366 TROPICAL NATURE v 
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 
