MANIFESTED IN THE ANIMAL KINGDOM 95 



rendered limp and exhausted by a hot, moist climate, 

 while another scarcely seems to notice it. We know that 

 the body is provided with a heat-regulating apparatus 

 which drives the blood into the interior of the body in 

 cold weather, and thus conserves heat ; while when the 

 weather is hot the capillaries on the surface of the body 

 expand, thus allowing the blood to come to the surface 

 and radiate heat. If in one individual or species the 

 regulating apparatus is very efficient and in another 

 relatively inefficient, there will obviously be a great 

 difference in the way they react to changes of climate, 

 and there may be a corresponding difference in the degree 

 of effect upon their fertility. And this will apply to other 

 factors beside climate. So these apparently capricious 

 results are in no way inconsistent with the general law, 

 and, in view of the tremendous complexity of the factors 

 at work, are to be expected. 



It may be pointed out here that we have only to assume 

 that human beings are subject to the same general biological 

 laws as animals and plants, and we have an adequate 

 explanation of the declining birthrate. For the main 

 effects of advancing civilisation and increasing prosperity 

 are cerebral development, with ever-increasing nervous 

 energy, high feeding, and diminished physical labour. 

 The human race is tending more and more to resemble 

 the highly bred race-horse, and to be subject to a similar 

 condition of feeding and training. So if sterility appears 

 in the race-horse in spite of the effects of careful selection, 

 what shall we expect in the human species when the 

 same tendencies and conditions are reproduced in a 

 highly accentuated form ? 



