INTEODUCTION. 
xiii 
The cases of variation among birds are, as far as we can see, analo/^^rMJs to tliosc of the 
human race; and in all species known to be intercrossed with others, tlie (^ffspiin^' is 
variable, and each brood contains widely differing individuals, though produc(;d under 
conditions the most favourable to uniformity, the same father, the same rrujther, the same 
birth ; whereas, in 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 in 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 in 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 ; 
for 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 
birds ; and to estabhsh 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 
groups based on outward similarity, or else imply consanguinity of the individuals so grouped 
together ; and the discussions about allied genera, and positions to be occupied in a natural 
arrangement have no meaning unless they infer community of descent. For presuming 
consanguinity we have nothing further to go on than similarity and the known possibility 
of hereditary variation ; and these we have shown to be no sufficient guide for position, and 
the difficulty before us in insisting on a natural arrangement is as great as ever ; for, as almost 
every complete organ can be shown to be produceable by minute hereditary gradations, we 
are left with no data whatever for the establishment of non-afiinity, 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 
followed is simply intended to facilitate the identification of the species, and not to imply 
relationships, which may or may not exist. 
For species the best limitation seems to be those birds which ordinarily interbreed only 
among each other, 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 m this 
limited graup to afford a standpoint from which to commence. Even here, however, there 
is a measure of indefiniteness ; for, to prevent confusion, we have been obliged to retain as 
distinct such closely resembling species as M. Hodgsoni and M. Uneata, M. cankejjs and 
M. inornata; and though as yet no proofs are forthcoming of the interbreeding ot these 
species, and no intermediate varieties have been found, we see no improbability of a future 
