Corresponding changes have taken place in the climate. At first the temperature of the earth was much warmer than now, and uniform in all parallels of latitude, as is shown by the fossil remains. Now we have a great diversity of climate, whether we contrast the polar with the torrid regions, or the different seasons of the temperate zone with each other.
The same law of increasing diversity obtains in the fauna and flora of the various periods of geological history. The earliest fossil record of animal life is witness to the simplicity of organic structure. Among vertebrated animals, fishes first appear, next reptiles, then birds; still higher, the lower type of animals which suckle their young; and as the strata become more recent, still higher forms of mammalia, till we reach the upper tertiary, in which geologists have discovered the remains of many animals of complex structure nearly allied to those which are now in existence. In the historic period appear many organic forms of still greater complexity, with man at the head of the zoological series.
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.