and Old World Birches. 189 
the coast of Alaska, the islands of Behring Sea, and adjacent 
Kamtschatka this depressed form with small orbicular or reni- 
form leaves retains its characteristics in a marked degree, and 
were the plant known only from that district it would stand as 
an undoubted species. 
Mr. Coville has called the attention of the writer to a shrub 
four to six feet high which is associated with the depressed 
Betula glandulosa, var. rotundifolia, and the Cook’s Inlet 
tree which is identified with B. pendula, var. japonica. This 
intermediate shrub is well represented in the National Herba- 
rium, and it is possible, as Mr. Coville suggests, that it is of 
hybrid origin, since similar hybrids of trees and dwarf shrubs 
have before been noted.* The material presents a strong super- 
ficial resemblance to Greenland specimens of B. alba, var. 
minor, although the strongly resiniferous branchlets hardly 
place it with that shrub. 
In conclusion, it should be emphasized that the specitic lines 
in Betula, as in Alnus, Quercus and Salia, are often too vague. 
It is quite possible to trace by a series of specimens a direct. 
connection between the dwarf Betula nana or B. glandulosa 
and the tall B. alba. Thus 2. nana in its larger development 
is separated with difficulty from the Scandinavian B. alpestris.. 
This shrub, in turn, is quite like glabrate states of the Ameri- 
can L. pumila, which, through its var. glandulifera, passes to 
LB. glandulosa, the larger developments of which pass in the 
Cascade Mts. to B. microphylla, and in the Saskatchewan 
region to B. alba, var. minor. The latter shrub is often 
inseparable on the New England mountains from &. alba, var. 
cordifolia, which on the lower slopes becomes a large tree and 
as L. papyracea. A very similar series is readily made to 
include B, pendula and B. humilis. But since it is obvyi- 
ously impracticable to regard all these forms as one species, it 
seems wiser to recognize the more marked centers of variation 
as species which are admitted to pass by exceptional tendencies 
to other forms ordinarily distinguished by marked charac- 
teristics. 
The American representatives of § Costatae, Betula nigra, 
B. lenta, and B. lutea, are represented by related species in 
Asia, but none of these trees are of very boreal range, and 
they appear well distinguished as endemic species. 
* B. pubescens [alba] x humilis, Warnst. Verh. Bot. Ver. Brandenb. xi, 
129 (1870). 
B. nana x verrucosa [pendula], Sael. Medd. Soc. Faun. et. Fl. Fenn. xiii, 
256 (1886). 
B. nana x ‘pubescens [alba], Koehne, Deutsche Dendr. 112 (1893). 
B. pumila x lenta, Jack, Gard. and For. viii, 243, fig. 36 (1895). 
