No. 464] STUDY OF THE SALICACEE. 523 
Again, comparisons with existing species are instructive. If 
all the various forms now recognized are to be regarded as valid 
species, then it would appear that the genus Populus must have 
attained to its full development in the later Cretaceous or pos- 
sibly early Tertiary time, since when it has been in a process of 
slow decline, or at least it has made no substantial progress. A 
very noteworthy feature of such comparisons appears in the 
somewhat abrupt increase among the Salicaceze in common with 
other Dicotyledons, in the Middle Cretaceous, especially in the 
Dakota group. A ready explanation of this phenomenon on 
purely botanical grounds might be found in the facility with 
which hybridization gives rise to very stable. forms, and it is in 
all probability true that such hybridization is accountable in 
large measure for the multiplication of forms or species in past 
times as it is known to be at the present day ; but to the geolo- 
gist this would probably not afford an adequate reason, since he 
recognizes the important extent to which “accidents ” as deter- 
mined by exposure of strata, methods of preservation, etc., con- 
stitute very definite and often controlling factors in the number 
and kinds of plants which may be yielded by a given formation, 
in which sense Dr. Knowlton has observed * that though some 
types of the Cenomanian, as shown by the leaves of the Dakota 
group, generally remain distinct and plainly defined in the 
vegetation of some of the subsequent formations, the chain of 
evidence is not always continuous. A number of these, for 
example, still remain unrecognized in the Upper Cretaceous, 
though present in more recent strata of the Laramie or of the 
Tertiary. We know very little as yet of the flora of the Senon- 
ian or of intermediate stages between the Dakota and the Lar- 
amie groups. But judging from recent discoveries in Wyoming, 
Montana, Canada, and Vancouver Island, we have been able to 
recognize in the scanty materials obtained, the presence and 
therefore the persistence of some of the primitive or more 
ancient types, and it is most. probable that further research will 
complete the evidence of the persistence and fepresentation of 
the types of the Dakota group up to the Laramie, as clearly as 
it is observable in this flora and through the different stages of 
the Tertiary to the present time (Lesquereux, '92). 
