atter is inert: some form of independent motion is our
most general test of life. Passing over the indefinite border-land
between the animal and vegetable kingdoms, we may roughly class plants
as organisms which, while they exhibit the kind of motion implied in
growth, are not only without locomotive power, but in nearly all cases
are without the power of moving their parts in relation to one another;
and thus are less differentiated from the inorganic world than animals.
Though in those microscopic _Protophyta_ and _Protozoa_ inhabiting the
water--the spores of algae, the gemmules of sponges, and the infusoria
generally--we see locomotion produced by ciliary action; yet this
locomotion, while rapid relatively to their sizes, is absolutely slow.
Of the _Coelenterata_, a great part are either permanently rooted or
habitually stationary, and so have scarcely any self-mobility but that
implied in the relative movements of parts; while the rest, of which the
common jelly-fish serves as a sample, have mostly but little ability to
move themselves through the water. Among the higher aquatic
_Invertebrata_,--cuttle-fishes and lobsters, for instance,--there is a
very considerable power of locomotion; and the aquatic _Vertebrata_ are,
considered as a class, much more active in their movements than the
other inhabitants of the water. But it is only when we come to
air-breathing creatures that we find the vital characteristic of
self-mobility manifested in the highest degree. Flying insects, mammals,
birds, travel with velocities far exceeding those attained by any of the
lower classes of animals; and so are more strongly contrasted with their
inert environments.

Thus, on contemplating the various grades of organisms in their
ascending order, we find them more and more distinguished from their
inanimate media in _structure_, in _form_, in _chemical composition_, in
_specific gravity_, in _temperature_, in _self-mobility_. It is true
that this generalization does not hold with regularity. Organisms which
are in some respects the most strongly contrasted with the inorganic
world, are in other respects less contrasted than inferior organisms. As
a class, mammals are higher than birds; and yet they are of lower
temperature, and have smaller powers of locomotion. The stationary
oyster is of higher organization than the free-swimming medusa; and the
cold-blooded and less heterogeneous fish is quicker in its movements
than the warm-blood