The Evidence from Sexual Characters. 229 



fence. The welfare of the race does not depend upon 

 the number of young which are born, but upon the num- 

 ber which grow up; and if we take two cases, one vari- 

 ety in which the male has special weapons which enable 

 him to drive away his rivals and thus to produce a great 

 number of children, and another variety in which the 

 female has special weapons which enable her to protect 

 her young from enemies, and thus rear them all in safety, 

 it certainly seems as if the modification would be most 

 sure of perpetuation in the second case, and that the 

 second variety should, in time, exterminate the first. 



As a, matter of fact we do find that the weapons of 

 mammals exist in many cases in the female, but they are 

 most developed and most modified in the male, and it is 

 hard to understand why variations of this kind should 

 not more frequently arise and become hereditary in the 

 female, unless something besides sexual selection deter- 

 mines that males should be more plastic than females. 



The modification of the female is certainly quite pos- 

 sible, for there are numbers of cases in all groups of the 

 animal kingdom where the females alone have some pe- 

 culiar characteristic which is not directly concerned in 

 reproduction. 



Thus Darwin says (Variation, Vol. I. p. 333): "The 

 tarsi of the front legs are dilated in many male beetles or 

 are furnished with broad cushions of hairs; and in many 

 genera of water-beetles they are armed with a round flat 

 anchor, so that the male may adhere to the slippery body 

 of the female. It is a much more unusual circumstance 

 that the females of some water beetles (Dytiscus) have 

 their elytra deeply grooved, and in Acilius sulcatus thick- 

 ly set with hairs, as an aid to the male." 



We have seen that the males of many species of crus- 

 tacea have various parts of their bodies especially modi- 



