NATURAL SELECTION. 88 



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 Diver- 

 gence of Character and Extinction, on the Descendants from a Com- 

 mon Parent, explains the Grouping of all Organic Beings — Advance 

 in Organization — Low Forms preserved — Convergence 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 efficiently. 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 domestication, 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 productions, is not directly pro- 

 duced, 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 infinitely varied diversities of struc- 

 ture might be of use to each being under changing condi* 



