426 THEORIES OF EVOLUTION 



to maturity and reproduce their kind. New variations, 

 in the form of mutations, appeared during the long his- 

 tory of the race. Where these resulted in longer necks 

 than before they were incorporated into the race. The 

 great height and exceedingly long neck of the giraffe is 

 therefore not the result of one variation in the history of 

 a deer-like ancestor. On the contrary the evolution of 

 the species is the result of the gradual accumulation of 

 many comparatively slight variations or mutations which 

 not only modified the race in reSpect to the neck but 

 enabled it to withstand the attacks of enemies and the 

 severe conditions of its surroundings. This same line of 

 reasoning is applicable in the case of every other species 

 of animal as well as plant. 



Sexual Selection. — In another connection (Chap. 

 XXVI) it has been shown that, among many kinds of 

 animals, marked differences exist between the males and 

 females. The reason for this fact appeared to Darwin to 

 lie more or less beyond the realm of Natural Selection, 

 and was rather to be explained on the basis of Sexual 

 Selection. According to this theory the males either 

 engage in a struggle among themselves for a particular 

 mate, or, among other species, they are chosen by the 

 opposite sex. When a battle ensues it is a well known 

 fact that in many instances those males with the greatest 

 strength and the most highly developed weapons come 

 off victorious and leave descendants. This is even more 

 apparent among polygamous animals, where one male 

 fights for the possession of several females as in the case 

 of the fur seal, sea lion, domestic fowl, and several other 

 species. In all of these cases there is a survival of the 

 fittest, and at the present time this phase of sexual selec- 

 tion is explained on the basis of Natural Selection. 



Among the birds, insects, the crabs and related species, 

 where the male is often highly colored or distinguished 

 by other secondary sexual characteristics, the female, 

 according to the theory of Sexual Selection, is believed to 



