16 ANIMALS CONTRASTED 



at once in favor of a vegetable nature, whatever might be the 

 attributes which seem to point to an opposite conclusion. The 

 power of living upon organic matter would seem to be less de- 

 cisive of an animal nature, for some fungi appear to derive sup- 

 port almost entirely from this source. 



II. There is, commonly, a marked difference in general 

 chemical composition between vegetables and animals, even in 

 their lowest forms ; for while the former consist mainly of a 

 substance containing carbon, hydrogen, and oxygen only, ar- 

 ranged so as to form a compound closely allied to starch, and 

 called cellulose, the latter are commonly composed in great 

 part of the three elements just named, together with a fourth, 

 nitrogen; the proximate principles formed from these being 

 identical, or nearly so, with albumen. It must not be supposed, 

 however, that either of these typical compounds alone, with 

 its allies, is confined to one kingdom of nature. Nitrogenous 

 or albuminous compounds are freely produced by vegetable 

 structures, although they form an infinitely smaller proportion 

 of the whole organism than cellulose or starch. And while 

 the presence of the latter in animals is much more rare than 

 is that of the former in vegetables, there are many animals in 

 which traces of it may be discovered, and some, the Ascidians, 

 in which it is found in considerable quantity. 



III. Inherent power of movement is a quality which we so 

 commonly consider an essential indication of animal nature, 

 that it is difficult at first to conceive it existing in any other. 

 The capability of simple motion is now known, however, to 

 exist in so many vegetable forms, that it can no longer be held 

 as ail essential distinction between them and animals, and 

 ceases to be a mark by which the one can be distinguished 

 from the other. Thus the zoospores of many of the Crypto- 

 gamia exhibit movements of a like kind to those seen in animal- 

 cules ; and even among the higher orders of plants, many ex- 

 hibit such motion, either at regular times, or on the application 

 of external irritation, as might lead one, were this fact taken 

 by itself, to regard them as sentient beings. Inherent power 

 of movement, then, although especially characteristic of ani- 

 mal nature, is, when taken by itself, no proof of it. Of course, 

 if the movement were such as to indicate any kind of purpose, 

 whether of getting food or any other, the case would be differ- 

 ent, and we should justly call a being exhibiting such motion, 

 an animal. But low down in the scale of life, where alone 

 there exists any difficulty in distinguishing the two classes, 

 movements, although almost always more lively, are scarcely 

 or not at all more purposive in one than the other ; and even 

 if we decide on the animal nature of a being, it by no means 



