X l l INTRODUCTION.
mixed race into groups on minor structural points must outrage a natural arrangement
based on affinity, and on the other hand a truly natural arrangement must necessarily
group together, in a very perplexing manner, individuals differing in points of structure
and feature,—and, further, that though, on account of the long lapse of time required to
modify an important structural detail, species differing in this respect may be safely assumed
to be very distantly related, if at all, yet the converse, that similarity in essential details of
structure necessarily implies close relationship, does not hold good: and in this lies a perpetual
stumbling-block in the way of classification based on affinities; for any species that
is capable of divergence from the type, must also be capable of convergence to it, and the
offspring of a single pair most divergent at one time, may have again at a later period both
had a tendency to revert to the original type and thus approach each other, and, though in
t h e present generation absolutely identical, may possibly be the direct descendants of the
most dissimilar forms ever assumed by their ancestors; so t h a t we have really no more right
to take for granted that two different kinds of Barbet are less remote, in derivation from a
parent stock, from each other than a Woodpecker is from either of them; and it is an
assumption for which we have no warrant at all to say that one genus or family is more
nearly related to any other particular genus or family than to t h e remainder, on account of
t h e greater similarity in s t r u c t u r e : and this point strikes at the root of all classification
based on affinity.
W i t h reference to this it is worthy of remark that, as long as the various races of mankind
are kept separate, each geographical district presents its own peculiar type of feature and
form, easily to be identified, and yet comprising individuals varying widely as to form as well
as h a b i t ; but as soon as new blood is introduced into any country by interbreeding, the
hard line of separation ceases, and a hybrid race is produced, the characteristic feature of
which is non-uniformity—that is, t h a t among the half-bred progeny the original types
reassert themselves in t h e most aberrant manner, and with a vagueness that defies prediction
as to t h e nature of the offspring from a study of the parents. This view of the case would
lead us to infer t h a t in classification feature is a surer guide than habit, an almost certain
guide in a pure species, but deteriorating greatly in value with a mixed race, and, secondly,
that geographical distribution is effective in modifying both colour and feature. The
vagueness of our knowledge of t h e past history of the animal kingdom renders it impossible
to distinguish clearly between cause and effect; and naturalists are thus enabled' to take
whichever seems best adapted to bear out their particular views: one, assuming habit as
t h e cause, explains by this means peculiar modifications of structure otherwise unaccountable
; another, equally anxious to arrive at the truth, assumes the points of structure as the
cause, and points out the habit as a natural consequence. It will be only by observations
extending over long series of years that any definite conclusion on these points will be
obtained; and for the present (in this particular the scientific world can never too highly
appreciate the services of Mr. Darwin in his life-long series of careful experiments) ornithologists
cannot perhaps be better employed than in clearing up t h e existing confusion in
synonymy, and thus paving the way for a comparison of the results of observations in all
parts of t h e world.
INTRODUCTION. xiu
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, t h e same
b i r th ; whereas, in t h e wild and presumably pure species, the offspring are almost invariably
practically similar both to each other and to t h e 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 ('( ntral-African; and the inhabitants of adjacent countries even differ quite as conspicuously
from each other as do the allied forms of Malayan Barbets. The chief point in
which the analogy fails is, t h a t the species of men interbreed more freely than the species of
b i r d s ; 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 difficulties
of the definition of scientific terms is felt. Few naturalists could give a clear and
comprehensive idea of what they mean by t h e words " species," " genus," " family," so constantly
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 t h e 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-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
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 in this
limited group 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 If. Ilodgsoni and M. Uneata, 21. caniceps and
M. inornata; 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