IX.] RETROGRESSIVE DEVELOPMENT IN NATURE, 7 



life can fall into disuse. Obviously, this can only happen 

 through a change in the conditions under which the animal 

 lives. When a bird which has been accustomed to seek its 

 food in trees and bushes, finds upon the ground supplies so 

 rich as to afford better sustenance, it will gradually come to live 

 more and more upon the ground, and less and less in trees, a 

 fact which taken alone will entirely alter the conditions of its 

 life. It will not require to fly, and will consequently fly less 

 and less often, and after the lapse of generations will cease to 

 fly altogether. And to bring all this about, the wood in which 

 it lives, the climate, the surrounding animals, need not have 

 undergone any changes ; merely the adoption of a new habit 

 by the bird itself will suffice. 



It is the same with animals removed from their original 

 habitat ; they may find themselves in circumstances so essen- 

 tially different as to render superfluous some organ which had 

 once been indispensable. For instance, if a species which had 

 always lived in the light, were to find its way into some new 

 habitat where there was complete darkness, its eyes would be- 

 come useless to it ; and accordingly we commonly find that in 

 such species the eyes have more or less completely atrophied. 



This is the case, for instance, with animals which live in 

 dark caves. In the limestone caverns of Carniola and Carinthia 

 a blind amphibian, the Proteus, is found in great numbers, and 

 there are also bhnd Crustacea (both isopods and amphipods), 

 blind insects and snails. In the Mammoth Cave of Kentucky 

 among other bhnd animals we find a blind fish and a bhnd 

 fresh-water crayfish. It is almost superfluous to offer any 

 further proof that these species are descended from ancestors 

 which possessed the power of sight, beyond the fact that the 

 caverns in question have not existed from the beginnings of 

 organic life, and that therefore the animals must have lived in 

 the hght before they entered them. Nevertheless, in many of 

 these animals direct proof exists in the fact that they still 

 possess vestiges of what have once been eyes. The Proteus 

 and the blind fish of the Mammoth Cave have small imperfectly- 

 developed eyes under the skin, which are no longer of any use 

 as organs of sight. In the case of the blind crayfish, the eyes 

 have entirely disappeared, although the moveable stalks upon 

 which they were placed still remain. 



