756 THE POPULAR SCIENCE MONTHLY. 



never touched at all, and, in the second place, we have no ground 

 whatever for supposing that those which are touched are thereby made 

 to grow, and to take those shapes which render them efficient. Plants 

 which are rendered uneatable by the thick woolly coatings of their 

 leaves, cannot have had these coatings produced by any process of 

 reaction against the action of enemies ; for there is no imaginable 

 reason why, if one part of a plant is eaten, the rest should thereafter 

 begin to develop the hairs on its surface. By what direct effect of 

 function on structure can the shell of a nut have been evolved ? Or 

 how can those seeds which contain essential oils, rendering them un- 

 palatable to birds, have been made to secrete such essential oils by 

 these actions of birds which they restrain ? Or how can the delicate 

 plumes borne by some seeds, and giving the wind power to waft them 

 to new stations, be due to any immediate 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 

 po excited as to produce a greatly thickened epidermal layer, some- 

 times 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 armor of the 

 armadillo, or the imbricated covering of the manis. The skins of 

 these animals are no more exposed to habitnal 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 der- 

 mal growths had resulted from that extra nutrition caused by extra 

 superficial circulation, we should still be without explanation of the 

 structure of a feather. Kor should we have any clue to the speciali- 

 ties of feathers — the crests of various birds, the tails sometimes so 

 enormous, the curiously placed plumes of the bird of paradise, etc. 

 Still more obviously impossible is it to explain as due to use or dis- 

 use the colors 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 

 oyes 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 how insufficient was the assigned cause. During their growth, 

 a deer's horns are not used at all ; and when, having been cleared of 

 the dead skin and dried-up blood-vessels covering them, they are ready 



