112 EVOL UTION AND NATURAL THEOLOGY 



Previous to the Glacial Period, the Fauna 

 and Flora of the whole world were of a much 

 more uniform character than at present. The 

 preglacial Fauna and Flora appear to have 

 been less completely exterminated or modified 

 in America than in the Old World, which 

 would account for American forms being con- 

 stituted on lower and older types. Species 

 may remain unchanged for any length of 

 time, if their surrounding conditions remain 

 the same ; and Murray believes that California 

 still exhibits a somewhat pre -glacial character.* 



The greater part of North America, north 

 of Mexico, appears, however, to have been 

 extensively colonised from Northern Asia, 

 subsequently to the Glacial Period, as many 

 Classes of its productions are so closely related 

 to the great Palsearctic Eegion that it could not 

 be separated from it as a distinct region, were 

 it not that other Classes present well -marked 

 characters of their own. There is often so 

 great a resemblance between the productions of 

 Europe, Asia north and west of the Himalayas, 

 and America north of Mexico, and west of the 



* " Geographical Distribution of Mammals," ch. iv. & v. 



