524 THE AMERICAN NATURALIST. (VoL. XXXIX. 
We are now led to ask: * What is the nature of the evi- 
dence, with respect to the particular form and value of the plant 
remains, upon which our knowledge of the geological succession 
is based?” Of the internal structure of the fossil Salicaceze we 
as yet know nothing, for with the exception of a few specimens 
from the Pleistocene, the wood of these plants has not yet been 
brought under examination, and we are therefore denied one of 
the most definite means of distinguishing with certainty not 
only the various species and genera, but also the relations which 
the members of this group bear to one another and to other 
groups. That fragments of the stems of willows and poplars 
must sooner or later be found and brought under examination, 
is altogether probable, and as an essential provision for the 
proper recognition of the various species at such times, it is 
important that accurate diagnoses of existing species should be 
made. 
The only evidence at present available for the recognition of 
members of the Salicacez, is in the remains of their leaves which 
are too often found in a very fragmentary condition and other- 
wise unsatisfactorily preserved. Recalling the extent to which 
hybrid and variable forms occur in this family, and the often 
extreme differences which may arise in the same species as 
shown by Ward ('88), Holm ('9o, '95), Berry (:01—:03, :02a, 
:02b), and Penhallow (: O4), itis readily seen that the numerical 
distribution of these leaf forms in geological time cannot be 
regarded as affording an accurate basis upon which to found a 
knowledge of succession, and such remains can never be of 
more than approximate value with reference to an exhibition of 
the most general relations. Under these circumstances, as 
pointed out by Holm ('95) some years since, “a careful study of 
the recent flora is absolutely necessary when it is desired to 
identify fossil leaves with even an approximate degree of correct- 
ness. The plant must be studied as it stands amidst the sur- 
roundings to which it has adapted itself and which its leaves 
reflect." It is altogether probable that a more complete and 
detailed knowledge of the Salicaceze would show that the pres- 
ent forms which are recognized as distinct species, in reality 
represent, in. many cases, only variations of the same species. 
