May 20, 1898.] 



SCIENCE. 



691 



here called the chief- differential. Now it 

 is frequently the case that different system- 

 atists working in the same groups select 

 different characters as the most distinctive, 

 and consequently, as the characters are not 

 perfectly correlative, the specific line is 

 drawn at different points. For instance, 

 one botanist may separate Thalictrum pur- 

 purascens, L. from T. polygamum, Muhl., by 

 the characters of the leaf, another by the 

 form of the stamens; one distinguish Calli- 

 triche verna, L. from C. heterophylla, Pursh by 

 the persistence of the stigmas ; another by 

 the shape of the fruit. It is necessary then 

 to have a method for determining the chief- 

 differential in order that the specific lines 

 be uniformly drawn and that the value of 



the species be justly estimated. The method 

 for obtaining the chief-differential may be 

 shown by taking a particular case. 



Two species of the marsh plant Typha 

 are found in the eastern United States and 

 are often confluent at the point where the 

 brackish marshes pass into fresh-water 

 swamps. Seven characters regarded as 

 probably specific were measured in about 

 250 specimens taken mainly from eastern 

 Massachusetts and selected one meter 

 apart, as nearly as possible, across the 

 swamps were found. These seven charac- 

 ters were each arranged separately by 

 classes, according to size and the relative 

 frequency of each class enumerated, as 

 shown in the following table : 



Table of Relative Frequency. 

 (Fig. 12.) I. Stem-Height, measured from the ground or water to base of Pistillate Spike. 

 Decimeters. I 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 



No. of Individuals' 



2 2 6 8 18 30 33 38 38 27 18 16 7 2 2 



Millimeters. 



(Fig. 13.) II. Base Diametee of Stem, including leaf-sheaths. 

 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 



No. of Individuals.] 2 20 45 2119 4 10 13 17 19 16 13 5 5 2 1 



Millimeters. 



(Fig. 14.) III. Mid-Stem Diametee, taken at half the height. 

 2 3 4 5 6 7 8 9 10 11 12 



No. of Individuals. I 5 10 68 43 29 8125 23 11 2 1 



(Fig. 15.) IV. Leaf-Width, largest leaf in widest part. 

 Millimeters. I 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 

 No. of Individuals. I 12 26 39 25 15 6 5 14 1116 15 20 15 12 7 8 1 1 2 



Centimeters. 



3 4 



(Fig. 16.) V. Pistillate Spike-Length. 

 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 



No. of Individuals. 



1 5 7 9 17 23 22 37 31 26 23 12 11 6 3 7 1 1 1 



(Fig. 17.) VI. Pistillate Spike-Diamktee. 

 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 



30 31 32 33 34 35 



No. of Individuals. 



1 7 3 6 6 12 13 30 29 19 3 5 2 10 6 6 16 8 H 15 5 10 6 3 1 1 1 



(Fig. 18.) VII. Interval between Staminate and Pistillate Spikes. 

 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 



No. of Individuals. I 110 15 7 16 37 40 9 4 2 1 1 1 



