It cannot be assumed that the stratum _A_ is therefore contemporaneous with _A´_, _B_ with _B´_, and _C_ with _C´_, but on the other hand, it must not be assumed that they are not contemporaneous. This is a statement which requires some comment. It has been urged that if the deposits _A_ and _A´_ in different localities contain the same fauna, this is a proof that the two are not contemporaneous, for some time must have elapsed in order to allow of the migration of the organisms from one area to another, it being justifiably assumed that they did not originate simultaneously in the two areas. But everything depends on the time taken for migration as compared with the period of existence of the fauna. If the former was extremely short as compared with the latter it may be practically ignored, for we might then speak of the strata as contemporaneous, just as a historian would rightly speak of events in the same way which occurred upon the same afternoon, though one might have happened an hour before the other. Let us then glance at the evidence which we have at our disposal, which bears upon this matter. The objection to identification of strata with similar faunas as contemporaneous was urged by Whewell, Herbert Spencer, and Huxley, and the latter suggested the term Homotaxis or similarity of arrangement as applicable to groups of strata in different areas, in which a similar succession of faunas was traceable, maintaining that though not contemporaneous the strata might be spoken of as homotaxial. Huxley went so far as to assert that "for anything that geology or palæontology are able to show to the contrary, a Devonian fauna and flora in the British Islands may have been contemporaneous with Silurian life in North America, and with a Carboniferous fauna and flora in Africa[13]," a statement which few if any living geologists will endorse. If the statement be true, and the fauna which we speak of as Devonian, when present be always found (as it is) above that which we in Britain know as Silurian and below that which we term Carboniferous, the faunas must have originated independently in the three centres, and disappeared before the appearance of the next fauna, or having originated at the same centre, each must have migrated in the same direction, spread over the world, and become extinct as it reached the point or line from which it started. Suppose for instance a fauna _A_ originates at the meridian of Greenwich, migrates eastward, and dies out again when it once more reaches Greenwich, that _B_ and _C_ do the same, at a later period, then the fauna _B_ will always be found above _A_ and _C_ above _B_, but if _B_ did not become extinct when it reached the Greenwich meridian, it would continue its eastward course, and _C_ having in the meantime started on its first round, the fossils of the fauna _B_ would be found both above and below those of _C_. It will be shown below that