COMPARISON BETWEEN PLANTS AND ANIMALS. 205 



the formation of sugar, which most probably is solely produced from 

 the metamorphosis of nitrogenous substances (vol. ii, p. 90), as I 

 have recently proved by careful observations."* We must remember 

 too what a number of substances are formed in the animal body 

 which never occur in the vegetable kingdom. It has indeed been 

 stated that these substances are only the products of a process of 

 oxidation, but what an essential difference there is between xanthine 

 or uric acid, and their homologues, theme and theobromine ! and 

 who could determine, on seeing taurine or cystine, whether it were 

 derived from the vegetable or the animal kingdom, if he were 

 ignorant of the origin of these substances ? Those complex sub- 

 stances, the biliary acids, have no analogues in the vegetable 

 kingdom, nor can we deny to the animal organism generally the 

 property of generating new organic matters within itself; but in 

 this respect the animal organism is very much in the same con- 

 dition as the chemist in his laboratory ; both require, for the most 

 part at least, ready-formed organic matter, from which to generate 

 new substances foreign to, but analogous with the products of the 

 vegetable kingdom. As many of the excreted matters of the 

 animal body contain somewhat complex atoms of organic matter, 

 the proposition that animals give off to the external world carbonic 

 acid, water, and ammonia, is only half true : for although we 

 regard the urine collectively as an ammoniacal salt, and even take 

 the same view regarding the taurine of the solid excrements and 

 although, further, we comprise under the same head, and as of 

 equal value with the carbonic acid and water, the formic, butyric, 

 acetic, and caproic acids of the sweat it yet cannot be denied that 

 men and animals daily give off directly to the external world no 

 inconsiderable amount of protein-bodies ; since the solid excre- 

 ments are never free from mucus, and since the desquamation of 

 the epithelium and the abrasion of other horny tissues occasion a 

 loss in these complex bodies, the amount of which may even be 

 ascertained by moderately careful investigations. 



It is also perfectly true that the vegetable cell, which is capable 

 of overpowering the strongest chemical combinations, eliminates 

 oxygen from the atmospheric ingredients, carbonic acid, and water, 

 and is able to fix the indifferent substance, nitrogen, whilst the 

 animal germ can only be developed by the co-operation of atmo- 

 spheric oxygen. It cannot be denied that the separation of oxygen 

 in the vegetable organism constitutes one of the principal momenta 

 of chemico-vital activity, and that a progressive motion is induced 

 * Ber. d. k. sachs. Ges. d. Wiss. zu Leipzig, 1851, S. 130-164. 



