SOME POSSIBLE CRITICISMS ANTICIPATED 157 



manner. Highly favourable conditions lead to sterility. 

 We should have expected that the decline of vitality due 

 to advancing age would have overcome the effects of 

 favourable conditions, and that the later periods of life 

 would have been most fertile. But again this is not the case. 

 Perhaps there is a genuine analogy between senile decay 

 among the higher organisms and the phenomena which 

 have received the same name among unicellular organisms. 

 Senile decay, then, and the exhaustion which results 

 from unfavourable conditions, may be looked upon as 

 representing two distinct principles. While the latter 

 favours conjugation, the former is inimical. Therefore, 

 in the multicellular organism any tendency to increased 

 fertility due to the exhaustion of the nervous charge as 

 the result of advancing age would be counteracted by the 

 opposing principle of senile decay. The exhaustion in 

 this case would, in fact, be directly due to senile decay, 

 and so would be more than counteracted by it. 



Spencer, in his Principles of Biology, directs some 

 criticisms against the theory of Doubleday, that a plethoric 

 condition of the organism is unfavourable to fertility, 

 and that a deplethoric condition is favourable. This is 

 a very different theory to the one outlined in these pages, 

 in which food is only one factor and not necessarily the 

 most important. Indeed, we have seen good reason to 

 believe that cerebral development and mental activity 

 are far more important than the supply of food. But 

 as similar criticisms may be directed against this theory, 

 it may be well to anticipate them. 



Spencer says : "It may be readily shown that such 

 an arrangement would be the reverse of self-adjusting. 

 Suppose a species too numerous for its food to be in the 

 resulting deplethoric state. It will, according to Mr. 



