154 ANIMALS AND PLANTS. [LECT. 



the whole body, has no permanent digestive cavity or 

 mouth, but takes in its food anywhere ; and digests, 

 so to speak, all over its body. 



But although Cuvier's leading diagnosis of the 

 animal 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 tak- 

 ing solid nutriment into the body and there digesting 

 it, the definition so changed will cover all animals, 

 except certain parasites, 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 

 distinctive mark when he admits that it is wanting in 

 the simpler animals. 



The third distinction is based on a completely 

 erroneous conception of the chemical differences and 

 resemblances between the constituents of animal and 

 vegetable organisms, for which Cuvier is not re- 

 sponsible, as it was current among contemporary 

 chemists. It is now established that nitrogen is as 

 essential a constituent 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 animals. Amylaceous and saccha- 

 rine substances are largely manufactured, even by 

 the highest animals ; cellulose is widespread as a con- 



