INTRODUCTORY ESSAY. 59 



apartments it is preserved. As all the duplicates were made 

 up into sets, ticketed, and distributed at home and abroad, 

 this herbarium has taken the place of a standard work of 

 reference, and it is impossible to over-estimate its value, or 

 the importance of the constant access which we have enjoyed 

 to its contents. The numbers attached to each plant have 

 been so cited by all monographists, that a reference to these, 

 in the great majority of instances, suffices for the identifica- 

 tion of the species ; and we have therefore constantly quoted 

 the catalogue numbers, carefully examining every specimen 

 before doing so, in order to avoid as much as possible the risk 

 of error. The distribution appears on the whole to have been 

 made with much care, though the limited time allotted to its 

 execution prevented that critical comparison without which 

 species of difficult genera cannot be discriminated. Hence we 

 occasionally find two or more species under the same num- 

 ber and letter, and far more frequently the same species under 

 two or more numbers. It is not easy to say how many spe- 

 cies are contained in the WaUichian collection ; but the 9000 

 numbers may, we think, be diminished by at least one-fourth, 

 as Dr. WaUich, being obliged to distribute without describ- 

 ing, very judiciously avoided unitiag apparently distinct forms. 

 For the present therefore we estimate this great collection at 

 between 6500 and 7000 species. The named specimens of 

 this Herbarium having been, as we have said, extensively dis- 

 tributed, it has been customary with botanists to retain the 

 names given by Dr. Wallich. We have been careful to do 

 the same ourselves for aU otherwise unpublished genera and 

 species; but where published names, accompanied with de- 

 scriptions, have come in contact with them, we have consi- 

 dered it to be our duty to follow the generally recognized rule 

 of priority, and to retain the published one ; except, of course, 

 in cases where the authors of these names had habitually 

 availed themselves of the WaUichian collections, and where 

 we feel justified in assuming that they would wish to have 

 adopted the WaUichian name had they recognized the plant. 



