other,
perhaps, in the whole range of natural history. We are all well
acquainted with the common frog, whose grander name is _Rana
temporaria._ We see it--and it is to be feared some of us kill it--in
our gardens, among strawberry-beds and damp vegetation. But, whereas
frogs feed upon those slugs and insects which are in the habit of
pasturing upon our plants, and are themselves indebted to us for not a
grain of vegetable matter, we ought by all means to be grateful to them.
So industrious are frogs in slug-hunting, that it would be quite worth
while to introduce them as sub-gardeners upon our flower-beds. In
catching insects, the frog suddenly darts out his tongue, which, at the
hinder part, is loose, and covered with a gummy matter. The insect is
caught, and the tongue returned with wonderful rapidity. The frog, when
it is first hatched, has the constitution of a fish: it is purely
aquatic; has a fish's heart, a fish's circulation, and a fish's gills.
The tadpole swims as a fish does--by the movement, side-ways, of its
tail. For the unassisted eye, and still more for the microscope, what
spectacle can be more marvelous than the gradual process of change by
which this tiny fish becomes a reptile? Legs bud; the fish-like gills
dwindle by a vital process of absorption; the fish-like air-bladder
becomes transmuted, as by a miracle, into the celled structure of lungs;
the tail grows daily shorter, not broken off, but absorbed; the heart
adds to its cells; the fish becomes a reptile as the tadpole changes to
a frog. The same process we observe in toads; and it is also the same in
our newts, excepting that in newts the tail remains. There is no
parallel in nature to this marvelous and instructive metamorphosis.

The perfectly-formed frog does not live of necessity in water, or near
it, but requires damp air occasionally. It breathes by lungs, as we have
said; but, as it has no ribs, there is no chest to heave mechanically.
The frog's air has to be swallowed, to be gulped down into the lungs.
That is not possible unless the mouth is shut; and, therefore, as we
might suffocate a man by keeping his mouth shut, so we should suffocate
a frog by keeping his mouth open. Yet we should not suffocate him
instantly; we should disable the lungs; but, in this class of animals
the whole skin is a breathing surface. A frog has lived a month after
his lungs had been extracted. All respiratory surfaces, like the inside
of our own lungs, can