The same physical agents are indispensable to the support of the animal body; but the animal cannot be sustained by these physical agents alone; for the maintenance of animal life, in some shape or other, vegetable or animal matter, or both in a certain state of combination, must be superadded: hence another distinction between the plant and the animal,—the necessity, on the part of the animal, of an elaborated aliment to maintain its existence. By the vital processes of its economy, the plant converts inorganic into organic matter; by the vital processes of its economy, the animal converts matter, already rendered organic, into its own proper substance. The plant is thus purveyor to the animal: but it is more than purveyor to it; for while it provides, it also prepares its food; it saves the animal one process, that of the transmutation of inorganic into organic matter. The ultimate end, or the final cause of the vital processes performed by the first class of living beings, is thus the elaboration of aliment for the second: the inferior life is spent in ministering, and the great object of its being is to minister to the existence of the superior. At the point at which organization commences structure is so simple that there is no manifest distinction of organs. Several functions are performed apparently by one single organized substance. The lowest plants and the lowest animals are equally without any separate organs, as far as it is in our power to distinguish them, for carrying on the vital actions they perform. An organized tissue, apparently of an homogeneous nature, containing fluid matter, is all that can be made out by which the most simply-constructed plant carries on its single set, and by which the most simply-constructed animal carries on its double set, of actions. But this simplicity of structure exists only at the very commencement of the organized world. Every advancement in the scale of organization is indicated by the construction of organs manifestly separate for the performance of individual functions; and, invariably, the higher the being, the more complete is this separation of function from function, and, consequently, the greater the multiplication of organs, and the more elaborate and complex the structure;—and hence another distinction between the plant and the animal. The simplicity of the structure of the plant is in striking contrast to the complexity of the structure of the animal; and this difference is not arbitrary; it is a matter of absolute necessity, and the reason of this necessity it will be instructive to contemplate.