THE HAUNTS OF LIFE 123 



insects, many spiders and crustaceans, various snails, and 

 so on. They tend to be somewhat dwarfed types, with 

 more or less degenerate eyes (except in the bats and mice), 

 with highly developed tactility, and with reduced pigmenta- 

 tion. In those cavernicolous animals in which the develop- 

 ment of the eye has been worked out, e.g. Proteus, Ambly- 

 opsis (a fish), and Cambarus (a crayfish), it has been shown 

 that the eye of the young form is relatively less degenerate 

 than that of the adult. 



Racovitza, who has made a special study of cave-animals, 

 gives an interesting account of an Isopod or wood-louse, 

 Spelseoniscus, from an Algerian cavern. It is colourless, 

 blind, and covered with tactile setse ; it has no longer any 

 near relatives living in the Ught of day ; it is an archaic 

 representative of a fauna which has disappeared. It was 

 in a sense a failure, Racovitza thinks, for whereas it can 

 roll itself up in a ball like many other Isopods, such as 

 the widely distributed ArmadilUdium vulgare, its antennae 

 are left sticking out and exposed to danger. So it had 

 to become a Troglodyte. Racovitza suggests that it is 

 not the only failure who has taken refuge in a cave, ' cet 

 asile que dame nature installa a peu de frais pour ses veil- 

 lards, ses impotents et ses rates'. 



VI. The Aerial Fauna 



The last haunt of life to be tenanted was the air, and 

 it is interesting to notice how many attempts have been 

 made to possess it. Among backboneless animals the 

 insects alone have attained to the power of true flight, but 

 among backboned animals there are three instances of 

 success — the extinct Pterodactyls, the Flying Birds, and 

 the Bats. Thus in each of the three great classes of air- 



