CHAP. II.] DOUBTFUL SPECIES. 37 



must be confessed that it is in the best known countries that we 

 find the greatest number of them. I have been struck with the 

 fact, that if any animal or plant in a state of nature be highly 

 useful to man, or from any cause closely attracts his attention, 

 varieties of it will almost 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 assign- 

 able 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. The 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 compara- 

 tively 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 surround the typical Quercus robur. Finally, De Candolle 



