174 THE LAW OF BIRTHS AND DEATHS 



he never does more than he can help and hard enough 

 to get him to do that without scamping it ; but an Irish- 

 man will work as if he'd die the moment he stopped." 



When the classes of evidence which have no bearing 

 on the variation in the degree of sexual fertility under 

 the direct influence of the environment are put aside, 

 and when the evidence which has been distorted by 

 special pleading has been readjusted to its proper per- 

 spective, the amount of evidence in favour of Spencer's 

 theory is microscopical. 



In an appendix to vol. i. of his Principles of Biology 

 Spencer gives what is really a much better attempt at 

 a scientific theory of animal fertility than the theory 

 criticised above. The essence of it is contained in the 

 following extract : 



" We found it to be the necessary law of the mainten- 

 ance of races that the ability to maintain individual 

 life and the ability to multiply vary inversely. But the 

 ability to maintain individual life is in all cases measured 

 by the development of the nervous system. If it be in a 

 good visceral organisation that the power of self-preser- 

 vation is shown, this implies some corresponding nervous 

 apparatus to secure sufficient food. If it be in strength, 

 there must be a provision of nerves and nervous centres 

 answering to the size of the muscles. If it be in swiftness 

 and agility, a proportionate development of the cerebellum 

 is presupposed. If it be in intelligence, this varies with 

 the size of the cerebrum. . . . Hence the nervous system 

 becomes the universal measure of the degree of co-ordina- 

 tion of actions, that is, of the life or the ability to maintain 

 life. And if the nervous system varies directly as the 

 ability to maintain life, it must vary inversely as the 

 ability to multiply." 1 



1 Principles of Biology, Spencer, vol i, Appendix A. 



