510 THE AMERICAN NATURALIST. "([Vor. XXXIX. 
families of Dicotyledons, it made its appearance very abruptly 
in the Cretaceous age, whence it has continued through suc- 
ceeding formations, appearing with prominence in the Pleis- 
tocene where its remains form a distinctly connecting link with 
existing species, with many of which they are more or less 
identical. 
An analysis of the Salicacee based upon the enumeration 
of the Index Kewensis, shows that there are at the present time 
no less than two hundred and seventy recognized species, an 
altogether unusual proportion for a family embracing such a 
limited number of genera. Of these, twenty-two belong to the 
genus Populus, while two hundred and forty-eight belong to the 
genus Salix. 
The poplars are preéminently a type peculiar to the northern 
hemisphere, and in their chief aspects they have a strong tend- 
ency to a boreal habit, the one exception to be met with in 
the subtropical P. mexicana offering no material contradiction 
of this law. P. ciliata of the Himalayas, though reaching a 
latitude far below that of most representatives of the genus, 
nevertheless conforms to the temperate habit of the group by 
virtue of the somewhat high elevations at which it grows. Six 
species range from Japan through China, Mongolia, and Siberia 
to Central Asia, while the western extension of the genus is 
chrried through northern and central Europe as far south as 
Spain and Italy. In the western hemisphere, no less than ten 
species are to be met with, ranging from Mexico as the extreme 
southern extension, thence northward as far as the arctic circle 
where P. balsamifera appears to establish the limit of develop- 
ment. The very remarkable isolation and localization of the 
various species is one of the most prominent features disclosed 
by an examination of these plants. P. mexicana is confined to 
Mexico and similarly P. ciliata is peculiar to the Himalayas. 
P. adenopoda is restricted to China as P. sieboldii is to Japan ; 
while P. przewalskii of Mongolia and P. pruinosa of Siberia, as 
also P. euphratica of Central Asia, give emphasis to this strong 
segregation, since they are as wholly distinct from the European 
and American species as are those of the last two regions from 
one another. In only one instance does there seem to be any 
