ox THE OEIGIX OF GENERA. 95 



geologic epoch to another by a change of generic form ; and has 

 not the genus been transferred from one epoch to another under 

 change of ordinal type, and as a consequence the same species ? 



As a reply I propose to render the affirmative of the first of 

 these questions highly probable. 



Paleontology only will be able to answer this question conclu- 

 sively, though, as we have abundant evidence that the relations of 

 species to genera and other higher groups were the same then as 

 now, we may look to the present status as furnishing important 

 evidence on the subject. ^Ye are turned at once to the probable 

 history of development in the separate zoological areas of the 

 earth's surface. The question may be asked. Are the present zo- 

 ological regions on an equal plane as to the geologic relations of 

 their faunae, or are they related as the different subdivisions of a 

 geologic period in time ? 



I have on a former occasion asserted that the latter of these 

 propositions was true.* 



a. Of Homologous Groups. 



Naturally following the admission of a developmental succes- 

 sion of organic beings is the question of its relation to the differ- 

 ent surfaces of land and water on the earth. The following con- 

 siderations bear on this subject. 



Among the higher groups of animals can be detected series 

 ''homologous" on the same principle as the alcohols (? com- 

 pound radicals) and their derivatives ; and the component types 

 of each can be, and have been in many instances, shown to be 

 "heterologous," as are the ethers, mercaptans, aldehydes, acids, 

 etc. Among Mammalia two partly homologous series have been 

 pointed out, Implacentialia and Placentialia ; possibly such are 

 the types Altrices and Prsecoces among Aves ; of a lesser grade 

 in this class are the parallel series of Pullastrae and Gallinae, of 

 Clamatores and Oscines. Among tortoises I have alluded to the 

 Pleurodira as compared with the remainder of the order, already 

 parallelized by Wagler ; and, of lesser grades, the series among 

 Lacertilia of Acrodonta and Ignania, parallelized by Dumeril and 

 Bibron, and of Teidai and Lacertidae, compared by Wiegmann. I 

 have discovered a full parallelism between the Eaniform and Arcif- 

 erous Anura. It is carried out between the Characini and a group of 

 remaining Physostomous Fishes, perhaps not yet well defined ; it 



*0a "Arciferous Anura," " Journ. Ac. Nat. Sci.," 1866, p. 108. 



