REPRODUCTION AND SEX 485 



physiological habit and length of life, which are very real, though 

 we cannot say much about their structural expression! 



Another consideration to be kept in mind in the classification of 

 sex differences is the degree of permanence in their expression. 

 An adult peacock can never be confused with a peahen, but there 

 are many birds, e.g. some ducks, which show great dissimilarity 

 between the sexes at the breeding season and great similarity at 

 other times. In many fishes, such as sticklebacks, the males are 

 conspicuously different from the females at the breeding time, but 

 inconspicuously different at other times. In short, there are many 

 nuptial characters which wax and wane according to the sexual 

 state of the organism. 



ILLUSTRATIONS OF SEX- CHARACTERS. —Convenient sur- 

 veys of sex differences are to be found in Darwin's Descent of Man 



Male (A) and Female (B) of the Stag-beetle {Lucanus cervus), from specimens. 

 MN, mandibles, which are enormously exaggerated in the male. 



and Cunningham's Sexual Dimorphism and our Sex (Home Uni- 

 versity Library). Among Mammals one recalls the gorilla's sagittal 

 crest, the mandrill's enormous canines, the elephant-seal's nasal 

 proboscis, the lion's mane, the narwhal's tusk, the stag's antlers, 

 the duckmole's spur. Among Birds one recalls the peacock's tail, 

 the wing-feathers of the Argus Pheasant, the decorations of Birds 



