372 EVOLUTION OF PHYSICAL CHAKACTEBS 



themselves as they have elsewhere in the Continent. In Mexico 

 Indian blood largely predominates over European, and the 

 explanation must be that Europeans are not so well physically 

 adapted to the climate as are the original inhabitants. In this 

 there is nothing surprising. We can understand how certain 

 types of respiratory, excretory, and circulatory organs might be 

 better adapted than others to certain conditions of temperature, 

 moisture, actinic rays, and so on, though we may be ignorant as to 

 what types of circulatory and other organs are best fitted to any 

 particular climatic zone. It would be very remarkable if it were 

 not so, if, that is to say, any type of constitution was equally 

 well fitted to any kind of climate. In this manner to differences 

 in habits and in climate we may attribute the origin of the 

 varieties of mankind. 



Nevertheless, differences between races are far from being 

 wholly due to selection on these grounds. Sexual selection is 

 of importance. There grows up within each race a more or less 

 clearly defined type of physical beauty, and differential fertility 

 working through polygamy would evidently favour such types. 

 That differences in colour to some extent represent adaptations 

 to climate is clear ; in part such differences are probably to be 

 explained as due to varying ideals of beauty, but there are probably 

 yet other factors entering into the evolution of racial types. 



Kecently Professor Keith has made an interesting contribution 

 to the problem of racial differences. Eeference has already been 

 made to the duct less_glands . It is now known that variations in 

 the functioning of these glands have a profound influence upon 

 the bodily organs. Keith has remarked that, if the peculiarities 

 characteristic of the chief racial types of man are considered 

 together, it appears that they are attributable to different degrees 

 of development of these glands in different races. It should be 

 observed that this suggestion does not involve the conclusion that 

 racial differences are of the nature of modifications. It is merely 

 supposed that the innate development of these glands is different 

 in different races that there is a predisposition in one race to the 

 peculiar development of one gland and in another race to the 

 peculiar development of another gland. The value of the sugges- 

 tion lies in the fact that it shows how many racial peculiarities, 

 for which there are at least no obvious explanations, may be 

 merely the accompaniments of a difference in the development 



