DOUBTFUL SPECIES. 45 



No one ever had more ample materials for the discrimina- 

 tion 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 vari- 

 ations. 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 labor, 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 pro- 

 visional. Just as we come to know them better, inter- 

 mediate 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 two- 

 thirds are provisional species, that is, are not known strictly 

 to fulfil the definition above given of a true species. It 

 should be added that De Candolle no longer believes that 

 species are immutable creations, but concludes that the 

 derivative theory is the most natural one, "and the most 

 accordant with the known facts in palaeontology, geographi- 

 cal botany, ancl zoology ? of anatomical structure &n4 classifi- 

 cation," 



