366 TROPICAL NATURE 



male to special developments of dermal appendages and colour, 

 quite independently of sexual or any other form of selection. 

 Thus " the hump on the male zebu cattle of India, the tail 

 of fat-tailed rams, the arched outline of the forehead in the 

 males of several breeds of sheep, and the mane, the long hairs 

 on the hind legs, and the dewlap of the male of the Berbura 

 goat " are all adduced by Mr. Darwin as instances of char- 

 acters peculiar to the male, yet not derived from any parent 

 ancestral form. Among domestic pigeons the character of 

 the different breeds is often most strongly manifested in the 

 male birds ; the wattles of the carriers and the eye- wattles of 

 the barbs are largest in the males, and male pouters distend 

 their crops to a much greater extent than do the females, 

 while the cock fantails often have a greater number of tail- 

 feathers than the females. There are also some varieties of 

 pigeons of which the males are striped or spotted with black, 

 while the females are never so spotted (Animals and Plants 

 under Domestication, i. 161) ; yet in the parent stock of these 

 pigeons there are no differences between the sexes either of 

 plumage or colour, and artificial selection has not been applied 

 to produce them. 



The greater intensity of coloration in the male, which may 

 be termed the normal sexual difference, would be further 

 developed by the combats of the males for the possession of 

 the females. The most vigorous and energetic usually being 

 able to rear most offspring, intensity of colour, if dependent 

 on, or correlated with vigour, would tend to increase. But 

 as differences of colour depend upon minute chemical or 

 structural differences in the organism, increasing vigour acting 

 unequally on different portions of the integument, and often 

 producing at the same time abnormal developments of hair, 

 horns, scales, feathers, etc., would almost necessarily lead also 

 to variable distribution of colour, and thus to the production 

 of new tints and markings. These acquired colours would, 

 as Mr. Darwin has shown, be transmitted to both sexes or 

 to one only, according as they first appeared at an early age, 

 or in adults of one sex ; and thus we may account for some 

 of the most marked differences in this respect. With the 

 exception of butterflies, the sexes are almost alike in the 

 great majority of insects. The same is the case in mammals 



