INSESSORES, OR PERCH ING BIRDS.
The inequality between natural groups, oconpying a corresponding or analogous
station in their own circles, is nowhere more apparent than on looking to the
Insessorial birds,—the typical order of this great division of vertebrated animals.
The Vertebrata will not bear the least comparison, in point of numbers,
with the Annulosa; the proportion of the former to the latter being, in all probability,
not more than 1 to 15; yet no one would think of denying to these divisions
an equal value in the scale of creation, merely because their contents are so
strikingly disproportionate. This inequality, moreover, is observable in another
way. It may generally be remarked, on comparing the typical with the subtypical
groups, that the contents of the latter are the most numerous. But
sometimes this proportion is reversed, and the typical circle exhibits a greater
diversity of forms than are to be met with in all the other groups put together.
The order of Insessores is of this description. It comprehends such a vast
assemblage of species, arranging themselves under subordinate tribes, so distinct
in themselves, that it becomes extremely difficult to select characters sufficiently
comprehensive to define the order. This difficulty, indeed, is so great, that some
naturalists have contented themselves with assigning negative rather than positive
distinctions to this order. For if we rest on their inhabiting the land rather than
the water, by what reason can we exclude the Gallinaceous tribes, and even many
of the Grallatores, whose chief haunts are upon dry moors and commons ? The
Cinclus, again, is amphibious. If we characterize the Insessores as exclusively
perching birds, the term, rigidly understood, is equally objectionable ; for several
families live entirely upon the ground; while others, among the wading and
swimming orders, habitually repose upon trees. These, and a thousand similar
difficulties, have ever embarrassed the speculations of those naturalists who seek for
absolute and unexceptionable distinctions in nature; where, in truth, as they do
not exist, so they have never been found. Without, therefore, dwelling further
upon such exceptions to the prominent features which characterize the order
before us, we may state that it is distinguished,—
First, by the feet being of that construction most adapted for perching or grasping
; the hallux or hind toe being always present, and articulated on the same plane
with the fore toes.
Secondly, by the absence of that strongly-defined tooth, which gives to the
rapacious birds the exclusive power of tearing or dividing their food, previous to
swallowing it.
Thirdly, by the presence, in the typical groups, of a small notch in one or
both mandibles, enabling the bird to hold, but not divide, its food, which is
swallowed almost universally in an entire state.
The two first distinctions are much more positive and universal than the last ;
but the whole will sufficiently detach the Perchers from all other birds.
The primary divisions of this order are so obvious, that the most inexperienced
student cannot fail to perceive them. The Dentirostres, Conirostres, Scan-
sores, Tenuirostres, and Fissirostres, are so many prominent groups, which
have been long recognised in the celebrated Prodromus of Illiger, the Règne
Animal of Cuvier, and sufficiently so in other works ; they have been accordingly
cited, with justice, to illustrate the circular nature of the Insessores. So far
as regards the admission of these groups, nothing more need be said, since
they have been universally adopted. This unanimity, however, among ornithologists
merely extends to the typical forms ; all agree, for instance, in classing
the Woodpeckers and Parrots as climbing birds. But because the scansorial
structure is exhibited under a different form in the Grimpereaux of M. Cuvier, we
find these birds occupying a station in the Règne Animal with the Tenuirostres.
Even this latter tribe, by some writers, is made to embrace every bird suspected
of having a filamentous tongue, however- strongly it may be allied, in all other
parts of its organization, with groups far differently situated. When, therefore,
it is said that the five sub-orders or tribes above named have been universally
admitted, it must be at the same time remembered that no two writers have yet
agreed on their definite nature. Hence it becomes advisable, before we proceed
further, to attempt their true definition.
We have already stated an opinion, that the Insessorial order first resolves
itself into three circular groups ; by which disposition the Scansores, the Tenuirostres,
and the Fissirostres, are united into one ; and that the two others, namely,
the Conirostres and the Dentirostres, constitute the typical and the subtypical
circles. Their prominent distinctions may be thus stated :—