INTEODIJCTION. 
discoveiy of such facts when the localities they inhabit shall have been more closely watched ; 
and it becomes necessary to stretch our definition still further, and assume that where dif 
ferent geographical distribution is found permanently associated with a difference of size and 
colouring, it will simplify matters to describe each form separately, even though interbreeding 
may take place at the line of junction. 
Having thus eliminated the question of affinity, the grouping of the birds into subfamilies 
and genera on arbitrarily selected differences in structure becomes a matter of extreme sim- 
plicity, and bases itself upon intelligible grounds; and we are thus enabled to make divisions 
on such features as the shape of the bill, and the amount of rictal bristles, which, though of 
comparatively small value in determining relationships, are particularly useful in arbitrary 
groups : they are easy of definition, sufficiently prominent to catch the eye ; and, last but not 
least, they are equally available to those who study the birds in life, and to the less fortu- 
nately situated naturalists whose acquaintance with most birds is necessarily limited to an 
examination of the dry and distorted skin. 
Of the two features which we have selected, one, the rictal bristles, possessed as they are 
by many widely differing groups of birds, completely puzzles us ; and after studying the birds 
for some years inside and outside, in life and in death, we are unable even to hazard a guess 
at the purpose they are meant to serve ; the other, the bill, so far from being a typical 
feature in the scansorial group, is the one which, as Mr. A. II. Wallace has pomted out in 
a very able paper on the natural affinities of birds, presents the greatest variety of form ; 
and had we professed to have been guided by affinities, the selection of these features might 
have laid us open to the charge of inconsistency. For the rest, though the grouping is 
arbitrary, it is for the most part familiar, and we have avoided as far as possible any inno- 
vations in the nomenclature. We should prefer ourselves sinking the generic names 
altogether, and retaining only the three subfamilies ; but the definitions would have lost in 
clearness and conciseness what they gained in limitation of quantity, and, as far as we can 
judge, identification will be simpler with the minor subdivisions and restricted genera 
retained ; and the advantage thus gained may be found adequate compensation for the extra 
tax on the memory. Those who do not think so will of course drop the minor subdivisions ; 
but those who do, will, we think, find the grouping sufficiently clear, and, though based on 
no affinity, still containing nothing to outrage the doctrines of the supporters of a natural 
theory. 
In conclusion a few remarks are necessary with regard to the word " type " so frequently 
used in this as in all other works on natural history. The word has two distinct applications :— 
one, the philosophical sense, in which community of descent is imphed, and which signifies 
either the common progenitor, or the existing species possessing its peculiar characteristics 
most strongly marked ; the other, the technical sense, meaning simply the species or genus 
to which the specific or generic name was first applied. It is in this latter sense that it has 
been used throughout the body of the work, while in this introductory chapter it has been 
used in the former or philosophical sense. 
