most universally considered
by other botanists to be varieties; and in this country the highest
botanical authorities and practical men can be quoted to show that the
sessile and pedunculated oaks are either good and distinct species or
mere varieties.

I may here allude to a remarkable memoir lately published by A. de
Candolle, on the oaks of the whole world. No one ever had more ample
materials for the discrimination of the species, or could have worked on
them with more zeal and sagacity. He first gives in detail all the many
points of structure which vary in the several species, and estimates
numerically the relative frequency of the variations. He specifies above
a dozen characters which may be found varying even on the same branch,
sometimes according to age or development, sometimes without any
assignable reason. Such characters are not of course of specific value,
but they are, as Asa Gray has remarked in commenting on this memoir,
such as generally enter into specific definitions. De Candolle then goes
on to say that he gives the rank of species to the forms that differ by
characters never varying on the same tree, and never found connected
by intermediate states. After this discussion, the result of so much
labour, he emphatically remarks: "They are mistaken, who repeat that the
greater part of our species are clearly limited, and that the doubtful
species are in a feeble minority. This seemed to be true, so long as
a genus was imperfectly known, and its species were founded upon a few
specimens, that is to say, were provisional. Just as we come to know
them better, intermediate forms flow in, and doubts as to specific
limits augment." He also adds that it is the best known species which
present the greatest number of spontaneous varieties and sub-varieties.
Thus Quercus robur has twenty-eight varieties, all of which, excepting
six, are clustered round three sub-species, namely Q. pedunculata,
sessiliflora and pubescens. The forms which connect these three
sub-species are comparatively rare; and, as Asa Gray again remarks, if
these connecting forms which are now rare were to become totally extinct
the three sub-species would hold exactly the same relation to each other
as do the four or five provisionally admitted species which closely
surround the typical Quercus robur. Finally, De Candolle admits that
out of the 300 species, which will be enumerated in his Prodromus
as belonging to the oak family, at least t