644 ^^^ POPULAR SCIENCE MONTHLY. 



ment into the body and there digesting it, tlie definition so changed 

 will cover all animals, excejDt certain parasites, and the few and ex- 

 ceptional 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 concep- 

 tion of the chemical difi'erences and resemblances between the con- 

 stituents of animal and vegetable organisms, for which Cuvier is not 

 responsible, 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, chemi- 

 cally 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. Amy- 

 laceous and saccharine substances are largely manufactured, even by 

 the highest animals ; cellulose is widespread as a constituent of the 

 skeletons of the lower animals ; and it is probable that amyloid sub- 

 stances are universally present in the animal organism, though not in 

 the precise form of starch. 



Moreover, although it remains true that there is an inverse re- 

 lation between the green plant in sunshine and the animal, in so far 

 as, under these circumstances, the green plant decomposes carbonic 

 acid, and exhales oxygen, while the animal absorbs oxygen and ex- 

 hales carbonic acid; yet the exact investigations of the modern chemi- 

 cal investigator of the physiological processes of plants have clearly 

 demonstrated the fallacy of attempting to draw any general distinc- 

 tion between animal and vegetable on this ground. In fact, the difler- 

 ence vanishes with the sunshine, even in the case of the green plant; 

 which, in the dark, absorbs oxygen and gives out carbonic acid like 

 any animal. While those plants, such as the fungi, which contain no 

 chlorophyl and are not green, are always, so far as respiration is con- 

 cerned, in the exact position of animals. They absorb oxygen and give 

 out carbonic acid. 



Thus, by the progress of knowledge, Cuvier's fourth distinction 

 between the animal and the plant has been as completely invalidated 

 as the third and second; and even the first can be retained only in a 

 modified form and subject to exceptions. 



But has the advance of biology simply tended to break down old 

 distinctions, without establishing new ones ? 



With a qualification, to be considered presently, the answer to this 

 question is undoubtedly in the affirmative. The famous researches of 

 Schwann and Schleiden, in 1837 and the following years, founded the 

 modern science of histology, or that branch of anatomy which deals 

 with the ultimate visible structure of organisms, as revealed by the 



