194 TROPICAL NATURE, AND OTHER ESSAYS. 



are the invariable accompaniments of health and energy. 

 The same rule applies to the feathers of birds, whose 

 colours are only seen in their purity during perfect 

 health ; and a similar phenomenon occurs even among 

 insects, for the bright hues of caterpillars begin to fade 

 as soon as they become inactive preparatory to under- 

 going their transformation. Even in the vegetable 

 kingdom we see the same thing ; for the tints of foliage 

 are deepest, and the colours of flowers and fruits richest, 

 on those plants which are in the most healthy and 

 vigorous condition. 



This intensity of coloration becomes most developed 

 in the male during the breeding season, when the 

 vitality is at a maximum. It is also very general in 

 those cases in which the male is smaller than the female, 

 as in the hawks and in most butterflies and moths. The 

 same phenomena occur, though in a less marked degree, 

 among mammalia. Whenever there is a difference of 

 colour between the sexes the male is the darker or more 

 strongly marked, and the difference of intensity is most 

 visible during the breeding season (Descent of Man, 

 p. 533). Numerous cases among domestic animals also 

 prove, that there is an inherent tendency in the 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 dew- 

 lap of the male of the Berbura goat," are all adduced by 

 Mr. Darwin as instances of characters peculiar to the 

 male, yet not derived from any parent ancestral form. 



