176 THE LAW OF BIRTHS AND DEATHS 



of man and regarded as a nuisance, is fiercely persecuted. 

 It consequently needs an enormous rate of reproduction 

 to maintain the species. 



The difference in the degree of fertility between the 

 wild pigeon and the pheasant or partridge cannot be 

 said to bear an inverse proportion to the difference in 

 the bulk of their nervous systems. Yet it represents 

 fairly accurately the difference in the survival-capacity 

 of the species owing to the greater flying powers of the 

 pigeon. Therefore, the mass of the nervous system is 

 not a reliable measure of the survival-capacity of a species, 

 but the rate of reproduction must, and does, vary in- 

 versely with the latter. Again, it may be pointed out 

 that the inherited potential degree of fertility will be 

 decided by the laws of variation and adaptation which 

 govern the rest of organic evolution. 



The mass of the nervous system cannot possibly act 

 as a measure of the variations of either the survival- 

 capacity or the reproductive capacity from day to day. 

 After the reproductive period has been reached, the ratio 

 between the nervous system and the rest of the organism 

 will remain fairly constant throughout life. The variations 

 in the survival-capacity of organisms can only be measured 

 by a factor which varies also. Had Spencer analysed 

 the factors of the problem a little more closely, he might 

 have stumbled on the hypothesis set forth in this work 

 and found such a varying factor within the nervous system 

 in the form of nervous energy. 



