534 THE EVOLUTION OF LIFE. 



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 a mollusk 

 cannot have arisen from direct reactions of the organism 

 against the external actions to which it is exposed; but it is 

 quite explicable as an effect of the survival, generation after 

 generation, of individuals whose thicker coverings protected 

 them against enemies. Similarly with such dermal struc- 

 ture as that of the tortoise. Though we have evidence that 

 the skin, where it is continually exposed to pressure and fric- 

 tion, may thicken, and so re-establish the equilibrium 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. ISTor, indeed, are the conditions 

 under which alone its production in such a manner 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 inexplic- 

 able as results of evolution, unless through the process of 

 natural selection. So, too, is it with the formation of odori- 

 ferous glands in some mammals, or the growth of such ex- 

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

 all those organs of animals which do not play active parts. 



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 in harmony 

 with his hypothesis, though stumbling-blocks to all other 

 hypotheses. The various differences which accompany differ- 

 ence of sex, sometimes slight, sometimes very great, are 

 similarly accounted for. As before suggested (§ 79), natural 

 selection appears capable of producing and maintaining the 

 right proportion of the sexes in each species ; and it requires 

 but to contemplate the bearings of the argument, to see that 



