330 ELEMENTS OF COMPARATIVE ZOOLOGY. 



animals that it becomes removed to a deeper position, which 

 necessitates the development of nerve-cords to connect it 

 with the sensory system and with the muscles and other 

 parts. It is interesting that in all animals, even in man, 

 no matter how deeply situated or how thoroughly protected 

 it may be in the adult, the central nervous system arises 

 from the outer surface, and secondarily attains its perma- 

 nent position. 



Since most animals must search for their food, we find 

 that one end becomes adapted for always going in front, 

 and in this way a head has come into existence, and here are 

 situated the brain and the most important sensory organs, 

 as well as the mouth, since this part of the body first comes 

 into the neighborhood of substances useful as food or likely 

 to be injurious to the animal. 



So far we have treated of the animal as an automatic self- 

 regulating machine, but in one respect it differs from all 

 machines of human production. No amount of fuel put 

 under the boiler of a steam-engine will cause this mechanism 

 to increase in size or to give rise to other bits of mechanism 

 like itself. The animal machine grows by the taking in of 

 food, and like the steam-engine, it wears out. It, however, 

 has the power of reproducing the kind, by the formation of 

 small parts (either buds or eggs) which eventually grow into 

 animals like the parent which produced them, and thus the 

 species is perpetuated, the young taking the place of the 

 generation which has worn itself out. 





