In this glance of zoological progress, we discover increasing complication of two kinds; for while the individual structure has been constantly becoming more complex, there are now in existence the analogues of the lowest fossil types, which, with the highest, and with all the intermediate, present a maze and vastness of complication, which, in comparison with the homogeneity of the aggregate of early structure, is sufficiently obvious and impressive. There is in this view, still another outline of increasing complexity. At first the same types prevailed all over the earth's surface; but as the soil, atmosphere, and climate changed, and the animal structure became more complex and varied, the limits of particular species became more and more localized, till the earth's surface presented zoological districts, with the fauna of each peculiar to itself. But, what of unitization? Here, there appears to be divergence only, and that continually increasing. Guyot says that 'the unity reappears with the creation of 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.)