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man, who combines in his physical nature all the perfections of the animal, and who is the end of all this long progression of organized beings.' Agassiz recognizes man alone as cosmopolite; and Comte regards him as the supreme head of the economy of nature, and representative of the fundamental unity of the anatomical scale. But another and more obvious example of unitization in complexity, is derivable from the consideration of the animal organism, and will soon be given. We will merely mention in passing, that the most complex animals, in the various stages of fetal development through which they pass, correspond to the types of structure which are permanent in the lower forms of animal life. Thus, in the zoological chain, there are beings of all grades, from the most simple in structure to the most complex; and the most complex animal, in its development from the ovum or egg, passes through all these grades of structure, ending in that which is above all, and distinctively its own. 'Without going into tedious details, man presents, as regards the most important of his constituent structures, his nervous system, the successive characteristics of an avertebrated animal, a fish, a turtle, a bird, a quadruped, a quadrumanous animal, before he assumes the special human characteristics.' (Draper.) 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 s
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