40 DOUBTFUL SPECIES. [Chap. II. 



attracts his attention, varieties of it will alrr.ost universally be 

 found recorded. These varieties, moreover, will often be ranked by 

 some authors as species. Look at the common oak, how closely it 

 has been studied ; yet a German author makes more than a dozen 

 species out of forms, which are almost 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 con- 

 nected 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 nivmber of spon- 

 taneous varieties and sub-varieties. Thus Quercus robur has twenty- 

 eight varieties, all of which, excepting sis, are clustered round three 

 sub-species, namely, Q. pedunculata, sessiliflora, and pubescena. 

 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 wholly 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 sur- 

 round the typical Quercus robur. Finally, De Candolle admits 

 that out of the 300 species, which will be enumerated in his Pro- 

 ■iromus as belonging to the oak family, at least two-thirds are 



