another barrier intervenes before the closely 

 safeguarded chromosomes are reached. 



Evidence on this problem is, unfortunately, of a 

 highly unsatisfactory kind. We know that chromo- 

 somes — genes — change. The abundant occurrence 

 of mutations in organisms used for experiment is 

 evidence of that. We know too that the change 

 may be associated with conditions of external 

 environment now that mutations are so readily 

 produced by various kinds of irradiation. But 

 when we consider the expression of characters in 

 the organism in response to Varying conditions of 

 environment we encounter conflicting opinions 

 which interfere seriously with our conclusions. 



Morgan ^^ has studied such a character in 



" Am. Nat, Vol. XLIX, pp. 285-429, 1915. 



