316 THE EVOLUTION THEORY 



a class of animals than feathers are of birds ? They alone are 

 enough to distinguish the class from all other living classes; an 

 animal with feathers can now be nothing but a bird, and yet the 

 feather is a skin-structure which has arisen through adaptation, 

 a reptilian scale which has been so transformed that an organ 

 of flight could develop from its anterior extremity. We find it thus 

 even in the two impressions of the primitive bird Archceopteryx^ 

 which have been preserved for us in the Solenhofen slate since the 

 Jurassic period in the history of the earth. And into what detail 

 does adaptation go in tlie case of the feathers ! Is not the whole 

 structure, with its quill, shaft, and vane, precisely adapted to its 

 function, although that is purely passive? What I liave just said 

 of the whole class of Birds holds true for this individual structure, 

 the feather ; everything al)0ut it is adaptation, and indeed illustrates 

 adaptation in two directions, for in the first place the feathers, by 

 spreading a broad, light, and yet resistant surface with which to beat 

 the air, act as organs of flight, while they are also the most effective 

 warmth-retaining covering conceivable. In both these directions 

 their achievements border on the marvellous. I need only recall the 

 most recent discovery in this domain, the proof recently given by the 

 Viennese physiologist, Sigmund Exner, that the feathers become 

 positively electric in their superficial layer, and negatively electric 

 in their deeper la^^er, whenever they rub against one another and 

 strike the air. But they are rubbed whenever the bird flies or moves, 

 and the consequence of the contrast in the electric charging of the 

 two layers is that the covering feathers are closely aj^posed over the 

 down-feathers, while, on the other hand, the similar charging of the 

 down-feathers makes them mutually repel each other, with the result 

 that a layer of air is retained between them, and thus there is 

 between the skin and the covering feathers a loose thicket of feathers 

 uniformly penetrated by air — the most effective warmth-preserver 

 imaginable. The electric characters of the feathers — and the same is 

 true of the hairs of animals — are thus not indifferent characters, but 

 with an appreciable biological importance, and the same is true of the 

 almost microscopical series of little booklets which attach the barbs 

 of the covering feathers to one another, and thus form a relatively 

 firm but exceedingly light wing-surface which offers a strong resist- 

 ance to the air. But as we must regard these booklets as adaptations, 

 so must we also regard the electrical characters of the feathers, and 

 we must think of them as having arisen through natural selection, as 

 Exner himself has insisted. 



If we are able to recognize all the more prominent features 



