183 
and feeds chiefly on the larve of timber-eating insects (such as 
the Cerambyx before us), which larve often reside quite in the 
heart of the wood. In the dead of the night, with its large erect 
ears directed forward, the Aye-Aye can hear the slightest noise. 
It taps with its claw upon the tree—it is made sensible of the 
movements of the grub, if there be one, beneath the bark—and 
then pares away the wood with its strong incisor teeth, till the 
' grub is exposed. The next step is to insert its long slender 
finger to probe the cavity, and with its claw it drags forth the 
prey, and pokes it into its gullet. Who can number the ages it 
must have taken to perfect this structure and its marvellous. 
adaptation to such a purpose? No other known living animal 
approaches the Aye-Aye in this peculiarity. 
As there are structures, such as those spoken of, which must. 
have been perfected by use, so there are others that have become 
completely atrophied by disuse. One of the most curious of these 
eases is that of the blind animals found in caves and subterranean 
waters both in Europe and America. The Mammoth cave in 
Kentucky—perhaps the most celebrated of these caves—is that. 
of which I shall here speak. This cave extends for many miles 
into the heart of a limestone rock, answering I believe, geologically, 
to our mountain-limestone. Within the cave there are waters, 
some having connection with the Green river, one of the 
tributaries of the Ohio, which passes near its mouth, so that fish 
(or any other aquatic animals) can easily enter the cave if they 
choose, or when in make their exit. About three or four miles 
from the mouth of this cave—(which has been penetrated for 
more than 10 miles without coming to an end)—there exists a 
peculiar Fauna consisting of fish, crustacea, insects, arachnids, &c., 
the number of species being very considerable, but the number of 
individuals of each species few and scattered. The insects, and 
other creatures not aquatic, are found on the walls and floor 
of the cavern—the fish and chief of the crustacea in the 
waters. Now the great peculiarity which distinguishes these 
