The theory of homologous groups furnishes important evidence in favor of derivation. Many orders of animals (probably all, when we come to know them) are divisible into two or more sections, which I have called _homologous_. These are series of genera or families, which differ from each other by some marked character, but whose contained genera or families differ from each other in the same points of detail, and in fact correspond exactly. So striking is this correspondence that were it not for the general and common character separating the homologous series, they would be regarded as the same, each to each. Now it is remarkable that where studied the difference common to all the terms of two homologous groups is found to be one of _inexact parallelism_, which has been shown above to be evidence of descent. Homologous groups always occupy different geographical areas on the earth’s surface, and their relation is precisely that which holds between successive groups of life in the periods of geologic time. In a word, we learn from this source that distinct geologic epochs coexist at the same time on the earth. I have been forced to this conclusion[47] by a study of the structure of terrestrial life, and it has been remarkably confirmed by the results of recent deep-sea dredgings made by the United States Coast Survey in the Gulf Stream, and by the British naturalists in the North Atlantic. These have brought to light types of Tertiary life, and of even the still more ancient Cretaceous periods, living at the present day. That this discovery invalidates in any wise the conclusions of geology respecting lapse of time is an unwarranted assumption that some are forward to make. If it changes the views of some respecting the parallelism or coëxistence of faunæ in different regions of the earth, it is only the anti-developmentalists whose position must be changed. Footnote 47: _Origin of Genera_, pages 70, 77, 79. For, if we find distinct geologic faunæ, or epochs defined by faunæ, coëxisting during the present period, and fading or emerging into one another as they do at their geographical boundaries, it is proof positive that the geologic epochs and periods of past ages had in like manner no trenchant boundaries, but also passed the one into the other. The assumption that the apparent interruptions are the result of transfer of life rather than destruction, or of want of opportunities of preservation, is no doubt the true one. δ. Rationale of Development.