36 SEXUAL DIMORPHISM 



new characters to the species. He calls this the law of 

 male preponderance. 



For my own part, I am inclined to doubt whether there is 

 any essential or constitutional difference between the sexes 

 with regard to the tendency to variation. In vast numbers 

 of species the individuals of opposite sexes are so much alike 

 that it is difficult to distinguish them without examination of 

 the generative organs. Examples of this fact among mammals 

 are furnished by the mouse, the cat, hyaena, rabbit, hare, 

 and many others. Among birds examples are very abundant. 

 Darwin found that there were six classes into which birds 

 could be divided according to the differences and similarities 

 in the characters of the sexes. In two of these classes the 

 adult male resembles the adult female, examples of which 

 condition are afforded by the robin, the kingfisher, many 

 parrots, crows and rooks, and the hedge-warbler. In fishes 

 the sexes are more often alike than different, and among 

 the lower animals special peculiarities in the males are by 

 no means universally present. 



Cases in which the exceptional characters, conspicuous or 

 extraordinary peculiarities, are confined to the females, though 

 less common than the opposite condition, are not wanting, as 

 in the species of Turnix among birds mentioned by Darwin. 



On the other hand, in either sex unisexual characters 

 have, as a general rule, some function or importance in the 

 special habits or conditions of life of the sex in which they 

 occur. The antlers of stags are certainly fighting weapons, 

 and are habitually employed by every healthy stag in combat 

 with his fellows. The special plumes of male birds are 

 habitually erected and displayed in courtship. The melody 

 of singing birds is intended for the ears of their mates, and 

 the note of the male bell-bird, the call-notes of birds in 

 general, are love-notes. So far there can be no doubt that 

 Darwin was perfectly right and his opponents all wrong. 



