206 COMPARATIVE ZOOLOGY. 
the hair of Mammals, may be termed a metamorphosis. 
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 ana 
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 antennz; when adult, the 
antennee 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. 
2. Alternate Generation. 
Sometimes a metamorphosis extending over several gen- 
erations is required to evolve the perfect animal; “in oth- 
er words, the parent finds no resemblance to himself in 
any of his progeny, until he comes down to the great- 
grandson.” Thus, the Jelly-fish, or Medusa, lays eggs 
which are hatched into larvee resembling Infusoria—little 
transparent oval bodies covered with cilia, by which they 
swim about for a time till they find a resting-place. One 
of them, for example, becoming fixed, develops rapidly ; 
it elongates and spreads at the upper end; a mouth is 
formed opening into a digestive cavity; and tentacles 
multiply till the mouth is surrounded by them. At this 
stage it resembles a Hydra. Then slight wrinkles appear 
along the body, which grow deeper and deeper, till the 
animal looks like “a pine- cone surmounted by a tuft of 
tentacles ;” and then like a pile of saucers (about a dozen 
in number) with scalloped edges. Next, the pile breaks 
