240 ANIMAL ACTIVITIES. 
have been answered. The Colorado beetle has been 
able to maintain itself under new conditions. The 
domestic (English) sparrow thrives in America. In 
these cases the creatures have adapted their mode of 
life to new habitats with marvellous rapidity and suc- 
cess. Foreign rats have practically exterminated 
American species, being able not only to survive, but 
to drive out animals already adapted to their environ- 
ment. Fishes in caverns have lost the use of their 
eyes, either from lack of use, or because eyes in abso- 
lute darkness are a disadvantage, and so disappear by 
the process of natural selection. 
Structure and Habitat. In the chapter on Insect 
Adaptations we have called attention to the fitness of 
organs for the work they must perform. At first 
thought it might seem as if these insects had been 
fitted from the start for their peculiar mode of life. 
But a little reflection must show that many of these 
adaptations, however ingenious they may seem, are 
really very imperfect. The breathing-organs of aquatic 
insects are clumsy, compared with the gills of a fish. 
In fact, they are soon seen to be modifications of organs 
intended for another purpose, namely, for breathing 
air. If, then, we were to hold that animals were orig- 
inally adapted for the localities in which we find them, 
the useless eye of the blind fish in a cavern would be 
to us aninsoluble problem. Fish made on purpose for, 
life in dark caverns should have no eyes, no optic 
nerves, and no useless muscles to move the eyeballs. 
The fact that all aquatic insects have trachez admits 
of no reasonable explanation unless we assume that 
these insects are descended from air-breathing ances- 
tors. Such ancestors may have entered the water in 
search of food, or to escape enemies. In either case, 
those whose tracheez were best fitted to use the air 
dissolved in the water survived, while their kindred 
perished. 
The obvious inference from the facts of distribution 
