IN THE VARIOUS GROUPS OF ANIMALS AND PLANTS. 99 



An account was next given of the properties of pure cry- 

 stallized Sugar, illustrated by specimens of sugar in that state, 



dom will be still more apparent, if we exclude from consideration those 

 which do not in reality form part of the healthy system. The four animal 

 substances above mentioned, are perhaps the most definite combinations 

 which are peculiar to animal chemistry ; but Cholesterine and Urea are 

 merely the results of excretory operations, the manifest design of which is 

 to remove useless or injurious matter from the system ; and the sugar of 

 milk forms part of a fluid which is not truly an element of the vital system 

 of the animal producing it, but is intended to be digested and converted into 

 principles more highly animalized, and (consequently ?) less definite, in the 

 stomach of its young. The only substance, among those above enumerated, 

 which occurs as an element of the healthy system, is fat, and that is pre- 

 cisely the one, which, so far as our present knowledge extends, is the least 

 definite of the whole. 



The case, however, is different, when, confining our attention to one of the 

 organic kingdoms, we compare with each other different groups of animals 

 or of vegetables, of higher and lower organization, respectively ; for it is the 

 highest of these only, in each kingdom, which furnish definite and stable 

 principles of this kind. Thus none but the Pertebrata and Annulosa, 

 among Animals, appear to yield non-metallic compounds so definite and 

 so stable as the Sugar of Milk, Urea, and Elytrine ; while none but the 

 Dlcotyledonea and Monocotyledonea, which correspond to them in station 

 among Vegetables, yield principles so definite and stable as the woody fibre, 

 sugar, and starch. It is necessary, however, in tracing analogies of chemi- 

 cal constitution between subjects of the two organic kingdoms, to bear in 

 mind the important fact, that although such analogies may as readily be 

 discovered, and are as perfect, as those by which every considerable group in 

 one kingdom, is represented by a group in the other, (as has been shown by 

 Desfontaines, MacLeay, and Agardh,) yet the series of chemical substances 

 which forms the organization of each kingdom is radically different. Thus, 

 the phosphate and carbonate of lime, constituting, either separately, or in 

 combination, the skeleton, in almost every group of animals, are repre- 

 sented, throughout the vegetable kingdom, by the woody fibre or lignin, or 

 by some compound of carbon, oxygen, and hydrogen nearly related to that 

 substance. If this essential difference of the series be not borne in mind, we 

 shall be likely to overlook analogies where they really exist, and to imagine 

 them where they have no real existence. 



With respect to those products of organization which occur also in the 

 mineral kingdom, or which can readily be produced by art, and to the com- 

 position of which an alkaline or earthy metal is essential, such as carbonate 

 and phosphate of lime, other laws seem to prevail. When simple binary 

 salts (as those just mentioned, for example) constitute animal textures, 

 they appear to differ in their state of aggregation only, in each animal or 

 group of animals. Thus, the carbonate of lime which forms the shell of the 

 Ckama Gigas among the Mollusca has nearly the structure of fine-grained 

 saccharoid marble ; while that which constitutes the tube of the Teredo 

 gigantea, among the Annelida, is formed of fibrous crystals. When two 

 or more such salts are united, or intimately mingled together, to form an 

 animal texture, as in the bones and the teeth of the fertebrata, the law 

 which applies to the non-metallic compounds remains valid, and is manifested 

 by a difference in the respective proportions of the salts. The bones of the 

 Lion, for example, contain different proportions of phosphate and carbonate 

 of lime from those of the Sheep ; and the teeth of the Mammalia differ in 

 the same manner from those of the Amphibia. 



Probably the several modes of existence of the proximate constituents of 



H 2 



