Contemporaneity

Contemporaneity, in Geology, has a somewhat more extended signification than the word bears in ordinary language. When a geologist speaks of the Silurian systems of America and Europe having been accumulated contemporaneously, he does not infer that they were precisely synchronous, but merely implies that each occupies the same relative position in the succession of systems. Each was preceded by a Cambrian and succeeded by a Devonian system; but for ought he can tell, the Silurian period may have commenced earlier or endured longer in one area than the other. Professor Huxley has suggested the term homotaxis ('similarity of order') as a substitute for contemporaneity. According to this view we should apply the term homotaxial instead of contemporaneous to widely separated systems which contain the same assemblages of fossils—all that the proposed term indicates being this, that the order of organic succession was the same in both regions. All the evidence, however, goes to show that in the earlier ages of the world the climate of the earth was not differentiated into zones as it is now, so that faunas and floras were enabled to extend themselves readily in every direction. Hence, although it is impossible to say that any particular bed of palæozoic or mesozoic limestone in Europe is of precisely the same age as a similar limestone in America, yet it seems in the highest degree probable that the great system to which both these limestones happen to belong was accumulated during one and the same long-continued period. Geologists seem to be justified, therefore, in describing the successive systems of the Old and New Worlds as actually contemporaneous.