ON THE GENUS POLYMORPHINA. 201 
to glide into each other by absolutely imperceptible gradations. This does not diminish 
the necessity for subdivision, but it alters the zoological significance of the constituent 
groups, which have no claim to rank as true species, although necessarily distinguished 
by trivial names. The headings under which the various modifications of the type have 
been arranged in the present memoir are based upon what we regard as the best repre- 
sentative specimens; and under one or other of these we have been able without much 
difficulty to place nearly all the forms described by the authors referred to. But when 
the most closely related individuals may differ in so many small particulars affecting their 
general appearance, and the successive links in the chain are so close, considerable lati- 
tude in minor points of variation may be properly allowed to each subgroup. Under 
such circumstances it is impossible entirely to avoid artificial distinctions; and the best 
that can be done is to accept only those that are really serviceable and do not interfere 
with natural sequence in general characters. Those who prefer an extended nomencla- 
ture based upon evervarying minute peculiarities, have ample field amongst the Poly- 
morphinæ for the exercise of their ingenuity ; indeed, if the subdivision be carried much 
further than the limits we have adopted, it can hardly stop short of naming every spe- 
eimen. There would be no difficulty in splitting up each of the groups which, with us, 
stand in the place of “ species” into half a dozen smaller sets; but if this were done, just 
as great necessity would appear again to subdivide, and so forth: and even such a pro- 
cess of multiplication of groups has found favour with some authors. 
The plan which we have adopted in reducing the nomenclature has been to go carefully 
over the whole of the published descriptions and figures to which we have had access, 
taking them as nearly as possible in the order of precedence, selecting in the process the 
varieties which afforded the best-defined characters, and only introducing new subtypes 
for specimens not referable to previously described forms. Upwards of a hundred me- 
moirs, dating from 1780 to the present time, have been worked over in this way; and 
no pains have been spared to place successive writers on a proper footing in respect to the 
forms they have described. In some instances scant justice may appear to have been 
done to the labours of an author in the non-adoption of trivial names founded on labori- 
ously worked-out diagnoses. Such omissions, alluded to more at length at a subsequent 
page, do not result from any lack of disposition to accept already published material ; 
and where names previously employed do not appear, it is due to the fact that we have 
failed to see any advantage likely to be gained by their retention. 
In almost every case we have adopted the name first given to a variety as its proper 
designation, on the simple ground of priority, making no distinction whether its original 
application was to a recent orto a fossil specimen ; butin one or two instances the descrip- 
tion and figures appended to the first mention of a “ species ” are ill defined or obscure, or 
in some way do not fairly represent the subgroup to which it belongs, whilst a later name 
given to a better representative may have been in general use and be already well under- 
stood. In these rare cases we have chosen the better type, or have merged the question 
of priority in that of practical convenience. 
The drawings of the different “species” have, as far as possible, been made from 
specimens; when copied, they have been carefully redrawn from the originally pub- 
