DOUBTFUL SPECIES. 47 



does not here permit me to discuss them. Close investiga- 

 tion, in many cases, will no doubt bring naturalists to agree 

 how to rank doubtful forms. Yet it 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 ^vill 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 vari- 

 eties; 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 pub- 

 lished by A. de Candolle, on the oaks of the whole world. 

 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, aud 

 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 provisional. Just as 

 we come to know them better, intermediate forms flow in. 



