354 Evolution and Adaptation 



One of the most familiar cases of degeneration is blind- 

 ness, associated with life in the dark. The most striking 

 cases are those of cave animals, but this is only an extreme 

 example of what is found everywhere amongst animals that 

 live concealed during the day under stones, etc. The blind 

 fish and the blind crayfish of the Mammoth Cave, the 

 blind proteus of the caves of Carniola, the blind mole that 

 burrows underground, the blind larvae of many insects that 

 live in the dark, are examples most often cited. Some noc- 

 turnal animals, like the earthworm, have no eyes, although 

 they are still able to distinguish light ; and some of the deep- 

 sea animals, that live below the depth to which light pene- 

 trates, have degenerate eyes. The workers of some ants, 

 that remain in the nests, are blind, but the males and the 

 queens of these forms have well-developed eyes, although the 

 eyes may be of use to them at only one short period of their 

 life, namely, at the time of the marriage flight. This fact 

 is significant and is underestimated by those who believe 

 that disuse accounts for the degeneration of organs. 



The wings of the ostrich and of the kiwi are rudimentary 

 structures no longer used for flight, and many insects, be- 

 longing to several different orders, have lost their wings, as 

 seen in fleas, some kinds of bugs, and moths, and even in 

 some grasshoppers. 



A curious case of degeneration is found in the abdomen of 

 the .hermit crab, which is protected by the appropriated shell 

 of a snail. The appendages of one side of the abdomen 

 have nearly disappeared in the male, although in the female 

 the abdominal appendages are used to carry the eggs as in 

 other decapod crustaceans. The abdomen, instead of being 

 covered by a hard cuticle, as in other members of this group, 

 is soft and unprotected except by the shell of the snail. 



Cases of these kinds could be added to almost indefinitely, 

 and the explanation of these degenerate structures has been 

 a source of contention amongst zoologists for a long time. 



