GEOGRAPHICAL DISTRIBUTION. 113 



Rocky Mountains and the Andes, that if we 

 confined ourselves to the consideration of those 

 groups of animals and plants in which this 

 resemblance is most marked, we should be com- 

 pelled to regard all the various Faunas and 

 Floras as offshoots of one stock, from Ireland 

 to Chili. Of course different regions of this 

 vast region have different characteristics, and 

 the temperate forms are only found under the 

 equator at a great height in the Andes ; but 

 the Andes, and further north, the table-land of 

 Mexico form barriers which they do not pass, as 

 we find no trace of them either in Mexico, the 

 West Indies, or South America, east of the 

 Andes. It may here be mentioned, that as 30 

 per cent, of tropical marine fish are met with on 

 both sides of the Isthmus of Panama, Lubbock 

 thinks it possible that the isthmus itself may be 

 of modern origin.* This suggestion is, how- 

 ever, rendered very doubtful both by the pre- 

 sence of European forms in Chili, and by 

 the purely South American character of the 

 Central American and Mexican Faunae, which 

 cannot have been derived from the West Indies, 



* " Prehistoric Times," p. 393. 



