40 THE ORIGIN OF SPECIES 



almost universally be found recorded. These varieties, moreover, 

 will often be ranked by some authors as species. Look at the com- 

 mon 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 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, 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 num- 

 ber 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 con- 

 necting 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 defi- 



