114 ^lASSACHUSETTS HORTICULTURAL SOCIETY. 



and as the glacial sheet advanced southward, the plants of central 

 Europe were caught between it and the increasing local glaciers 

 from the Pyrenees, Alps, Apennines, and Caucasus ; there 

 being little chance for escape mau}^ of the forms were extermi- 

 nated, particularly those not having natural means of wide 

 dissemination. And many of those which escaped the icy jaws of 

 the glaciers were pushed across the Mediterranean. This they 

 were unable to recross when the glacial ice-sheet retreated, and but 

 few of the exiled forms could return to their homes again. That 

 this was the true course of events is rendered nearly certain by 

 the fact that these missing forms are found fossil in late deposits 

 in central Europe, showing that they existed there anterior to the 

 Glacial epoch. Central Asia has to some extent suffered in a 

 similar way, and Greenland has its flora swept off into the deep 

 sea, and it has not now the plants to which the climate of its 

 southern part entitles it. But the conditions Avere very different in 

 eastern America, and in eastern Asia. Here the mountain ranges 

 run north and south, or nearly so, and in Asia the great ranges 

 end before they reach the sea. Thus a free passage was open 

 along which the plants travelled south out of harm's way, and 

 north again when the enemy had retreated. Hence the floras of 

 eastern America and eastern Asia are both exceedingly rich in 

 species and most strikingly alike, each having preserved nearly all 

 of its species through the vicissitudes of glacial times. 



But why is not the flora of western America rich and like that 

 of eastern America? This question is not so easy to answer. It 

 is true the Rockies run north and south ; why were not all the 

 western plants preserved? Several causes have doubtless con- 

 tributed to destroy them, one of which is the extreme narrowness of 

 the region west of the great mountains, and, hence, the crowding 

 and extermination of some species, and another the fact that the very 

 uniform climate of California is not so favorable to the northern 

 species, accustomed to more variable conditions, as it is to the 

 more southern forms of Mexico ; and these latter have travelled 

 in large numbers into California and possibly by direct competi- 

 tion have exterminated some species. Certain other geological 

 conditions have doubtless contributed somewhat to the same end. 

 But the fact remains that like that of Europe, the flora of the 

 Californian region is poor in temperate species, while those of 

 eastern America and Japan are rich and nuich alike. 



