CHAPTER II 



THE PRINCIPLE DEDUCED 



IN considering the problem of animal fertility the subject 

 may be divided into two parts : 



(1) The variation of the inherited potential degree of 

 fertility as between species and species ; and 



(2) The variation of the actual degree of fertility of a 

 given species under the direct influence of the environment. 



The second of these is the really important question. 

 The first is only important from our present point of 

 view from the fact that its solution may lead to the 

 solution of the second. If we can obtain a generalisation 

 accurately defining the facts of the first problem, we 

 may be able to deduce therefrom the solution of the 

 second. 



The theory of evolution assumes the development of 

 the higher forms of life, through an ascending grade of 

 types, from the most primitive life-cells. A rate of 

 reproduction suitable for the lower types will not be 

 suitable for the higher. As the most primitive life-cells 

 have a rate of reproduction which makes an individual 

 the ancestor of billions in the course of a few days, it 

 follows that from these up to man and the still slower- 

 breeding elephant the rate of reproduction must be 

 graduated down according to definite principles. Our task 

 is to express these principles in a single generalisation. 



The necessary condition of the continued existence of 

 any species is that its reproductive capacity should be 



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