180 DARWINIANA. 



ing to their obvious resemblanceSj into groups of ap- 

 parently identical or nearly identical forms, wliich 

 were severally examined and compared. Where speci- 

 mens were few, as from countries httle explored, the 

 work was easy, but the conclusions, as will be seen, of 

 small value. The fewer the materials, the smaller the 

 likelihood of forms intermediate between any two, 

 and — what does not appear being treated upon the old 

 law-maxim as non-existent — species are readily enough 

 defined. Where, however, specimens abound, as in 

 the case of the oaks of Europe, of the Orient, and of 

 the United States, of which the specimens amounted 

 to hundreds, collected at different ages, in varied local- 

 ities, by botanists of all sorts of views and predilec- 

 tions — here alone were data fit to draw useful conclu- 

 sions from. Here, as De Candolle remarks, he had 

 every advantage, being furnished with materials more 

 complete than any one person could have procm*ed 

 from his own herborizations, more varied than if he 

 had observed a hundred times over the same forms in 

 the same district, and more impartial than if they had 

 all been amassed by one person with his own ideas or 

 predispositions. So that vast herbaria, into which con- 

 tributions from every source have flowed for years, 

 furnish the best possible data — at least are far better 

 than any practicable amount of personal herborization 

 — for the comparative study of related forms occur- 

 ring over wide tracts of temtory. But as the materials 

 increase, so do the difficulties. Forms, which appeared 

 totally distinct, approach or blend through interme- 

 diate gradations ; characters, stable in a limited num- 

 ber of instances or in a limited district, prove unstable 



