BOOKS AND CURRENT LITERATURE 271 



type locality, or even to the description of new forms from living plants. 

 In many other cases it is due to the difference of opinion which exists 

 between the authors and other cactus students regarding the limits 

 between the perplexing array of closely related forms that may be found 

 in the field in any region where these plants are abundant. The opun- 

 tias are preeminently a group in which prolonged and careful field work 

 is necessary to determine the influence of the age of the plant, the rela- 

 tive age of different joints, recent climatic conditions, character of soil, 

 and many other conditions, on the general habit of the p lant, the form 

 and size of the joints, the character of the armature and even the color 

 of the flowers. Among those to whom a taxonomic grouping is the in- 

 strument of work, rather than the material, there will be much satis- 

 faction in the appearance of a monograph in which the "lumping" of 

 species has been generously done. We have been made to believe in 

 recent years, however, that it is the business of systematic botany to 

 describe and classify all plant forms which possess even a single well- 

 marked and constant character differentiating them from their nearest 

 of kin. Anyone to whom the cacti of the United States are familiar 

 under their native conditions will be likely to ask whether this depart- 

 ure from recent taxonomic practise has been begun in the right place 

 and whether it has been impartially applied to the southeastern and 

 West Indian forms as well as to the southwestern ones. When such 

 indistinguishable forms as Opuntia littoralis and 0. occidentalis, of the 

 California coast, have been retained in separate series of the genus, and 

 when 0. pollardii, of the Mississippi coast, has been retained as separate 

 from 0. opuntia, partly on such a character as the turgidity of the 

 joints, one may well ask why such a form as 0. blakeana has been re- 

 duced to synonomy with O.phaeacantha, in spite of its well marked char- 

 acters and distributional separation, why the very local and clearly dis- 

 tinct 0. dillei has been relegated to the "lump" known as 0. engelmanii, 

 or why O.fulgida and 0. mamillata have been combined in spite of their 

 differences in habitat and vertical distribution and the fact that they 

 can be readily distinguished in the field at a distance of 200 feet. 



In spite of these and many other inevitable differences of opinion 

 that are sure to exist among students of cacti, there will nevertheless 

 be a very general appreciation of the masterly work which Britton and 

 Rose have done in an extremely difficult group. — Forrest Shreve. 



