48 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



of development of the egg. As these chromosomes 

 throughout the entire life history of an individual 

 seem to be the lineal descendants of those present 

 in the original fertilized egg, we have only to assume 

 that in heredity some chromosomes or some combi- 

 nations of chromosomal parts stand for one set of 

 characters, while other chromosomes or parts of 

 chromosomes stand for different ones. This assump- 

 tion, which appears entirely reasonable, gives us the 

 simplest and clearest possible physical basis for the 

 explanation of the Mendelian inheritance. It there- 

 fore appears that the complicated facts of heredity are 

 growing more intelligible in the light of a mechanistic 

 hypothesis. Is there, indeed, any other explanation 

 which can compete with this one in probability ? 



Let us consider now the bearing of the mechanistic 

 hypothesis on the extremely interesting but baffling 

 problem of the determination of sex. Does an 

 animal become a male and not a female, or a female 

 and not a male, because of some obscure accident 

 or through the operation of some definite mechanism ? 

 The researches of Professor Wilson and other 

 biologists show in an unmistakable manner that a 

 quite definite relation exists in some of the lower 

 animals between sex and certain chromosomal 

 peculiarities. The most significant evidence yet ob- 

 tained can be stated in a few words. The funda- 

 mental fact is that in some insects there are two 

 kinds of germ cells, which produce males and females 

 respectively. In a very few cases this peculiarity 



