1 70 PROBLEMS OF EVOLUTION 



males would drive off the shorter ones, who, except that a 

 meagre diet had reduced their strength, might be their equals 

 or superiors. The fact that the males have a superiority in 

 height supports the view that Sexual Selection has co-operated 

 with Natural Selection. Superiority of strength and stature is 

 not throughout the animal world the privilege of the same sex. 

 Among hawks the hens are much larger than the cocks, the 

 female spider in many species is a giant compared with her 

 partner. What superiority in height the male giraffe has, we 

 must attribute to Sexual Selection. The fact that the superiority 

 is no greater is what we have to account for. Now, it is known 

 that when a particular character is developed by Sexual Selection, 

 it is often transmitted in a modified form to the other sex, unless 

 it involves a danger. Dr Russel Wallace has shown that the 

 brilliant plumage of the cock-bird is often shared by the hen, 

 if, owing to the nesting habits of the species, it does not bring 

 her into much danger. In the case of the giraffe, the masculine 

 character, length of neck and fore-legs, so far from being a 

 danger is an advantage, and therefore we should expect an 

 approach to equality in the sexes. 



If Sexual Selection has worked in the way I have suggested, 

 we need not assume so many recurring famines, since at each 

 recurrence it was necessary for a male, if he was not to be 

 eliminated (discomfited, that is, and driven from the herd), not 

 only to be just tall enough to keep life in him by reaching the 

 lowest food available which is all that Natural Selection pure 

 and simple would insist upon but tall enough to get food in 

 plenty and so hold his own against tall and well fed rivals. 

 Migratory One more example to show how Sexual often co-operates 

 birds w i t h Natural Selection. In the spring migratory birds that have 

 been wintering in the south begin to think of their northern 

 home. A strong impulse takes possession of them. They set 

 out and put forth all their powers. It is a race and there are 

 prizes. The best flyers among the cock-birds arrive first, and 

 if they are songsters they make the woods ring with their 

 pealing notes. If they are but twitterers like the swallows, 

 their flying to and fro announces their arrival Before long, 



