98 NATUEAL SELECTION. [Chap. TV. 



our domestic productions, is not directly produced, as 

 Hooker and Asa Gray have well remarked, by man ; he 

 can neither originate varieties, nor prevent their occur- 

 rence ; he can only preserve and accumulate such as do 

 occur. Unintentionally he exposes organic beings to 

 new and changing conditions of life, and variability 

 ensues ; but similar changes of conditions might and 

 do occur under nature. Let it also be borne in mind 

 how infinitely complex and close-fitting are the mutual 

 relations of all organic beings to each other and to their 

 physical conditions of life ; and consequently what in- 

 finitely varied diversities of structure might be of use to 

 each being under changing conditions of life. Can it, 

 then, be thought improbable, seeing that variations useful 

 to man have undoubtedly occurred, that other variations 

 useful in some way to each being in the great and com- 

 plex battle of life, should occur in the course of many 

 successive generations ? If such do occur, can we doubt 

 (remembering that many more individuals are bom than 

 can possibly survive) that individuals having any ad- 

 vantage, however slight, over others, would have the 

 best chance of surviving and of procreating their kind ? 

 On the other hand, we may feel sure that any variation 

 in the least degree injurious would be rigidly destroyed. 

 This preservation of favourable individual differences 

 and variations, and the destruction of those which are 

 injurious, I have called Natural Selection, or the Survival 

 of the Fittest. Variations neither useful nor injurious 

 would not be affected by natural selection, and would 

 be left either a fluctuating element, as perhaps we see 

 in certain polymorphic species, or would ultimately 

 became fixed, owing to the nature of the organism and 

 the nature of the conditions. 



