INTRODUCTION 
XVII 
It is felt that, in the case of a well-worked area like the British Islands, some middle course 
is desirable ; and accordingly the species in the present work are much wider than those of Jordan 
and considerably narrower than those of Bentham. We believe that the adoption of this middle 
course will commend itself to the great majority of botanists. 
How species are subdivided into varieties 
We also desire to make clear our position with regard to the subdivision of species into 
varieties. Here there are two plans each of which finds favour in certain circles. One is to 
regard a certain form of a species as typical of that species, and to regard any deviations from 
that type as varieties. The second plan is to subdivide the same species wholly into varieties, 
just as a genus is wholly subdivided into species. Populus tremula may be taken as an illus- 
tration. Two varieties of this are recognised as British. One is a form whose young leaves 
are silky, and the other a form whose leaves (excepting the leaves of the suckers) are always 
glabrous or almost glabrous. If the first of the above plans be adopted, it becomes necessary 
to decide which of the two varieties shall be regarded as the type. Supposing the silky variety 
be regarded as the type, the British forms would be written thus : — 
Populus tremula 
(b) var. glabra. 
If the glabrous variety be regarded as the type, then the British forms would be written thus : — 
Populus tremula 
(b) var. sericea. 
However we ourselves have decided not to adopt this first plan but the second ; and accordingly 
we write the British forms thus : — 
Populus tremula 
(a) var. sericea 
(b) var. glabra. 
We have decided on this plan for two reasons. First, it is (so far as we are able to judge) 
quite arbitrary in many cases to decide which of the forms of a species is the type ; and it is 
unusual to find agreement among botanists as to which form is to be regarded as the 
type and which the deviation from the type. We frequently find that the form which a 
botanist regards as the type is merely the form which he happens to have come to know 
first, or the form which is more abundant in the district which he usually investigates ; and 
we find that this view of the type of the species sometimes prevents him from taking a broad 
view of the relationships of the different forms of the species. Secondly, it is impossible, if the 
first plan be chosen, for a botanist to record definitely the existence of a species in a given 
locality without committing himself to the recording of a particular form of that species, and of 
a form, it may be, of whose distinguishing characters he is wholly ignorant. By adopting the 
second plan, it is possible to record the existence of a species in a particular locality without 
being so committed ; and, if it be desired to make the additional observation that the species 
exists in that locality in a particular form, it is only necessary to add the name of the par- 
ticular variety, whichever it may happen to be, to that of the species. 
Subvarieties and formae 
Subvarieties and formae are prefixed by Greek letters, varieties by Roman letters. A 
subvariety is distinguished by a single character which is known or presumed to be constant, 
and is not related to habitat-conditions. A forma is known or presumed to be due to habitat- 
conditions, and reverts to the normal form of the variety or species when transplanted to the 
ordinary habitat of that variety or species. 
Sign of certainty 
A note of exclamation (!) after a synonym indicates that an authentic specimen has been 
seen, and that if more than one such specimen has been seen all the specimens are alike. 
