in higher groups, as orders, etc., may be accounted for. 



The theory of homologous groups furnishes impor- 

 tant 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 corres- 

 pond exactly. So striking is this correspondence that 

 were it not for the" general and common character sepa- 

 rating 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 differ- 

 ent geographical areas on the earth's surface, and their 

 relation is precisely that which holds between succes- 

 sive groups of life in the periods of geologic time. 



In a word, we learn from this source that distinct ge- 

 ologic epochs coexist at the same time on the earth. I 

 have been forced to this conclusion* 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 At- 

 lantic. These have brought to light types of Tertiary 

 life, and of even the still m6re ancient Cretaceous pe- 

 riods, living at the present day. That this discovery 

 invalidates in any wise the conclusions of geology re- 



* Origin of Genera, pages 70, 77, 79. 



