This system of classification is founded exclusively upon the anatomical structure of animals, whence comparative anatomy is the source of its materials. It not only rejects the habits and properties of animals as immaterial and transient, but it also leaves out of consideration their mental endowments, which, how ever important in other respects, were incapable of affording a basis of classification. Under its clear and definite discriminations all the species of each of the four great branches of the animal kingdom are seen in intelligible and harmonious relations, notwithstanding their striking diversities of form. Unity of type runs through the structural organization of all the individuals comprised in each Of these branches. The grandeur of this fourfold plan of creation is not more impressive than the wonderful adaptation of the sur rounding elements to the condition and wants of the multitude of animal organisms which God has made.