146 THE LAW OF BIRTHS AND DEATHS 



out the body of the organism, and there is no obvious 

 reason why the reproductive cells should be exempt 

 from its influence. The distribution of nutrition to the 

 various parts of the body appears to be governed by the 

 nervous system. Therefore the nervous system should 

 determine the nutritive condition of the ovum. 



There is considerable variation in the receptive capacity 

 of the egg cells of a given individual, as experiments by 

 Hertwig and others have shown. There would certainly 

 also be great variation in the average receptive capacity 

 of the egg cells of different individuals when exposed 

 to the same conditions. It is also a well established fact 

 that there is great variation in the penetrating powers 

 of the sperm cells as between different individuals, for 

 many stud animals are conspicuously good stock getters 

 as compared with others. Thus there is provision for 

 an enormous range of variation in individual cases. We 

 might have the case of a male whose sperm cells were 

 exceptionally penetrating matched with a female whose 

 egg cells were exceptionally receptive. Or we might 

 have the case of a male whose sperm cells were deficient 

 in penetrating power matched with a female whose egg 

 cells were exceptionally unreceptive. If these two 

 couples were exposed to the same conditions, one couple 

 would prove exceptionally fertile as compared with the 

 other. We can thus understand how it is that, while 

 the birthrate steadily declines with the increase of wealth 

 and education, yet individual rich families are sometimes 

 very fertile, and why poor people are sometimes childless 

 or have very few children. We can also understand why 

 some animals breed freely in captivity, whilst other 

 individuals of the same variety are sterile. But when 

 statistics are compiled for large classes the law of averages 

 levels up these individual variations and shows clearly 

 the operation of the principle. 



