The plant, as has been shown, performs only one set of functions, the organic; while the animal performs two sets of functions, the organic and the animal. The animal, then, performs more functions than the plant, and functions of a higher order; it carries on its functions with a greater degree of energy; its functions have a more extended range, and all its functions bear a certain relation to each other, maintaining an harmonious action. The number, the superiority, the relation, the range, and the energy of the functions performed by the animal are, then, so many conditions, which render it absolutely indispensable that it should possess a greater complexity of structure than the plant. 1. To build up structure is to create, to arrange, and to connect organs. Organs are the instruments by which functions are performed, and without the instrument there can be no action. With as many more organs than the plant possesses the animal must, therefore, be provided, as are necessary to carry on the additional functions it performs. Organs, for its organic functions, it must have as well as the plant; but to these must be superadded organs of another class, for which the plant has no need, namely, organs for its animal functions. Two sets of organs must, therefore, be provided for the animal, while the plant requires but one. 2. Some functions performed by the animal are of a higher order than any performed by the plant, and the superior function requires a higher organization. The construction of an organ is complex as its action is elevated; the instrument is elaborately prepared in proportion to the nobleness of its office. [Illustration: Fig. I.] [Illustration: Fig. II.] [Illustration: Fig. III.] [Illustration: Fig. IV.]