Our purpose being to show that while complexity of structure is constantly increasing, unitization, or the organized dependence of one part on another, is, at the same time, becoming more complete, we shall refer briefly to the comparative anatomy and physiology of animals. There is in this connection such wealth of material--a long chain of animal beings with all grades of structure from very simple to very complex; each complex animal, in its development from the ovum, passing through all the lower types of structure in succession; so many new organs and functions arising in the course of this development; each organ so arising, becoming, in its turn, more complex in structure, more specialized in function, and more dependent on the office of other organs;--in the midst, I say, of all this wealth of material, indicated here in a great general and imperfect manner, the difficulty, in so brief an exposition as this, is to know what facts to seize upon as calculated to illustrate most aptly the principle under consideration. The development of the senses, with reference to their organs, nerves, and functions, presents a striking illustration of increasing complexity. In the lowest forms of animal life, we find general sensibility only, and it is claimed that this exists in the lowest forms, without even the presence of nerves. But as we rise higher in the scale, the special organs of sense gradually become developed--one new sense after another appears; but this is not the only line of increasing complexity. When an organ of sense first appears, its function is of the simplest character; and it is only when we reach the highest types of animal life that it performs the greatest variety of offices peculiar thereto. That of touch is, at first, but crude and simple, becoming delicate and complicated only in the highest types. The sense of pain is a differentiated function, possessed only in a slight degree by reptiles and fishes, and probably not at all by animals still lower in the scale. The eye-spots of star fishes and jelly fishes simply distinguish light from darkness, much as we do with our eyes closed. There are many degrees of development from this condition of the inferior organism to that of the human eye, which distinguishes the nicest shades of color, distance, form, and size of objects, and the play of passion on the human countenance.