Geographical Distribution of Indigenous Plants. 65 
Cardamine hirsuta, var. sylvatica. Polygonum dumetorum, var. scan- 
Lepigonum medium, var. macrocar- dens. : 
a. Castanea vesca, var. americana. 
Oxalis corniculata, var. stricta. Cyperus rotunda var. hydra. 
Salicornia fruticosa, var. ambigua. 
There remains now of list No. IV. but four species. Of these 
Salicornia virginica has been rather doubtfully identified with S. 
mucronata of the coast of Spain. Polygonum erectum, a very common 
plant around all houses, has doubtless been introduced by man, Carex 
limula has been rather doubtfully identified with the Lapland plant, 
by Wm. Boott.* There then remains only Convallaria majalis. This 
is known to grow in this country only on the mountains of Georgia and 
North Carolina, “ and extends north to the peaks of Otter, in Virginia, 
latitude 374 deg., at an altitude of 4,000 feet ; but it is not known to 
occur anywhere beyond this; while in western Europe it extends 
nearly to latitude 70deg.”’; This certainly is aremarkable case, and is 
difficult to account for. It is hardly possible it has been overlooked, 
for it is too striking a plant. I have a specimen, collected in Massa- 
chusetts, escaped from cultivation, and it is given in the catalogue of 
Plants of Wisconsin. It seems to be in much the same position on 
the Atlantic coast, as Sequota is on the Pacific ; now confined to 
limited localities, though once ranging over wide tracts of country. 
Enumerated in the four preceding lists, there are 360 species and 
varieties of plants. A reasonable explanation of why they are found 
in both Europe and America has been given. But as a foundation for — 
the explanation, two things must be admitted. First, that the region 
surrounding the North Pole has been the source from which has been 
- derived a good part of the floras of Europe, North America, and Asia ; 
and, second, the occurrence of glacial and inter-glacial (warm) periods 
both north and south. | 
As for the first, the region close around the North Pole is, as yet, 
a terra incognita. We know enough of it, however, to say with cer- 
tainty that there is considerable land clustered in its vicinity. We 
may very reasonably suppose, that during the warm period at the North, 
the ocean was ata lower level than it is now,{ and that a land con- 
nection formerly existed between the northwest coast of America, and 
* Gray’s Manual, p. 582. 
7 Am. Jour. l. c., vol. xxiii., p. 64. 
‘t In consequence of the withdrawal of large quantities of water to form the Antarctic ice 
cap; and because the shifting of the earth’s cénter of gravity caused by this ice, would have 
a tendency to draw the water toward the south, thus leaving much dry land at the north. 
Croll, ‘* Climate and Time.’’ 
