132 REVIEWS. 



specimens in hand were distributed, according to their ob- 

 vious resemblances, into groups of apparently identical or 

 nearly identical forms, which were severally examined and 

 compared. Where specimens were few, as from countries 

 little 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 Eu- 

 rope, of the Orient, and of the United States, of which the 

 specimens amounted to hundreds, collected at different ages, 

 in varied localities, by botanists of all sorts of views and pre- 

 dilections, — 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 procured from his own herboriza- 

 tions, 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 

 contributions from every source have flowed for years, furnish 

 the best possible data — at least are far better than any prac- 

 ticable amount of personal herborization — for the compara- 

 tive study of related forms occurring over wide tracts of terri- 

 tory. But as the materials increase, so do the difficulties. 

 Forms, which appeared totally distinct, approach or blend 

 through intermediate gradations ; characters, stable in a lim- 

 ited number of instances or in a limited district, prove unsta- 

 ble occasionally, or when observed over a wider area ; and the 

 practical question is forced upon the investigator, — what here 

 is probably fixed and specific, and what is variant, pertaining 

 to individual, variety, or race ? 



In the examination of these rich materials, certain char- 

 acters were found to vary upon the same branch, or upon the 

 same tree, sometimes according to age or develojDment, some- 

 times irrespective of such relations or of any assignable rea- 

 sons. Such characters, of course, are not specific, although 



