CO DOUBTFUL SPECIES. Chap. II. 



state of nature be liigbly useful to man, or from any cause 

 closely attract his attention, varieties of it will almost univer- 

 sally 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 

 imiversally considered as varieties ; and in this country the 

 highest botanical authorities and practical men can Ix) 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 struc- 

 ture which vary in the species, and estimates numerically the 

 relative frequency of the variations. He specifics above a 

 dozen characters Avliich niay 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 saj^- 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 arc 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 Avere fovuidcd 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 spe- 

 cific 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. peduuculata, sessilifiora, and 

 pubescens. The forms which connect these three sulvspecies 

 are comparatively rare ; and, as Asa Gray remarks, if these 

 connecting forms, whi(;h are now rare, were to become wholly 

 extinct, the three sub-species Avould hold exactly the same 

 relation to each other, as do the four or five provisionally- 



