In any inquiry into the history of the earth as a whole, we are met at the outset by a serious difficulty. In human affairs a general view of history, not confined to a single country, would be practically impossible, were we ignorant of the relations of the various eras from which different races reckon their dates: thus, it would be impossible to write a connected account of the history of Europe in the classical period were it not possible to determine the relation of the Olympian era to that dating from the foundation of the city of Eome. Yet the supposed case is not unlike that to which the geologist addresses himself when he endeavours to make a connected survey of such widely-separated regions as Europe, India, Australia, and America.

In the supposed case of the Greek and Roman eras, there are numerous points of contact, principally dates of battles, which, having been recorded by both nations according to their own system, enable us to compare the two, and so to determine what would be the date of any event, recorded by the one, had it been recorded by the other. But in geology we have no such points of contact; there is a very general tendency to regard any two series of beds, in which a few fossil forms specifically identical are found, as of contemporaneous origin. That this view is erroneous, and that it would be nearer the truth to say that two widely-separated beds, in which the same forms are found, could not be of contemporaneous origin, was long ago pointed out by Forbes and Huxley, the word homotaxis being invented by the latter to express the relation existing.