
No. 464] STUDY OF THE SALICACEE. 513 
Community of Species of Salıx. 
Europe and Asia à : ; i ; ; ; à 14 
Europe and North America i : : i 7 
Europe and Africa and Asia . ; : ; ; ; I 
Asia and Africa : 3 i : I 
N 
Qə 
In determining these relations, the very pronounced isolation 
of the Japanese and Himalayan species stands out with great 
prominence and lends emphasis to the idea of segregation in a 
previously continuous flora. From the facts thus presented, it 
is obvious that the willows are preëminently an Old World 
genus, and that, although their dispersion has a tendency to 
greater diffuseness than in the case of the poplars, like them 
they essentially belong to temperate regions, and their tendency 
is on the whole boreal, rather than austral; so that with respect 
to the Salicaceæ as a whole, it may be said to be a distinctly 
temperate and boreal family of wide dispersion within the north- 
ern hemisphere, with its center of distribution in Asia where it 
presumably had its origin. A summary of these geographical 
relations will serve to make the foregoing analysis more clear. 
In the light of these facts it becomes pertinent to ask whether 
the family as now known represents a type of vegetation which 
is yet in process of evolution, if it is a side line which has 
recently attained to the full limit of development and which is 
therefore terminal, or if it attained the culmination of its devel- 
opment in some previous geological age and is now in a state of 
decline comparable with that exhibited by the Lycopodiaceæ 
and the Equisetaceæ. The comparatively recent origin of these 
plants, if we are to judge them by the standards which have 
been set by other groups, the general phylogeny of which is 
fairly well known, would naturally lead us to support the first 
hypothesis or at least the second, and to reject the third as 
improbable. But such generalizations are unsafe unless sup- 
ported by other well established data, and it therefore becomes 
necessary to inquire somewhat particularly into the nature of the 
evidence afforded by the geological history of these plants. 
