Page:The Journal of geology (IA journalofgeology21894univers).pdf/384

As long ago as 1853 Pictet, in his then celebrated Traité de Paléontologie, presented a number of general principles, among them being one, the so-called eighth law, which bears directly upon the present question. It is as follows: "Contemporaneous deposits, or those formed at the same epoch, contain identical fossils. Conversely: deposits which contain identical fossils are contemporaneous." This was modified by Schimper,^[1] the celebrated French paleobotanist, who added that deposits "formed at the same epoch, contain floras, if not completely identical, at least homologous, and consequently deposits that contain identical or homologous floras are contemporaneous." But Huxley appears to have been the first (1862) to formulate clearly the objections to this law. He pointed out that while the succession of life in widely separated localities may be shown to have been similar, it by no means follows that the identical elements in these widely separated localities were strictly contemporaneous. To this he applied the term homotaxis, which implies that the plants and animals of widely separated places may have had practically the same process of development or succession, yet when the element of time is considered they may have been far from identical. As an example it may be mentioned that the most abundant and typical genus of plants in the Carboniferous rocks of Australia and Tasmania is Glossopteris, a genus which is not represented in rocks of similar age in Europe, but occurs in Upper Mesozoic beds of that region.

This, it will be readily understood, applies to localities widely separated, as for example between continents that are not intimately connected, or that are now and have been for a long geological period separated by insurmountable barriers to immigration, such as oceans and mountain chains. The plants originating within a given area or the ones inhabiting a locality adapt themselves to the environment, and these can only extend their distribution readily to areas in which the conditions are similar. Hence if the particular locality in which a species has been developed is separated from other areas, perhaps as well suited