66 ORIGIN OF SPECIES 



Many of the cases of strongly-marked varieties or doubtful 

 species well deserve consideration ; for several interesting 

 lines of argument, from geographical distribution, analogical 

 variation, hybridism, &c., have been brought to bear in the 

 attempt to determine their rank ; but space does not here per- 

 mit me to discuss them. Close investigation, in many cases, 

 v^^ill no doubt bring naturalists to agree how to rank doubt- 

 ful 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 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 authori- 

 ties 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 CandoUe, 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 several species, and estimates numeri- 

 cally 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 develop- 

 ment, sometimes without any assignable reason. Such char- 

 acters 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 re- 

 marks: "They are mistaken, who repeat that the greater 

 part of our species are clearly limited, and that the doubtful 



