THE LIVING WORLD. 
279 
and others which are unable to take wing, not only is the wing imperfectly 
developed, but the bones are almost solid, a fact which operates to confirm the 
theory here advanced, or at least lends great plausibility to it. 
Next to the feathers, in point of wonderful adaptation and structure, is 
the eye of a bird, and in this, too,, we observe another analogy, or correspond¬ 
ence, with reptiles. In all mammals the eye is protected by a double lid, an 
upper and a lower, but in birds there is a third lid, technically called the 
nictitating membrane. This so-called lid is controlled by a muscle united to it 
at the outer corner of the cornea, by which the membrane may be drawn quickly 
across the globe of the eye and back again, by which motion the eye is kept 
moistened and clear of foreign substances, as is done by the winking of the lids 
in mammals. This nictitating membrane is noticeable in the eye-structure of 
a large number of reptiles, but is never present in fishes, insects or mam¬ 
mals. By this provision the vision of a bird and of the reptiles that possess it 
is rendered much more acute and superior to that of man. 
As classification is much a matter of individual or arbitrary arrangement 
—beyond the confinement of species under appropriate heads—we will preserve 
a consistency in the general plan of this work by introducing, not what can be 
called the lowest orders, for all are equally perfect, but by first describing the 
smaller species, and maintaining a gradual ascent from the least to the greatest, 
in which respect it will bear the semblance at least of progression, if not develop¬ 
ment. Under this arrangement, therefore, we will first introduce the 
TROCHILID.®, OR HUMMING-BIRDS. 
It is a matter for some wonder that humming-birds are confined exclusively 
to the Western Continent, particularly since their range of latitude is almost 
equal to that of North and South America. The species are very numerous, some 
of which may be found widely distributed, while others are restricted to a single 
locality less than a dozen miles in diameter. This variable distribution is due 
to the fact that those of widest range are swift of wing, and can sustain them¬ 
selves in a long flight, while those whose habitat is circumscribed are feeble, 
have short wings, and are otherwise incapable of wandering any considerable 
distance. 
Wherever found, the humming-bird , so called from the whirring sound pro¬ 
duced by its wings, is a creature that excites our admiration as being com¬ 
parable to an animated flower. Not all have gorgeous plumage, but the delicacy 
of structure, and the gracefulness with which they poise above a flower, when 
extracting the nectar from its chambers; the manner in which they flit from 
bloom to bloom, or dart off like a bee in arrowy flight, bearing away in their 
crops a drink upon which the gods were once believed to regale themselves, is 
a habit peculiar to the several species. 
In the British Museum is a collection of every known species of humming¬ 
bird , more than four hundred in number, but the total number of specimens 
there shown is perhaps half a million, a very large wing of the building being 
entirely donated to these beautiful creatures. 
Though we generally observe these birds hovering amid the flowers, honey 
is not their only food, for experiments have shown that none of the genus can 
subsist without at least an occasional diet of insects—spiders and flies being their 
preferable animal food Some of the least attractive species seem to live chiefly 
