SEX-CHROMOSOMES AND INHERITANCE 197 



others into pancreas cells, etc. There is nothing inconsist- 

 ent in such a view with the theory that in other cases a 

 different mechanism produces different kinds of germ- 

 cells. Logically, this viewpoint is consistent, but I can 

 sympathize with efforts that are continually being made to 

 find an explanation that makes use of the same kind of 

 process in genetic segregation and in embryonic differen- 

 tiation. In fact, in 1902, while still under the influence of 

 the then recent advances in the field of experimental em- 

 bryology (developmental mechanics), I suggested that one 

 might attempt to treat the phenomenon of segregation 

 from the same theoretical standpoint {viz., the realization 

 of alternative states) as was then appealed to for embry- 

 onic differentiation. It soon became apparent to me, how- 

 ever, that (1) the two kinds of results depended upon 

 entirely different situations, and therefore need not have a 

 common explanation ; (2) that the genetic evidence showed 

 the improbability of explaining segregation and differ- 

 entiation in the same way; (3) that special tests that I 

 carried out failed to support the supposition of a common 

 explanation; (4) that while no detailed explanation is 

 possible at present for the general phenomena of specific 

 differentiation, yet for Mendelian segregation the reduc- 

 tion division supplies all that the results call for. 



Sex Ratios 



The theory of sex-determination has been deduced 

 from the evidence of equality of males and females as 

 well as from the cytological evidence. It remains to 

 explain why in some cases the machine fails to give 

 equality of the two sexes ; why, for example, all fertilized 

 eggs of phylloxerans and aphids, or daphnians, or roti- 

 fers, or bees, are female ; why certain mutant races of flies 

 give twice as many daughters as sons; why other races 

 of flies produce nearly all sons ; why the sex ratio in man 

 is about 106 males to 100 females. 



It is perhaps needless to point out that if, in a species 



