[Illustration: FIG. 3.—Bluff on Beaver Creek, Fremont Co., Wyoming. The White River beds were deposited on the worn and weathered surface of the Uinta, the heavy, broken line marking the separation between them. The valley was carved out long after the deposition of the White River strata.] In the second place, there are great differences in the contemporary life of separate regions and such geographical differences there have always been, so far as we can trace back the history of animals and plants. A new organism does not originate simultaneously all over the world but, normally at least, in a single area and spreads from that centre until it encounters insuperable obstacles. Such spreading is a slow process and hence it is that new forms often appear in one region much earlier than in others and in the very process of extending their range, the advancing species may themselves be considerably modified and reach their new and distant homes as different species from those which originated the movement. Extinction, likewise, seldom occurs simultaneously over the range of a group, but now here and now there in a way that to our ignorance appears to be arbitrary and capricious. The process may go on until extinction is total, or may merely result in a great restriction of the range of a given group, or may break up that range into two or more distinct areas. Of such incomplete extinctions many instances might be given, but one must suffice. The camel-tribe, strange as it may appear, originated in North America and was long confined to that continent, while at the present day it is represented only by the llamas of South America and the true camels of Asia, having completely vanished from its early home. These facts and a host of similar ones make plain how necessary it is to take geographical considerations into account in all problems that deal with the synchronizing of the rocks of separate areas and continents. Properly to estimate the significance of a difference in the fossils of two regions and to determine how far it is geographical, due to a separation in space, or geological and caused by separation in time, is often a very difficult matter and requires a vast amount of minute and detailed study. Once more, the principle involved is illustrated by the study of manuscripts. Down to the time when the printing press superseded the copyist, each of the nations of Europe had its own traditions and its more or less independent course of handwriting development. A great monastery, in which the work of copying manuscripts went on century after century, became an independent geographical centre with its particular styles. Thus the palæographer, like the geologist, is confronted by geographical problems as well as by those of change and development in general.