Chap. IV.

NATUR AL SELECTION.

83

CHAPTER IY.

Natural Selection.

Natural Selection — its power compared with man’s selection — itspower on characters of trifling importance — its power at all agesand on both sexes — Sexual Selection — On the generality ofintercrosses between individuals of the same species — Circum-stances favourable and unfavourable to Natural Selection, namely,intercrossing, isolation, number of individuals — Slow action —Extinction caused by Natural Selection — Divergence of Cha-racter, related to the diversity of inhabitants of any small area,and to naturalisation — Action of Natural Selection, throughDivergence of Character and Extinction, on the descendants froma common parent — Explains the Grouping of all organic beings— Advance in organisation — Low forms preserved — Objectionsconsidered — Indefinite multiplication of species — Summary.

How will tlie struggle for existence, discussed too brieflyin the last chapter, act in regard to variation ? Canthe principle of selection, which we have seen is sopotent in the hands of man, apply in nature ? I thinkwe shall see that it can act most effectually. Let it heborne in mind in what an endless number of strangepeculiarities our domestic productions, and, in a lesserdegree, those under nature, vary; and how strong thehereditary tendency is. Under domestication, it may betruly said that the whole organisation becomes in somedegree plastic. But the variability, which we almostuniversally meet with in our domestic productions, isnot directly produced, as Hooker and Asa Gray havewell remarked, by man; he can neither originatevarieties, nor prevent their occurrence ; he can only pre-serve and accumulate such as do occur; unintentionally