The Evidence from Intellectual Differences. 253 



transmission of the established hereditary features of the 

 species, we should expect the female to gradually ac- 

 quire a tendency to develop these general characteristics 

 more perfectly than the male. The male organism would 

 thus gradually become the variable organism, as well as 

 the transmitter of variations, and the female organism 

 would become the conservative organism, as well as the 

 originator of the conservative element in reproduction. 



The study of the higher forms of life shows that this 

 specialization has actually taken place in many cases, and 

 that, in nearly all cases in which the sexes differ in pe- 

 culiarities not actually concerned in reproduction, the 

 male has varied more than the female. The amount of 

 variation which any organism has lately undergone may 

 be learned in two ways by a comparison of allied spe- 

 cies, and by a comparison of the adult with the young. 

 In a genus which comprises several species the charac- 

 teristics which these species have in common are due to 

 heredity from a common ancestor, and are therefore older 

 than features which are confined to any one species. 

 Now, it is a well-known ornithological law that the fe- 

 males of allied species of birds are very much more alike 

 than the males, and that in some cases where the females 

 can hardly be distinguished the males are very conspic- 

 uously different so much so that there is not the least 

 danger of confounding them. Countless examples will 

 present themselves to any one who is at all familiar with 

 birds, and those who are not can at once find ample 

 proof by glancing through any illustrated work on orni- 

 thology Gould's " Humming-Birds," for example. 



The greater variability of the male is also shown by a 

 comparison of the adult male and female with the im- 

 mature birds of both sexes. Since the growing animal 

 tends to recapitulate, during its own development, the 



