84 NATUEAL SELECTION. Ciiap. IV. 



• CHAPTER IV. 



NATURAL SELECTION, OK THE SURVIVAL OF THE FITTEST. 



Natural Selection— its Power compared with jMan's Selection— its Power on Char- 

 acters of trifling Importance— its Power at all A<;e8 and on both Soxes— Sexual 

 Selection— On the CJenenility of Intercrosses between Individuals of the same 

 Species — Circumstances favorable and unfavorable to the Results of Natural Se- 

 lection, namely. Intercrossing:, Isolation, Nnnibor 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 Nat- 

 ural Selection, through Divergence of Character and Extinction, on the Descend- 

 ants from a C(<mmou Parent— Explains the Grouping of all Organic Heings— Ad- 

 vance in Organization — Low Forms preserved — Objections considered— Uniform- 

 ity of certain Characters due to their Unimportance and to their not having been 

 acted on by Natural Selection— Indefinite Multiplication of Species — Summary. 



How will the struggle for existence, briefly discussed in 

 tlie last chapter, act in regard to variation ? Can the principle 

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

 man, apply in Nature ? I think we shall see that it can act 

 most effectually. Let the endless number of peculiar varia- 

 tions in our domestic productions, and, in a lesser degree, in 

 those under Nature, be borne in ^ind ; as well as the strength 

 of the hereditary tendency. Under domestication, it may be 

 tridy said that the Avliole organization becomes in some degree 

 plastic. But the variability, which we almost universally meet 

 with in 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 occurrence ; he 

 can onlv preserve and acciunulate such as do occur; uninten- 

 tionally he exjioses organic beings to new and changing con- 

 ditions of life, and variability ensues; but similar changes of 

 conditions might and do occur under Nature. Let it als«> be 

 borne in mind how inlinitely complex and close-fitting are the 

 mutual relations of all organic beings to each other and to 

 tlieir physical conditions of life ; and consequently what in- 

 finitely-varied diversities of structure may be of use to each 

 l)cing under changing conditions of life. Can it, then, be 

 thought improbable, seeing that variations usefid to man have 

 undoubtedly occurred, that other variations useful in some way 



