448 THE EVOLUTION OF LIFE. 



rents, or not so well, but some few of which fit the conditions 

 better; and doing so, are enabled the better to preserve them 

 selves, and to produce offspring similarly capable of preserv 

 ing themselves. Among animals the like process re 

 sults in the like development of various structures which 

 cannot have been affected by the performance of functions 

 their functions being purely passive. The thick shell of &amp;lt;i 

 niollusk, is inexplicable as a result of direct reactions of the 

 organism against the external actions to which it is exposed ; 

 but it is quite explicable as a result of the survival, genera 

 tion after generation, of individuals whose thicker coverings 

 protected them against enemies. Similarly with such a 

 dermal structure as that of the tortoise. Though we have evi 

 dence that the skin where it is continually exposed to pres 

 sure and friction may thicken, and so re-establish the equi 

 librium, by opposing a greater inner force to a greater outer 

 force ; yet we have no evidence that a coat of armour like 

 that of the tortoise can be so produced. Nor, indeed, are the 

 conditions under which only its production in such a man 

 ner could be accounted for, fulfilled ; since the surface of the 

 tortoise is not exposed to greater pressure and friction than 

 the surfaces of other creatures. This massive carapace, and 

 the strangely-adapted osseous frame-work which supports it, 

 are unaccountable as results of evolution, unless through the 

 process of natural selection. Thus, too, is it with the pro 

 duction of colours in birds and in insects ; the formation of 

 odoriferous glands in mammals ; the growth of such excres 

 cences as those of the camel. Thus, in short, is it with all 

 those organs of animals, which do not play active parts in the 

 compound rhythms of their functions. 



Besides giving us explanations of structural characters 

 that are otherwise unaccountable, Mr Darwin shows how 

 natural selection explains peculiar relations between indi 

 viduals in certain species. Such facts as the dimorphism of 

 the primrose and other flowers, he proves to be quite in har 

 mony with his hypothesis, though stumbling-blocks to aU 



