2. In different countries, isolated from one another and under different conditions, evolution takes different _directions_ and different _rates_, producing geographical diversity in each geological period. This diversity increases with time as long as the isolation continues. If this were all, the geographical diversity by continued divergence would have become so great that it would be impossible even approximately to determine any geological horizon. The history of each country must be studied for itself. A general history of the earth would be impossible. But this also is not true. There is therefore still another element. 3. From time to time, at long intervals--i. e., _critical periods_--there are widespread readjustments of the crust to internal strain, determining changes of physical geography and of climate, and therefore wide migrations of species with mingling and conflict of faunas. This would produce more rapid movement of evolution, but at the same time more or less complete obliteration of geographical diversity. 4. After these periods of migrations and minglings there would be re-isolations in new localities, and the process of diversification would recommence and increase as long as the isolation continues. The last of these critical periods of migrations and minglings and struggles for life among competing species was the _Glacial epoch_ or ice age. Therefore the present geographical distribution of species was largely determined by the extensive migrations of that time. II. COSMOPOLITAN AND PROVINCIAL FAUNAS.--There are apparently in the history of the earth periods of widespread or cosmopolitan faunas, alternating with localized or provincial faunas. The cosmopolitan periods are usually times of prevalence of limestones or organic sediments, and the fossils are very abundant. The provincial periods are usually characterized by sandstones and shales or mechanical sediments, and are comparatively poor in fossils. Moreover, it is believed that the cosmopolitan limestone periods are oceanic periods--i. e., periods of wide oceans and lower and smaller continents and little erosive activity, while the sandstone periods, characterized by provincial faunas, are periods of higher and larger continents, and therefore of great erosion and abundant mechanical sedimentation.