xxii INTRODUCTION. 



employed in any of our British works on the subject, of giving^ in 

 italics the salient and most distinctive generic and specific 

 characters. This will, I believe, be found a help by the student, 

 but it must be remembered that it is intended rather as a practical 

 assistance in identification than as an indication of what are, from 

 a systematic standpoint, characters of importance ; for though as 

 a rule the two run side by side it not unfrequently happens that a 

 feature of such slight structural importance as colour or size may 

 separate two species at a glance, but the italicising of these 

 characters must not be looked upon as necessarily implying that 

 they form the most important distinction between the two. 



Another feature of the work which is undoubtedly in the 

 bryological literature of our islands something of an innovation, is 

 the introduction of sub-species. I have not employed this method 

 of classification without considerable deliberation and some 

 hesitancy ; nor am I unaware that it has its disadvantages, and 

 perhaps its dangers. The temptation to subdivide genera into 

 sub-genera, and species into sub-species, varieties, sub-varieties, 

 forms and sub-forms, becomes increasingly great with the growth 

 of more accurate knowledge and the closer study of forms. That 

 these terms represent actual and existing degrees of affinity there 

 can be no doubt, nor can there be any as to the importance of a 

 recognition of the fact ; but I hold that such minute sub-divisions 

 are quite out of place in a general work of this kind, and are only 

 justified in works dealing with special and limited groups, and 

 even then only when they are dealt with from a somewhat 

 different point of view, in short when the object of the work is of 

 a theoretical rather than of a practical nature. I should therefore 

 greatly deprecate the introduction of such a chain of terms, and 

 their inevitably greatly multiplied nomenclature, into works 

 intended primarily as hand-books for the student, and secondarily 

 only as text-books for the systematise I do not however think 

 that the introduction of sub-species alone is open to the same 

 objection, while it certainly meets some of the difficulties which 

 constantly present themselves to the classifier who has no middle 

 choice between species and varieties. While the method pursued 

 here gives, I believe, a truer view of the relationships of the 

 plants in question than if they were treated either as independent 

 species or as merely varieties, I do not think it will be found to 

 render the classification in any way more complex or more 

 cumbersome. I have retained the ordinary binomial designation 

 as in the case of full species, and the only practical difference will 

 be found to be that the sub-species are not numbered in the 

 headings, but indicated by an asterisk. 



