Animals, as a class, are completely dependent upon vegetables for their existence. There is every reason to believe that the most lowly organised beings in the scale of animal life, even those of so simple a structure as to have been long regarded as vegetables or as plant-animals, are incapable of organising mineral matter. The so-called vegetative life of animals--for I believe the term to be exceedingly inexact--is applied to their growth, that is, to the increase in their weight. This increase takes place by their power of reorganising, or of assimilating to the nature of their own organisms, certain of the substances elaborated by plants, and destined to become food for animals. SECTION II. COMPOSITION OF ORGANIC SUBSTANCES. _Elements of Organic Bodies._--The number of distinct kinds of substances--each distinguishable from all the others by the peculiarity of its properties, taken as a whole--is exceedingly great, yet all these substances are resolvable into a very small number of bodies. As an illustration, I shall take a well-known substance, common green copperas, or, as the chemists term it, protosulphate of iron. By submitting this compound to the process termed chemical analysis, two other kinds of matter may be obtained from it, namely, oxide of iron and oil of vitrol, or sulphuric acid. If we continued this process--if we submitted the acid and the oxide to analysis--we could separate the former into sulphur and oxygen, and the latter into iron and oxygen. Now, by these means we could demonstrate the compound nature of copperas; we could prove that it was _proximately_ composed of sulphuric acid and oxide of iron; and, _ultimately_, of iron, sulphur, and oxygen.