122 GENERAL EVOLUTION. 



know of extinct sturgeons in the Southern Hemisphere. Indeed, 

 the Ganoid series is not well defined or known as yet. If, as 

 Agassiz states, the Siluroids pertain to it, it is cosmopolitan, 

 though least represented in the Palasarctic. 



Second, the Tailed or Urodele Batrachia. This order, entirely 

 characteristic of the Northern Hemisphere, is a group which com- 

 bines characters of Anura with those of the ancient forms, and 

 possesses in its Nearctic types many of low development. The 

 Gymnophidia of the Southern HemisjDhere can not be considered 

 inferior to them. In the possession of this group the Northern 

 Hemisphere presents its first element of inferiority. 



The preceding comparisons indicate also the relations of the 

 regions proper to each other. It is obvious enough that the Ethi- 

 opian is much superior to the two others of the Southern Hemi- 

 sphere. As to the Australian and Neotropical, the former must 

 still be regarded as probably the most ancient, though possessing 

 at the same time a much stronger admixture of northern forms. 

 I liave already presented the relations, with the inferior forms of 

 each, thus : * 



R. Australis. — Inferior in possessing Monotrematous and Marsu- 

 pial Mammalia, Pullastriform and Struthious Birds, Serpent- 

 iform Pleurodont Lacertilia, Arciferous Batrachia, Pleuro- 

 dire Tortoises, its Elapid venomous snakes, and the whole 

 Flora, according to linger. 



B, Neotropica. — Marsupial and Edentate Mammalia, Inferior 

 Rodentia and Quadrumana, Pleurodire Tortoises, Pleurodont 

 Lacertilia, Arciferous Batrachia, Clamatorial and Pullas- 

 triform Birds, Characin and Erythrinid Malacopterigii. 



Conclusions, 



The following may be looked upon as conclusions which have 

 been indicated in the preceding pages : 



I. Species have developed from pre-existent species by an in- 

 herent tendency to variation, and have been preserved in given 

 directions and repressed in others by the operation of the law of 

 natural selection (Darwin). 



II. Genera have been produced by a system of retardation or 

 acceleration in the development of individuals ; the former on 

 pre-established, the latter on preconceived lines of direction. Or, 



«■ " 



Jour. Acad. Nat. Sci.," Philada., 1866, p. 109. 



