XVI] THE PROBLEM OF ADAPTATION %1 



are discovered to be insecure, and in the end no man troubles to 

 controvert them*. 



But of a very different order from all such "adaptations" as 

 these are those very perfect adaptations of form which, for instance, 

 fit a fish for swimming or a bird for flight. Here we are far above 

 the region of mere hypothesis, for we have to deal with questions 

 of mechanical efficiency where statical and dynamical considerations 

 can be appHed and established in detail. The naval architect learns 

 a great part of his lesson from the stream-lining of a fish|; the 

 yachtsman learns that his sails are nothing more than a great bird's 

 wing, causing the slender hull to fly along % ; and the mathematical 

 study of the stream-lines of a bird, and of the principles underlying 

 the areas and curvatures of its wings and tail, has helped to lay the 

 very foundations of 'the modern science of aeronautics. 



We know, for example, how in strict accord with theorv i\t was 

 George Cayley who explained it first) the wing, whether of bird or 

 insect, stands stiff along its "leading edge," like the mast before the 

 sail; and how, conversely, it thins out exquisitely fine along its rear 

 or "trailing edge," where sharp discontinuity favours the formation 

 of uplifting eddies. And pQ see how, alike in the flying wing, in 

 the penguin's swimming wing and in the whale's flipper, the same 

 design of stiff fore-edge and thin fine trailing edge, both curving away 

 evenly to meet at the tip, is continually exemphfied. 



We learn how lifting power not only depends on area but has 

 a linear factor besides, such that a long narrow wing is more stable 

 and effective both for speedy and for soaring flight than a short 

 and broad one of equal area ; and how in this respect the hawkmoth 

 differs from the butterfly, the swallow from the thrush. We are 

 taught how every wing, and every kite or sail, must have a certain 



* The influence of environment on coloration is one thing, and the hypothesis 

 of protective colouring is quite another. That arctic animals are often white, 

 and desert animals sandy-hued or isabelline, are simple and undisputed facts; 

 but such field-naturalists as Theodore Roosevelt, Selous (in his African Nature 

 Notes), Buxton {Animal Life in Deserts), and Abel Chapman {Savage Sudan, and 

 Retrospect) reject with one accord the theory of colour-protection. 



t No creature shews more perfect stream-lining than a fur-seal swimming. 

 Every curve is a continuous curve, the very ears and eye-slits and whiskers falling 

 into the scheme, and the flippers folding close against the body. 



J Cf. Manfred Curry, Yacht Racing, and the Aerodynamics of Sails, London, 

 1928. 



