AOTMALS AND PLANTS. 161 



ity or mouth, but takes in its food anywhere ; and di- 

 gests, so to speak, all over its body. 



But although Cuvier's leading diagnosis of the ani- 

 mal from the plant will not stand a strict test, it remains 

 one of the most constant of the distinctive characters 

 of animals. And, if we substitute for the possession of 

 an alimentary cavity, the power of taking solid nutri- 

 ment into the body and there digesting it, the definition 

 so changed will cover all animals, except certain para- 

 sites, and the few and exceptional cases of non-parasitic 

 animals which do not feed at all. On the other hand, 

 the definition thus amended will exclude all ordinary 

 vegetable organisms. 



Cuvier himself practically gives up his second dis- 

 tinctive mark when he admits that it is wanting in the 

 simpler animals. 



The third distinction is based on a completely er- 

 roneous conception of the chemical differences and re- 

 semblances between the constituents of animal and vege- 

 table organisms, for which Cuvier is not responsible, as 

 it was current among contemporary chemists. It is 

 now established that nitrogen is as essential a constitu- 

 ent of vegetable as of animal living matter; and that 

 the latter is, chemically speaking, just as complicated as 

 the former. Starchy substances, cellulose and sugar, 

 once supposed to be exclusively confined to plants, are 

 now known to be regular and normal products of ani- 

 mals. Amylaceous and saccharine substances are largely 

 manufactured, even by the highest animals ; cellulose is 



