HATUBAL SELEOTION, 73 



CHAPTER IV. 



NATURAL SELECTION"; OR THE SURVIVAL OF THE FITTEST. 



Natural Selection — its power compared with man's selection — its 

 power on characters of trifling importance — its power at all ages 

 and on both sexes — Sexual Selection — On the generality of inter- 

 crosses between individuals of the same species — Circumstances 

 favorable and unfavorable to the results of Natural Selection, 

 namely, intercrossing, isolation, number of individuals — Slow 

 action — Extinction caused by Natural Selection — Divergence of 

 Character, related to the diversity of inhabitants of any small 

 area and to naturalization — Action of Natural Selection, through 

 Divergence of Character and Extinction, on the descendants 

 from a common parent — Explains the grouping of all organic 

 beings — Advance in organization — Low fonns preserved — Con- 

 vergence of character — Indefinite multiplication of species — 

 Summary. 



How will the struggle for existence, briefly discussed in 

 the last chapter, act in regard to variation? Can the prin- 

 ciple of selection, which we have seen is so potent in the 

 hands of man, apply under nature? I think we shall 

 see that it can act most efi&ciently. Let the endless 

 number of slight variations and individual differences 

 occurring in our domestic productions, and, in a lesser 

 degree, in those under nature, be borne in mind; as well as 

 the strength of the hereditary tendency. Under domesti- 

 cation, it may truly be said that the whole organization 

 becomes in some degree plastic. But the variability, which 

 we almost universally meet with in our domestic produc- 

 tions is not directly produced, as Hooker and Asa Gray 

 have well remarked, by man; he can neither originate 

 varieties nor prevent their occurrence; he can only pre- 

 serve and accumulate such as do occur. Unintentionally 

 he exposes organic beings to new and changiug conditions 

 of life, and variability ensues; but similar changes of con- 

 ditions might and do occur under nature. Let it also be 



