The Nature of Animal Life. 



The butterfly or the silkworm moth, beginning life as a 

 caterpillar and changing into a chrysalis, from which the 

 perfect insect emerges, is a familiar instance. And hosts 

 of the marine invertebrates have larval forms which have 

 but little resemblance to their adult parents. 



Such a series of changes as is undergone by the frog 

 is called metamorphosis, which essentially consists in the 

 temporary development of certain provisional embryonic 

 organs (such as gills and a powerful swimming tail) and 

 the appearance of adult organs (such as lungs and legs) to 

 take their place. In metamorphosis these changes occur 

 during the free life of the organism. But beneath the 

 eggshell of birds and within the womb of mammals 

 scarcely less wonderful changes are slowly but surely 

 effected, though they are hidden from our view. There 

 is no metamorphosis during the free life of the organism, 

 but there is a prenatal transformation. The little embryo 

 of a bird or mammal has no gills like the tadpole (though 

 it has for a while gill-slits, pointing unmistakably to its 

 fishy ancestry), but it has a temporary provisional 

 breathing organ, called the allantois, pending the full 

 development and functional use of its lungs. 



All the higher animals, in fact — the dog, the chick, the 

 serpent, the frog, the fish, the lobster, the butterfly, the 

 worm, the star-fish, the mollusc, it matters not which we 

 select — take their origin from an apparently unorganized 

 egg. They all, therefore, pass during their growth from a 

 comparatively simple condition to a comparatively complex 

 condition by a process of change which is called develop- 

 ment. But there are certain lowly forms, consisting 

 throughout life of little more than specks of jelly-like life- 

 stuff, in which such development, if it occurs at all, is not 

 conspicuous. 



6. They move about and sleep. This is true of our 

 familiar domestic pets. The dog and the cat, after periods 

 of restless activity, curl themselves up and sleep. The 

 canary that has all day been hopping about its cage, or 

 perhaps been allowed the freedom of the dining-room, tucks 



