56 THE THEORY OF NATURAL SELECTION 



pointed out, there are insuperable difficulties in the 

 way of adopting the belief that such a power can have 

 been acquired through the action of natural selection. 

 Let us consider the power of regenerating a lost tail, 

 such as is shown by the common gecko, or wall-lizard, 

 of the tropics. To account for this power by natural 

 selection we should have to suppose, firstly, that every 

 stage in the growth of a partly regenerated tail, even 

 its first small rudiment, was useful to the animal ; and, 

 secondly, that there was so much competition between 

 lizards which had lost their tails, that those which could 

 regenerate them a little better would survive rather 

 than the others. The first of these suppositions as to 

 the utility of a partly regenerated tail is in the highest 

 degree improbable ; but against the second there is an 

 entirely fatal argument, since, if the lizards which 

 regenerated badly were exterminated owing to com- 

 petition with those which had better powers of re- 

 generation, much more would all the injured lizards 

 be exterminated in competition with those which had 

 escaped injury. 



The theory of sexual selection constitutes an im- 

 portant branch of the Darwinian account of the origin 

 of specific structures. We are here concerned with 

 this hypothesis only in so far as it leads to a criticism 

 of the efficacy of natural selection from another point 

 of view. By the theory of sexual selection Darwin at- 

 tempted to explain the origin of two sorts of characters 

 in particular, one or other of which frequently appears 

 in the male sex only of many of the higher animals. 



