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. 
