SUMMARY OF THE ESSAY. 69 



II. Slight variations of all parts of the organism occur in 

 nature, and if a being could select with reference to the whole 

 structure, what changes might he not effect in the almost 

 unlimited time of which geology assures us. 



III. Animals increase so fast that, but for extermination, 

 the earth would not hold the progeny of even the slowest 

 breeding animal. Only a few r in each generation can live ; hence 

 the struggle for life, which has never yet been sufficiently 

 appreciated. " What a trifling difference must often determine 

 which shall survive and which perish ! " Thus is supplied the 

 " unerring power " of " Natural Selection . . . which selects 

 exclusively for the good of each organic being." 



IV. If a country were changing the altered conditions 

 would tend to cause variation, " not but what I believe most 

 beings vary at all times enough for selection to act on." 

 Extermination would expose the remainder to "the mutual 

 action of a different set of inhabitants, which I believe to be 

 more important to the life of each being than mere climate." 

 In the infinite complexity of the struggle for life " I cannot 

 doubt that during millions of generations individuals of a 

 species will be born with some slight variation profitable to 

 some part of its economy ; such will have a better chance of 

 surviving and propagating this variation, which again will 

 be slowly increased by the accumulative action of natural 

 selection ; and the variety thus formed will either coexist with, 

 or more commonly will exterminate its parent form." Thus 

 complex adaptations like those of woodpecker or mistletoe may 

 be produced. 



V. Numerous difficulties can be answered satisfactorily 

 in time. The supposed changes are only very gradual, and very 

 slow, " only a few undergoing change at any one time." The 

 imperfection of the geological record accounts for deficient 

 direct evidence of change. 



VI. Divergence during evolution will be an advantage. 

 "The same spot will support more life if occupied by very 

 diverse forms." Hence during the increase of species into its 

 offspring varieties, or sub-species, or true species, the latter 

 " will try (only few will succeed) to seize on as many and as 

 diverse places in the economy of nature as possible," and so 



