SPENCER 175 



Thus Spencer clearly grasps the fact that fertility must, 

 in the long run, vary inversely with the survival-capacity 

 of the species, and he argues from this that it must vary 

 inversely with the mass of the nervous system. This is 

 a much closer approximation to the truth, but he again 

 fails to analyse accurately the essential factors of the 

 problem. He again fails to distinguish between the 

 variation in the inherited potential degree of fertility as 

 between species and species and the variation in the 

 actual degree of fertility under the direct influence of 

 the environment. Let us consider the theory in its 

 relation to the first problem. 



It is true that cerebral development is the most powerful 

 factor making for survival, but it is not the only factor. 

 Organs of locomotion, weapons of offence and defence, 

 and many other factors play a part. It is no doubt 

 true that each of these factors implies a corresponding 

 development of nerves and nervous centres, but the 

 bulk of the nervous development will not necessarily 

 correspond with their life-preserving efficiency. The 

 skunk, for example, has the power of ejecting an intoler- 

 ably evil-smelling fluid over its enemies. So overpowering 

 is the stench, that the animal is usually left severely 

 alone. It is obvious that the necessary nervous develop- 

 ment would not correspond in bulk with the life-preserving 

 power of such a weapon. Moreover, the character of the 

 environment is of not less importance than the character 

 of the organism. Some organisms are much more favoured 

 by their environment and habits of life than others. 

 The brown rat is probably a more powerful and cunning 

 animal than the water rat. But it lives in an environ- 

 ment which leads to a heavy deathrate. The water 

 rat, living almost unmolested along the banks of streams, 

 can maintain its numbers with a very low rate of repro- 

 duction. The brown rat, living among the habitations 



