CHAP. IV.] NATURAL, SELECTION. 57 



should we know what to do. This ought to convince us of our 

 ignorance on the mutual relations of all organic beings ; a con- 

 viction as necessary, as it is difficult to acquire. All that we 

 can do, is to keep steadily in mind that each organic being is 

 striving to increase in a geometrical ratio ; that each at some 

 period of its life, during some season of the year, during 

 generation or at intervals, has to struggle for life and to suffer 

 great destruction. When we reflect on this struggle, we may con 

 sole ourselves witlr"the full belief, that the war of nature is not 

 incessant, that no fear is felt, that death is generally _prompt, 

 and thar the vigorous, the healthy, and the happy survive and 



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 intercrosses between indi- 

 viduals of the same species Circumstances favourable and unfavourable 

 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 naturalisation 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 organisation 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 principle of selection, 

 which we have seen is so potent in the hands of man, apply under 

 nature 1 ? 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 be truly said that the whole organisation 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 

 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 



