Vol. 27 



BULLETIN 



^ No. 5*- 



OF THE 



TORREY BOTANICAL CLUB 



MAY 1900 



Revision of the North American Species of Pseudoleskea* 



By G. N. Best 



* 



(Plates 7 and 8) 



While it is true that some mosses are so fully differentiated as 

 to fall readily in well-defined species there are others, probably a 

 larger number, that do not behave in this way but appear in 

 groups or series of closely intergrading forms. A satisfactory ar- 

 rangement of these tries the skill of the systematist to the utmost. 

 If such a group or series be taken as one species a description 

 broad enough to cover the varying forms is generally too indef- 

 inite for practical purposes ; if species be made of each of the ex- 

 tremes the intermediate forms are but poorly described, and if each 

 of these forms be allowed specific rank nomenclature becomes 

 burdensome to say the least. In general terms and within certain 

 limits it may be stated that the more species made of such a series 

 or group the more unstable are the characters by which they are 

 separated, or supposed to be separated, and the more difficult it 

 becomes to place a given plant. By a careful study however of 

 considerable material it is usually possible to acquire a conception 

 of a central type, which is of value in that it enables the systema- 

 tist to fix a starting point from which he may gauge the variations. 



What constitutes a species is indeed a vexed question, one not 

 likely to be answered soon to the satisfaction of all. It may be 

 safely said however, that a species should represent a fixed point 

 in the evolution of a plant. In other words that it should mean 



*The material on which this revision is based was furnished chiefly by Columbia 

 University, the Geological Survey of Canada and Harvard University. 



[Issued 26 May] 221 





