| 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 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 of course are not 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 it
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 remarks,
if these connecting forms, which are now rare, were to become 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 two-thirds are provisional species, that is, are not known strictly to fulfil the definition above given of a true species.
For it
should be added that De Candolle no longer believes that species are immutable creations, but concludes that the derivative theory of
the succession of forms is the
most natural one, "and the most accordant with the known facts in palæontology, geographical botany and zoology, of anatomical structure and classification;"
but, he adds, direct proof is still wanting.
|
