512 THE AMERICAN NATURALIST. (VoL. XXXIX. 
Like the poplars, the willows are a type essentially peculiar to 
the temperate regions of the northern hemisphere, though they 
represent adaptation to a greater diversity of climatic conditions 
and thereby greatly extend the northern and southern range of 
the family as a whole. With the exception of about 2 percent 
—an altogether insignificant proportion—they are of a conti- 
nental character. Nevertheless, in the S. junghuhniana of 
Java, S. sumatrana of Sumatra, S. canariensis of the Canary 
Islands, S. madagascariensis of Madagascar, and S. occidentalis 
of the West Indies, types which are strongly segregated and 
localized, their very restricted range is in close proximity to the 
corresponding vegetation of the continental areas in which they 
must have had their origin and from which they must have been 
isolated at a comparatively late period as shown by Wallace ('80). 
It is these insular forms which are chiefly concerned in the 
extreme southern extension of the genus, since the six Mexican 
and eight north African species range considerably farther north. 
The extreme isolation of S. Zumboldtiaua in South America 
affords at once an illustration of the extent to which dispersal 
must have been carried in former geological times, and of the 
potency of the influences which have tended to a greater restric- 
tion of geographieal area. 
Of the one hundred species common to Asia, twenty-four are 
found within the Himalayan region, while seventy-three are more 
peculiar to the temperate and boreal regions of Japan, China, 
Siberia, and Central Asia. In Europe no less than eighty-four 
species give a range to the genus which extends from the Medi- 
terranean on the south to the polar regions where the very 
diminutive S. Polaris with an extreme height of about 3.3 cm., 
defines the northern limit of growth. In North America, apart 
from Mexico, seventy-two species similarly extend the genus over 
a wide range of latitude, their northern limits being defined by 
the boreal S. ovalifolia which also ranges southward on the sum- 
mits of lofty mountains, as far as Mount Washington in New 
Hampshire. Ubiquitous species are much more common than 
among the poplars, and there are no less than 23 which are 
more or less common to the several continental regions as exhib- 
ited by the following synopsis. - 
