IX.] WESTERN DIVISION. 345 



In America, probably owing to the north and south trend of 

 the mountain-ranges, the glacial period has had an even more 

 marked effect than in the Old World. On this subject Dr 

 Merriam 1 writes that "not only are the pre-Plistocene animals and 

 plants now represented imperfectly and in greatly reduced num- 

 bers, but the areas at present inhabited by their descendants, 

 except in the case of the Boreal forms, are insignificant in com- 

 parison with their former extent. It should be remembered that 

 the refrigeration of the glacial epoch has only in part disappeared. 

 In earlier Pliocene times, characteristic representatives of sub- 

 tropical faunas and floras existed northwards over much of the 

 United States and Canada, and in still earlier times reached the 

 Arctic circle. During the advance of cold in the glacial epoch 

 these forms were either exterminated or driven southward into the 

 narrow tropical parts of Mexico and Central America. The retreat 

 of cold at the termination of this period was not complete, and 

 our continent has never regained its former warmth. Hence the 

 expelled species were not permitted to advance more than a short 

 distance into the region formerly occupied by them, and the 

 tropical species have been held back, and at the present day are 

 not found except along the extreme southern confines of our 

 territory [the United States]. For example, peccaries in early 

 Plistocene times ranged northward over a large part of western 

 North America, while at present they are restricted to parts of 

 Texas and Louisiana below the Red River of the south ; and 

 capivaras, tapirs and other tropical forms whose fossil remains 

 have been found in many parts of the United States have not been 

 able to return. The same is true of plants, for the palms, tree- 

 ferns, and numerous other tropical types that formerly ranged over 

 much of our country are now either altogether extinct or exist 

 only in the tropics. 



"The llama and many plants now inhabiting the Andes may 

 be looked upon as representing a class of cases in which Boreal 

 forms were driven so far south that they actually reached the great 

 mountain-system of South America and spread southward over its 

 elevated plateaus and declivities to the extreme end of the conti- 

 nent in Patagonia and Tierra del Fuego." 



1 Appendix, No. 19, p. 44. 



