4 THE FACTOKS OP ORGANIC EVOLUTION. 



power to waft them to new stations, be due to any imme 

 diate influences of surrounding conditions ? Clearly in these 

 and in countless other cases, change of structure cannot 

 have been directly caused by change of function. So 

 is it with animals to a large extent, if not to the same 

 extent. Though we have proof that by rough usage the 

 dermal layer may be so excited as to produce a greatly 

 thickened epidermal layer, sometimes quite horny; and 

 though it is a feasible hypothesis that an effect of this kind 

 persistently produced may be inherited ; yet no such cause 

 can explain the carapace of the turtle, the armour of the 

 armadillo, or the imbricated covering of the manis. The 

 skins of these animals are no more exposed to habitual 

 hard usage than are those of animals covered by hair. 

 The strange excrescences which distinguish the heads of 

 the hornbills, cannot possibly have arisen from any reaction 

 against the action of surrounding forces; for even were 

 they clearly protective, there is no reason to suppose that 

 the heads of these birds need protection more than the 

 heads of other birds. If, led by the evidence that in 

 animals the amount of covering is in some cases affected by 

 the degree of exposure, it were admitted as imaginable that 

 the development of feathers from preceding dermal growths 

 had resulted from that extra nutrition caused by extra 

 superficial circulation, we should still be without explana 

 tion of the structure of a feather. Nor should we have any 

 clue to the specialities of feathers the crests of various 

 birds, the tails sometimes so enormous, the curiously placed 

 plumes of the bird of paradise, &c., &c. Still more 

 obviously impossible is it to explain as due to use or disuse 

 the colours of animals. No direct adaptation to function 

 could have produced the blue protuberances on a mandril s 

 face, or the striped hide of a tiger, or the gorgeous plumage 

 of a kingfisher, or the eyes in a peacock s tail, or the 

 multitudinous patterns of insects wings. One single case, 

 that of a deer s horns, might alone have sufficed to show 



