140 GENERAL EVOLUTION". 



common origin. The relation of inexact parallelism is readily ex- 

 plained as follows : With a case of exact imrallelism in the mind, 

 let the repression producing the character of form B parallelize 

 the latter with a stage of form A in which a second part is not 

 quite mature : we will have a slight want of correspondence be- 

 tween the two. Form B will be immature in but one point, the 

 incompleteness of A higher being seen in two points. If we 

 suppose the immaturity to consist in a repression at a still earlier 

 point in the history of the higher, the latter will be undeveloped 

 in other points also : thus, the spike-horned deer of South Amer- 

 ica have the horn of the second year of the North American genus. 

 They would be generically identical with that stage of the latter, 

 were it not that these still possess their milk dentition at two 

 years of age. In the same way the nature of the parallelisms seen 

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



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 sec- 

 tions, which I have called Jiomologous. These are series of genera 

 or families, which differ from each other by some marked charac- 

 ter, 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 ho- 

 mologous 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 Avord, we learn from this source that distinct geologic 

 epochs co-exist at the same time on the earth. I have been forced 

 to this conclusion f 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 extinct family of the NimravidiB, which is homologous with the existing 

 family of Felid<B, has been discovered since this was written. (Ed. 1886.) 

 f " Origin of Genera," pp. TO, 11, 79. 



