164 Distribution of the North American Flora. [March, 
side of America, being found nowhere else in Arctic or Sub- 
arctic America. 
My explanation of these anomalies was, that at a period pre- 
vious to the glacial, a flora common to Scandinavia and Greenland 
was spread over the American polar area, and that on the acces- 
sion of the cold of that period this flora was driven southwards, 
and was affected differently in different longitudes. In Greenland 
many species were exterminated, being as it were driven into the 
sea at the southern extremity of the peninsula, where only the 
hardiest survived. On the return of warmth the Greenland sur- 
vivors migrated northward, peopling the peninsula with the 
hardiest of the species of its former flora, unmixed with American 
species; and unchanged in aspect from never having been 
brought into competition with those of any other flora. On the 
other hand, the same Scandinavian plants when driven south on 
the plains of the continent multiplied there in individuals, and 
being brought into competition with American species descend- 
ing from the continental mountains on to the plains, assumed 
varietal forms. On the return of warmth, therefore, many Scan- 
dinavian species that had been exterminated in Greenland would, 
having survived on the continent, travel northwards on it, some 
unchanged, others under varietal forms, accompanied with the 
American species that had descended from the mountains during 
the cooling of the continent. Lastly, as some of the Scandina- 
vian species were no doubt local, and confined to near the merid- 
ian of Greenland, it is not surprising to find that a few such 
should survive only in Greenland and on the eastern alps of 
North America. 
Thus only could I satisfactorily account for the almost complete 
identity of the Greenland flora with the Scandinavian after such 
changed conditions of climate; for the paucity of its species ; 
for the absence in it of varieties; for the rarity in it of peculiarly 
American species; for the few species which extra-arctic Green- 
land adds to its arctic flora; and for certain of its plants being 
limited in range to Greenland and the eastern American alps. 
North Asiatic and North American Floras—The rélationship 
between the flora of North-east Asia and Eastern North America 
has been fully explained by Dr. Asa Gray, in an essay on the 
flora of Japan, which is the first entirely satisfactory contribution 
_of its kind to the science of botanical geography known to me. 
