CHAPTER I. 

 THE FACTORS* 



§ 315. If organisms have been evolved, their respective 

 powers of multiplication must have been determined by 

 natural causes. Grant that the countless specialities of 

 structure and function in plants and animals, have arisen 

 from the actions and reactions between them and their 

 environments, continued from generation to generation ; and 

 it follows that from these actions and reactions have also 

 arisen those countless degrees of fertility which we see 

 among them. As in all other respects an adaptation of each 

 species to its conditions of existence is directly or indirectly 

 brought about ; so must there be directly or indirectly 

 brought about an adaptation of its reproductive activity to 

 its conditions of existence. 



We may expect to find, too, that permanent and temporary 

 diflPerences of fertility have the same general interpretation. 

 If the small variations of structure and function that arise 

 within the limits of each species, are due to actions like those 



* An outline of the doctrine set forth in the following chapters, was 

 originally' published in the Westminster Review for April, 1852, under the 

 title of, A Theory of Population deduced from the General Law of Animal 

 Fertility ; and was shortly afterwards republished with a prefatory note, to 

 the effect that it must be accepted as a sketch which 1 hoped at sonte future 

 time to elaborate. In novr revising and completing it, I have omitted a non- 

 essential part of the argument, while I have expanded the remainder by 

 adding to the number of facts put in evidence, by meeting objections which 

 want of space before obliged me to pass over, and by drawing various 

 secondary conclusions. 



