BRITISH SPECIES OF THE GENUS MNIUM. i 3 
I have tried to apply a similar method to the deseription of living species 
by measuring 38 characters of about 90 species and 20 varieties of the 
genus Carabus. For each property I have determined the minimal, median, 
and maximal value in each species. The figures, set in order in tables, 
enabled me to describe and to identify the species and varieties more 
accurately than by the usual method of description. Unfortunately, the war 
prevented me from finishing and publishing my work. 
I have tried to carry out similar work with plants, in the Cryptogamic 
Laboratory of the University of Manchester. On the suggestion. of 
Prof. Lang, I took mosses of the genus Mnium. I have measured fourteen 
different characters of 10 British species of this genus (altogether more than 
30,000 measurements have been made). Four other species of Mniwn 
belong to the British flora, but they are very rare in these islands and could 
not be measured for want of material. 
I hope to publish later a more extensive work on the theoretical part 
of this quantitative method. In the present paper I confine myself to a 
quantitative description of the ten studied species, giving only the theoretical 
indications which are necessary to make the given descriptions com- 
prehensible. 
I read a short abstract of my work at the Meeting of the British Asso- 
ciation for the Advancement of Science, in Manchester, on September 7th, 
1915*. I may perhaps be allowed to reply here in a few words to some 
remarks which were made on that occasion. 
The first criticism was to the effect that I was using a rather complicated 
method for the identification ot species which are easily distinguished by 
every student of British mosses. I give the deseription of British species 
of Mnium as a first example of the application of the quantitative method, 
because I wanted to begin with well-known, easily recognizable species, in 
order to put the method to the test and to establish a starting-point for 
further application. 
. Secondly, it was said that the use of the method is too laborious to allow 
of an extensive application. It is true that the quantitative description of 
a certain number of species is a long and rather tedious task, but this is not a 
sufficient reason to cause us to be discouraged. When the descriptions and 
the tables of identification are made (once for all), the measurements which 
are needed for the identification of a given specimen of Mniuin may be made 
in less than an hour, and for the use of the tables no more than about ten 
minutes are needed. Let us remember that the exact measurement of one 
single physical constant is also a delicate and sometimes a very long task. 
* “On the Expression by Measurement of Specific Characters, with Special Reference to 
Mosses,” Report of the Meeting of the British Association for the Advancement of Science, 
held at Manchester in September 1915. 
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