DEVELOPMENT AND GENESIS. 445 



surplus nutriment consisting almost wholly of those nitro- 

 genous materials from which the bodies of young ones are 

 mainly formed. But, allowing for all other differences, it 

 appears not improbable that the smallness of human fertility 

 compared with the fertility of large feline animals, is due to 

 the greater complexity of the human organization — more 

 especially the organization of the nervous system. Taking 

 degree of nervous organization as the chief correlative of 

 mental capacity ; and remembering the physiological cost of 

 that discipline whereby high mental capacity is reached ; we 

 may suspect that nervous organization is very expensive : the 

 inference being that bringing it up to the level it reaches in 

 Man, whose digestive system, by no means large, has at the 

 same time to supply materials for general growth and daily 

 waste, involves a great retardation of maturity and sexual 

 genesis. 



