276 ANIMAL COLORATION. 



Mr. Wallace would explain the secondary differences in 

 colour between the two sexes as due to a divergence in two- 

 opposite directions. 



The hen bird nearly always sits upon her eggs ; instances to 

 the contrary are rare. During the period of incubation, and 

 for a short time after the young birds are hatched, the mother 

 is subject to special dangers which are not shared by the 

 active male ; she is more liable to attacks from predatory 

 animals, which circumstances make it difficult to resist. An 

 inconspicuous coloration is thus clearly a desideratum. There 

 is also a remarkable relation between the sex coloration and 

 the nesting habits. Birds which build their nests in exposed 

 situations appear to be almost always dull-coloured : on the 

 other hand, the females of those species which build in holes r 

 or construct elaborate nests that completely conceal them, are 

 generally as brightly coloured in one sex as in the other. 



The, difference between the two sexes is furthur accentuated 

 by the " greater vigour and excitability " of the male, which 

 leads to a more pronounced development of colouring ; but the 

 further consideration of Mr. Wallace's important views upon 

 this important branch of the subject will be deferred, until 

 after some account of Mr. Stolzmann's theory of sexual dimor- 

 phism has been given ; for it is more fitting to conclude this 

 chapter with the most probable theory. 



Mr. Stolzmann's Views. 



Mr. Wallace's views of sexual dimorphism attribute, as. 

 has already been mentioned, the difference between the sexe 

 partly to a need for protection of the female bird. He replaces, 

 in fact, " sexual " by " natural selection," but only as regards the 

 female bird. Mr. Stolzmann * has endeavoured to account for 

 * Proc. ZooLSoc^. 1885, p. G15.. 



