40 THE LAW OF BIRTHS AND DEATHS 



be fixed by the general principles of variation and adapta- 

 tion which govern evolution as a whole. Into the nature 

 of these principles it is no part of our purpose to enter 

 here. Suffice to say, that as the factors which make for 

 survival develop fertility must diminish. The chief of 

 these factors will be cerebral development, size, weapons 

 of defence and organs of locomotion. All of these will be 

 important, but, as a factor making for survival, cerebral 

 development overshadows all the rest. Man, who is 

 deficient in all the other factors, is nevertheless enabled 

 to increase rapidly in numbers with a rate of reproduction 

 lower than that of any animal save the elephant. The 

 paramount importance of this factor should be borne in 

 mind in considering the next part of our subject. 



We now come to the really important part of the 

 problem. The question which we have to answer is : 

 What law governs the variation of the degree of fertility 

 in response to the direct action of the environment ? 

 We have seen that it is a necessary condition of the 

 success of the evolutionary scheme that the variation of 

 the inherited potential degree of fertility between species 

 and species must bear an inverse proportion to their 

 capacity for survival. But the actual degree of fertility 

 is decided by the direct action of the environment, and if 

 this generalisation is to hold good for the larger problem, 

 it must also hold good for the net result of the variation 

 of the degree of fertility under the direct action of the 

 environment. And this may be effected in one of two 

 ways : the birthrate and deathrate may rise and fall 

 together ; or periods of relative barrenness and great 

 mortality may alternate with periods of great fertility, 

 as we see among the lower organisms. 



To produce the first result it is necessary that the same 

 combination of causes which results in a high deathrate 



