10 . ZOOLOGY. 
the form of the teeth presupposes either a ruminant or carni~ 
vore. Hence this prime law of comparative anatomy led to: 
the establishment by Cuvier of the fundamental laws of 
paleontology, by which the comparative anatomist is en- 
abled to restore from isolated teeth or bones the probable 
form of the original possessor. Of course the more perfect. 
the series of bones and teeth, or the more complete the re- 
mains of insects or mollusks, the more perfect will be our 
knowledge, and the less room will there be for error in re-. 
storing extinct animals. 
Adaptation.—An organ with a certain normal use or 
function may be adapted, in consequence of a change in the: 
habits of the animal, to another use than the original one. 
To take an extreme case, the Anadas, or climbing fish, may’ 
use its fins to aid it in ascending trees. On the other hand, 
by disuse organs become aborted or rudimentary. The 
teeth of the whalebone whale are rudimentary in the young, 
and are replaced by whalebone, which is more useful to the: 
animal ; the eyes of the blind-fish are rudimentary, func- 
tionless. Those of certain cave-insects are entirely wanting, 
being lost through disuse, owing to a change of life from. 
the light, outer world to totally dark caverns, and the con-. 
sequent disuse of their eyes. Nature is economical. Every 
thing that is not of use as a rule disappears. It would be a. 
waste of material to nourish and care for an organ in a cave- 
animal, or a parasitic insect or crustacean, which would be 
of no use to the animal. On the other hand, if the leg or 
tail of a newt is snipped off by some rapacious fish, it 
grows out again. 
Moreover, the animal organism is far more pliable than is 
generally supposed. Not only is nature continually repair-. 
ing wounds and waste, not only is the body being contin-. 
ually made over again, but certain animals undergo a 
change of form, either generally or in particular parts. It 
the environment is unchanged, the animal remains true to 
its species. The dogma of the invariability or stability of 
species is a fallacy. Change the climate, moisture or dryness, 
the nature of the soil ; introduce the natural enemies of the 
animal or remove them ; destroy the balance of nature, in 
