INTRODUCTION. 
xm 
The cases of variation among birds are, as far as we can see, analogous to those of the 
human race; and in all species known to be intercrossed with others, the offspring is 
variable, and each brood contains widely differing individuals, though produced under 
conditions the most favourable to uniformity, the same father, the same mother, the same 
bn-th ; whereas, m the wild and presumably pure species, the offspring are almost invariably 
practically similar both to each other and to the parents; and the presumption is, we think, 
fairly m favour of the conclusion that such wild species as the ruff and florikin &c., which 
do produce varied offspring, are mixed and not pure races. The occurrence of allied species 
of birds m the various geographical districts of the earth, and more especially in adjacent 
islands and groups of islands, is analogous to the occurrence of the different races of man ; 
foi the Saxon is not more different from the Negro than the North-Indian Barbets are from 
the Central- African ; and the inhabitants of adjacent countries even differ quite as conspi- 
cuously from each other as do the allied forms of Malayan Barbets. The chief point in 
which the analogy fails is, that the species of men interbreed more freely than the species of 
nxls ; and to establish the origin with certainty is difficult enough with the former, ten 
times more so with the latter. 
When an attempt is made to fix the limits of a single group, the full force of the diffi- 
culties of the definition of scientific terms is felt. Few naturalists could give a clear and 
comprehensive idea of what they mean by the words “ species,” “ genus,” “ family,” so con- 
stantly in their mouths. It seems clear that they must either mean arbitrary divisions into 
(-.loups asec on outwaid similarity, or else imply consanguinity of the individuals so grouped 
oge er ; an the discussions about allied genera, and positions to be occupied in a natural 
rrangement have no meaning unless they infer community of descent. For presuming 
consanguinity we have nothing further to go on than similarity and the known possibilit^ 
: be - “ — *» 
every complete organ can be shown to be producel^ minute Wdi^J 
are left with no data whatever for the establishment of non-affinity, and there is we repeat 
no more solid reason than similarity why we should place Barbets near Toucans (as the 
expression is) rather than near Bee-eaters or any other birds. This being the case, we have 
left out all consideration of possible affinities in our arrangement, and based the limitation 
of both families and genera on mere structural resemblances; and the classification we have 
oued is simply intended to facilitate the identification of the species, and not to imnlv 
relationships, which may or may not exist. 1 3 
lor species the best limitation seems to be those birds which ordinarily interbreed onlv 
am°n 0 each othei, and of which the offspring are similar both to each other and to the 
parents. This definition cannot of course be taken to be universal ; but it is sufficient in this 
.united group to afford a standpoint from which to commence. Even here, however there 
is a measure of mdefimteness ; for, to prevent confusion, we have been obliged to retain as 
distinct such closely resembling species as M. Hodgsoni and M. lineata, M. canicevs and 
. tnwncc a, and though as yet no proofs are forthcoming of the interbreeding of these 
species, and no intermediate varieties have been found, we see no improbability of a future 
