EXTEXSIONS OF DARWl.XLSM i'8l 



are wonderfully varied throughout the whole of th(^ VLricbrate 

 sub-kingdom. Yet the more or less use of the teeth cannot 

 be shown to have any tendency to change their fnnii or 

 structure in the special ways in which they have been again 

 and again changed, though it might possibly have induced 

 growth and increased size. Yet again, the scales or plates of 

 reptiles, the feathers of birds, and the hairy covering of mam- 

 mal?, have never been shown to have their special textures, 

 shape?, or density modified by the mere act of use. One 

 common error is that cold produces length and density of 

 hair, heat the reverse ; but the purely tropical monkey-tribe 

 are, as a rule, quite as well clothed with dense fur as most 

 of the temperate or arctic mammals, while no birds are more 

 luxuriantly feather-clad than those of the tropics. XeitlnT 

 is it certain that increased gazing improves the eyes, or loud 

 noises the ears, or increased eating the stomach ; so that we 

 must conclude that this aid to the powers of natural selection 

 is very partial in its action, and that it has no claim to the 

 important position sometimes given it. 



(3) Germinal Seleclion, an Important Eximfiion of the 



Theory of Natural Selection 



Although I was at first inclined to accept Darwin's view of 

 the influence of female choice in determining the development 

 of ornamental colour or appendages in the males, yet, when 

 he had a(hhiced his wonderful array of facts bearing upon 

 the question in the Descent of Man, the evidence for any such 

 effective choice appeared so very scanty, and the ellects im- 

 puted to it so amazingly improbable, that T felt certain that 

 some other cause was at work. Tn my Tropical Nature 

 (1878) and in my Darwinism (1889) I treated the subject 

 at considerable length, adducing many facts to prove that, even 

 in birds, the colours and ornamental })lunies of the males were 

 not in themselves attractive, but served merely as signs of 

 sexual maturity and vigour. Tn the case of insects, especially 

 in butterflies, where the phenomena of colour, and to some 



