THE 
AMERICAN NATURALIST. 



Vor. XXXIX. August, 1905. No. 464. 

A SYSTEMATIC STUDY OF THE SALICACE. 
D. P. PENHALLOW. 
In attempting a discussion of the Salicacez from the stand- 
point of their ancestry, as presented in the woody structure of 
the mature stem, a first step involves comparison with previously 
existing forms and with closely related types now extant, in 
order to determine the general phylogenetic sequence and the 
position which the group at present occupies. We are there- 
fore led, at the very outset, to ask: (1) what is the nature of 
the evidence to be derived from paleontology, and what is the 
bearing of such evidence; and (2) what is the nature and bear- 
ing of the evidence to be derived from the structure of existing 
species? The Salicacez as we now know them, are altogether 
confined to two genera— Populus and Salix — of very wide 
: distribution in north temperate latitudes whence they extend 
beyond the extreme northern limit of tree growth within the 
polar circle, being there reduced to prostrate shrubs. It is 
quite possible that the family may have been more extensive in 
early or middle Mesozoic time, but the evidence now available 
as derived from their fossil remains, leads us to the supposition 
"that it has not been, at any previous time in its history, of a 
more comprehensive character. In common with many other 

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