422 CORRELATION OF PHYSICAL AND VITAL FORCES. 



period occurs, in the different tribes of animals, at very dis- 

 similar stages of the entire developmental process. In many 

 of the lower classes, the embryo comes forth from the egg, 

 and commences its independent existence, in a condition 

 which, as compared with the adult form, would be as if a 

 human embryo were to be thrown upon the world to obtain 

 its own subsistence only a few weeks after conception ; and its 

 whole subsequent growth and development takes place at the 

 expense of the nutriment which it ingests for itself. 



We have examples of this in the class of insects, many of 

 which come forth from the egg in the state of extremely simple 

 and minute worms, having scarcely any power of movement, 

 but an extraordinary voracity. The eggs having been depos- 

 ited in situations fitted to afford an ample supply of appropriate 

 nutriment (those of the flesh-fly, for example, being laid in 

 carcases, and those of the cabbage-butterfly upon a cabbage- 

 leaf) , each larva on its emersion is as well provided with ali- 

 mentary material as if it had been furnished with a large sup- 

 plemental yolk of its own ; and by availing itself of this, it 

 speedily grows to many hundred or even many thousand 

 times its original size, without making any considerable ad- 

 vance in development. But having thus laid up in its tissues a 

 large additional store of material, it passes into a state which, 

 so far as the external manifestations of life are concerned, 

 is one of torpor, but which is really one of great develop- 

 mental activity : for it is during the pupa state that those new 

 parts are evolved, which are characteristic of the perfect in- 

 sect, and of which scarcely a trace was discoverable in the 

 larva ; so that the assumption of this state may be likened in 

 many respects to a reentrance of the larva into the ovum. 

 On its termination, the imago or perfect insect comes forth 

 complete in all its parts, and soon manifests the locomotive 

 and sensorial powers by which it is specially distinguished, 

 and of which the extraordinary predominance seems to jus- 

 tify our regarding insects as the types of purely animal life. 



