8 RETROGRESSIVE DEVELOPMENT IN NATURE. [IX. 



Caves are not the only places where animals are known to 

 live in the dark ; in deep wells and at the bottom of the sea 

 and of lakes complete darkness reigns. To Professor Forel 

 of Morges we owe the discovery of the depth to which light 

 can penetrate. Photographic plates were sunk at night to a 

 certain depth, and after being suspended to a buoy, were 

 exposed, for a period of from twenty-one to twenty-four hours, 

 to such light as could reach them. By this means Forel found 

 that even in the transparent water of the Lake of Geneva, 

 the light in winter, when the water is clearest, only penetrated 

 to a depth of loo metres, and scarcely 50 metres in summer. 

 Later experiments by Fol and Sarasin, with more perfect 

 apparatus and more highly sensitive plates, proved, however, 

 that light penetrates the Lake of Geneva to the greater depth 

 of 170 metres. On a bright day there is about as much light 

 at such a depth as we are accustomed to see on a starlight 

 night, when there is no moon. Below this there is utter 

 darkness ; and we find blind animals from these downwards 

 to the greatest depths (300 metres), at which, for example, a 

 blind isopod and ah a'mphipod exist. In the sea, when the 

 water is undisturbed, light penetrates as far as 400 metres, but 

 as we now know that animal life exists in the sea at a depth of 

 4000 me,tres, there still remains a vast region in which darkness 

 reigns, and in which numberless blind animals are found, — 

 blind fish, blind crustaceans of various species, blind molluscs 

 and worms. Forms nearly related to all these live where the 

 light penetrates, and possess eyes. 



Burrowing animals, too, have, for the most part, either poorly- 

 developed eyes or none at all. Thus earthworms are sightless, 

 while closely-allied pelagic species generally possess e3^es, 

 often very highly developed, and of complex structure. The 

 common mole has indeed eyes, although very small ones, 

 completely hidden in its close fur, but in Africa there are 

 moles which are devoid of eyes and therefore entirely blind. 



Many other instances might be brought forward to prove 

 that the disuse of the organs of vision results in their disappear- 

 ance. And the same conclusion holds good for other organs ; 

 experience teaches that, as soon as any organ falls into disuse, 

 it degenerates and is finally lost altogether. 



We find interesting confirmation of this fact in the other 



