30 HANDBOOK OF BRITISH MOSSES. 



form no exception to the rule. Races, species, subspecies, and 

 varieties alike scarcely admit of accurate definition, and have 

 different meanings according to the point of view from which 

 they are regarded. While some Mosses admit at once of accu- 

 rate separation from all others, without any intervening links, 

 there are natural groups, especially amongst Hypnei, which 

 admit of subdivision into others possessed of more or less 

 predominant characters, but where it is almost impossible to 

 say what is or is not a species. Take for example the com- 

 mon Hypnum cupressiforme, and you will find some of its 

 acknowledged varieties more palpably different from each 

 other than some of the neighbouring species which are ad- 

 mitted as distinct. Indeed this species may be pointed as ex- 

 hibiting the greater part of the changes to which Mosses are 

 subject. Besides difference of size, the stem with the leaves 

 varies from compressed to nearly cylindrical, and in its mode 

 of branching and length ; the leaves differ in size, form, and 

 direction, in the presence or total suppression of the nerve, 

 and in the condition of the margin ; the sporangium, in its 

 inclination, length, and form, and the lid, in the degree of its 

 development. All these, and other differences, occur in a single 

 species. But differences occur also amongst Mosses in the areo- 

 lation of the leaves, the length and curvature of the fruitstalk, 

 the size of the apophysis, — whether belonging to the peduncle 

 or sporangium, — the nature of the inflorescence, and, what is of 

 the utmost importance as regards generic distinctions in the 

 condition of the peristome, which in the same species, as in 

 Encalypta vulgaris (Plate 22, fig. 1), may be present or entirely 

 wanting ; or, as in Orthotrichum anomalum (Plate 20, fig. 5), 

 there may be rudiments of an inner peristome, while in other 

 cases there may be none. Great differences may also exist 

 in the condition of the outer teeth, whether as regards their 



