Chap. Y. 
USE AND DISUSE. 
137 
The eyes of moles and of some burrowing rodents are 
rudimentary in size, and in some cases are quite covered 
up by skin and fur. This state of the eyes is probably 
due to gradual reduction from disuse, but aided perhaps 
by natural selection. In South America, a burrowing 
rodent, the tuco-tuco, or Ctenomys, is even more subter¬ 
ranean in its habits than the mole; and I was assured 
by a Spaniard, who had often caught them, that they 
were frequently blind; one which I kept alive was cer¬ 
tainly in this condition, the cause, as appeared on dis¬ 
section, having been inflammation of the nictitating 
membrane. As frequent inflammation of the eyes must 
be injurious to any animal, and as eyes are certainly 
not indispensable to animals with subterranean habits, 
a reduction in their size with the adhesion of the eye¬ 
lids and growth of fur over them, might in such case be 
an advantage; and if so, natural selection would con¬ 
stantly aid the effects of disuse. 
It is well known that several animals, belonging to the 
most different classes, which inhabit the caves of Styria 
and of Kentucky, are blind. In some of the crabs the 
foot-stalk for the eye remains, though the eye is gone; 
the stand for the telescope is there, though the telescope 
with its glasses has been lost. As it is difficult to ima¬ 
gine that eyes, though useless, could be in any way 
injurious to animals living in darkness, I attribute their 
loss wholly to disuse. In one of the blind animals, 
namely, the cave-rat, the eyes are of immense size ; and 
^Professor Silliman thought that it regained, after living 
some days in the light, some slight power of vision. In 
the same manner as in Madeira the wings of some of 
the insects have been enlarged, and the wings of others 
have been reduced by natural selection aided by use 
and disuse, so in the case of the cave-rat natural selec¬ 
tion seems to have struggled with the loss of light and 
