THE DETERMINATION OF SEX 331 



which brought aljoiit abnormal parthenogenesis was usually so 

 severe as to cause early death. 



The Purpose of Sexes. The foregoing data show that the 

 different germ cells are fundamentalh^ similar in so far as the 

 fact of reproduction is concerned, and different only in accord 

 with the parts which they play in the details of its accomplishment. 

 The sexes which produce them cannot, therefore, have different 

 capacities with respect to the fundamental fact of reproduction, 

 but only with respect to the degree of specialization of the process 

 in the species to which they belong. 



This being the case, it is apparent that the differentiation of 

 the sexes should be commensurate with their roles in normal 

 reproduction. The similarit}' of sexes in the lower organisms and 

 the occurrence of hermaphroditic animals are therefore not sur- 

 prising. Nor is the extreme diiferentiation of sexes in the higher 

 animals and the concomitant necessity for sexual reproduction in 

 a great majority of species any more than a normal development 

 in harmony with the high degree of differentiation characteristic 

 of these organisms. 



Differentiation of Sexes. When a phenomenon is constant in 

 occurrence and apparently necessary to the existence of the species 

 some provision for its perpetuation is to be expected, for it is 

 quite in harmony with the fundamental principles of biology. 

 The chromosome theory of sex determination therefore deals with 

 facts which seem entirely normal. It is not entirely clear how 

 the presence of two chromosomes in the cells of an individual 

 can cause it to belong to one sex while the presence of either 

 alone would cause its development into the opposite sex, but the 

 association of chromosomes with sex is well established. 



The fact that the allosomes in one sex are the same as those 

 in the other indicates one important consideration. While we are 

 forced to the conclusion that allelomorphic characters are based 

 upon qualitatively different genes, there is nothing to indicate 

 that the sexes are in any way allelomorphic. The chromosomal 

 difference between them is apparently quantitative. Under ordi- 

 nary conditions it has a very definite effect upon the differentiation 

 of the sexes, but we must recognize that the same active con- 

 stituents are present in the allosomes of the two sexes. This 

 interpretation is favored by the occurrence of hermaphrodites and 

 abnormal individuals intermediate in sex. 



