210 
COMPARATIVE ZOOLOGY. 
tlie Toad, the scales of Snakes, the feathers of Birds, and 
the hair of Mammals, may be termed a metamorphosis. 
Fig. 174.—Metamorphosis of the Newt. 
The change from milk-teeth to a permanent set is another 
example. 
An animal rises in organization as development ad¬ 
vances. Thus, a Caterpillar’s life has nothing nobler 
about it than the ability to eat, while the Butterfly ex¬ 
pends the power garnered up by the larva in a gay and 
busy life. But there are seeming reversals of this law. 
Some mature animals appear lower in the scale than their 
young. The larval Cirripede has a pair of magnificent 
compound eyes and complex antennae; when adult, the 
antennae are gone, and the eyes are reduced to a single, 
simple, minute eye-spot. So the germs of the sedentary 
Sponge and Oyster are free and active. The adult ani¬ 
mal, however, is always superior in alone possessing the 
power of reproduction. Such a process is known as retro¬ 
grade metamorphosis . 
There are certain larval forms so characteristic of the 
