112 MASSACHUSETTS HORTICtTLTURAL SOCIETY. 



the species themselves is hardly ever sutficieut to explain this. 

 But the solution is undoubtedly found in late geological historj'. 

 In the glacial period, geologically very late, as you know, there 

 descended from the north a great ice sheet of enormous thickness, 

 which drove all vegetation before it. It came so far down that 

 Arctic plants were driven to the south of the region in which we 

 are now living and arctic plants flourished all across the United 

 States just south of a line from about 40° north latitude on the 

 Atlantic coast, running diagonally north- wes toward, and in corres- 

 ponding regions in the eastern hemisphere. Along the high 

 mountain ranges these plants extended much farther south, finding 

 upon them their natural conditions of heat and moisture. Then 

 the ice-sheet retreated ; the arctic plants followed it, and they in 

 turn were followed b}' the north temperate, — they by the warm 

 temperate, and so on, each tending to return to its own latitudes. 

 But along the high mountains the 'conditions did not change much, 

 and the arctic plants found upon them, as they find todaj^, congen- 

 ial homes, where the species of the lowlands could not compete 

 with them. In the valleys, however, the more southern species 

 were more at home and soon drove out the arctic intruders. So 

 wide-spread and definite was this agency that there has not been a 

 mountain range explored in the northern hemisphere upon which 

 traces of these arctic plants have not been found ; moreover, the 

 lower ranges, those, like our own White Mountains, not high 

 enough to reach the line of perpetual snow, have not the extreme 

 arctic forms upon their tops, but sub-arctic or very cold temperate 

 forms, and these extend south along the hill ranges, ascending as 

 they go. A very interesting point in connection with this is the 

 fact that in the glacial period some of the arctic and sub-arctic 

 forms managed to cross the equator along the highest mountain 

 chains of the old and new world and establish themselves in the 

 southern hemisphere, and are there found on the mountains at the 

 present day. Hence we have a great natural group of plants 

 called the Arclic-alpine Flora, consisting chiefly of low herbs, the 

 distribution of which is all around the northern part of the 

 northern hemisphere upon the shores of the Arctic ocean, aiid 

 southward upon nearly all mountains high enough to reach the 

 limit of perpetual snow, and even extending sparingly on the great 

 ranges into the southern hemisphere, at length reaching the sea 

 level near the Antarctic ocean. 



