20 THE EVOLUTION THEORY 



prompted an animal of a given species to use certain parts of its body 

 in a new way, more vigorously, or less actively, or even not at all, 

 and the more vigorous use, or, conversely, the disuse, has brought 

 about variations in the organ in question. 



Thus the whales lost their teeth when they abandoned their fish 

 diet, and acquired the habit of feeding on minute and delicate 

 molluscs, which they swallowed whole without seizure or mastication. 

 Thus, too, the eyes of the mple degenerated through its life in the 

 dark, and a still greater degeneration of the eyes has taken place in 

 animals, like the proteus-salamander, which always inhabit lightless 

 caves. In mussels both head and eyes degenerated because the 

 animals could no longer use them after they became enclosed in 

 opaque mantles and shells. In the same way snakes lost their legs 

 pari passu with the acquisition of the habit of moving along by 

 wriggling their long bodies, and of creeping through narrow fissures 

 and holes. On the other hand, Lamarck interpreted the evolution of 

 the web-feet of swimming birds by supposing that some land-bird or 

 other had formed the habit of going into the water to seek for food, 

 and consequently of spreading out its toes as widely as possible so as 

 to strike the water more vigorously. In' this way the fold of skin 

 between the toes was stretched, and as the extension of the toes was 

 very frequent and was" continued through many generations, the web 

 expanded and grew larger, and thus formed the web-foot. 



In the same way the long legs of the wading birds have been, 

 according to Lamarck, gradually evolved by the continual stretching 

 of the limbs by wading in deeper and deeper water, and similarly for 

 the long necks and bills of the waders, the herons and the storks. 

 Finally we may mention the case of the giraffe, whose enormously 

 long neck and tall forelegs are interpreted as due to the fact that the 

 animal feeds on the foliage of trees, and was always stretching as far 

 as possible, in order to reach the higher leaves. 



We shall see later in what a different way Charles Darwin 

 explained this case of the giraffe. Lamarck's idea is at once clear ; it 

 is true that exercising an organ strengthens it, that disuse makes 

 it weaker. Through much gymnastic exercise the muscles of the arm 

 become thicker and more capable, and memory too may be improved, 

 that is to say, even a definite part of the brain may be considerably 

 strengthened by use. Indeed, we may now go so far as to admit that 

 every organ is strengthened by use and weakened by disuse, and so 

 far the foundations of Lamarck's interpretations are sound. But he 

 presupposes something that cannot be admitted so readily, namely, 

 that such ' functional' improvement or diminution in the strength of 



