210 



COMPARATIVE ZOOLOGY. 



the Toad, the scales of Snakes, the feathers of Birds, and 

 the hair of Mammals, may be termed a metamorphosis. 



FIG. 17k 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 



