102 THE PROBLEMS OF EVOLUTION 



white facets. At first thought this evidence seems 

 conclusive, but more careful consideration shakes 

 one's confidence. We know of localization factors 

 in the inheritance of colors in mammals. May they 

 not also occur in arthropods .^^ Spencer ^^ has re- 

 corded such a possibility in connection with this 

 very condition of spotted eyes in a sequence of 

 generations of Drosophila, Obviously there is 

 more to be said on the subject for which we have 

 no sound evidence at present. 



One condition would decide this matter finally. 

 If the cells in a developing body which express a 

 given hereditary character could be deprived of 

 the genes for that character, then we might see 

 whether the similar genes in other cells could bring 

 it to expression. It seems impossible to obtain 

 such a condition. The whole matter is closely 

 linked with cytoplasmic differentiation during 

 embryonic development, which presents such a 

 marked contrast to the uniform distribution of the 

 chromosomes. We know relatively little about the 

 relationship existing here. 



If we approach the problem from the point of 

 view that the cell is the unit of living matter, 

 then its actual development would seem to be 

 determined from within. Its susceptibility to in- 

 fluences outside of itself would seem to be entirely 

 in the nature of a response to environmental con- 



» Am. Nat., Vol. LX, pp. 282-285, 1926. 



