72 HEREDITY AND SEX 



We lack here the certain evidence from cytology that 

 we have in the case of the insects. Indeed, there is 

 some cytological evidence to show that the male bird 

 is heterozygous for the sex chromosome. But the 

 evidence does not seem to me well established ; while 

 the experimental evidence is definite and has been 

 independently obtained by Bateson, Pearl, Sturtevant, 

 Davenport, Goodale and myself. However this may be, 

 the results show very clearly that here also sex is con- 

 nected with an internal mechanism that is concerned 

 with other characters also. This is the mechanism of 

 Mendelian heredity. Whether the chromosomes suffice 

 or do not suffice to explain Mendelian heredity, the 

 fact remains that sex follows the same route taken by 

 characters that are recognized as Mendelian. 



To sum up : The facts that we have considered 

 furnish, I believe, demonstrative evidence in favor 

 of the view that sex is regulated by an internal mech- 

 anism. The mechanism appears, moreover, to be the 

 same mechanism that regulates the distribution of cer- 

 tain characters that follow Mendel's law of inheritance. 



