essicated _Protophyta_ and _Protozoa_ shrink into
mere dust; and among the acalephes we find but a few grains of solid
matter to a pound of water. The higher aquatic plants, in common with
the higher aquatic animals, possessing as they do much greater tenacity
of substance, also contain a greater proportion of the organic elements;
and so are chemically more unlike their medium. And when we pass to the
superior classes of organisms--land plants and land animals--we find
that, chemically considered, they have little in common either with the
earth on which they stand or the air which surrounds them.

In _specific gravity_, too, we may note the like. The very simplest
forms, in common with the spores and gemmules of the higher ones, are as
nearly as may be of the same specific gravity as the water in which they
float; and though it cannot be said that among aquatic creatures
superior specific gravity is a standard of general superiority, yet we
may fairly say that the superior orders of them, when divested of the
appliances by which their specific gravity is regulated, differ more
from water in their relative weights than do the lower. In terrestrial
organisms, the contrast becomes extremely marked. Trees and plants, in
common with insects, reptiles, mammals, birds, are all of a specific
gravity considerably less than the earth and immensely greater than the
air.

We see the law similarly fulfilled in respect of _temperature_. Plants
generate but an extremely small quantity of heat, which is to be
detected only by delicate experiments; and practically they may be
considered as being in this respect like their environment. Aquatic
animals rise very little above the surrounding water in temperature:
that of the invertebrata being mostly less than a degree above it, and
that of fishes not exceeding it by more than two or three degrees, save
in the case of some large red-blooded fishes, as the tunny, which exceed
it by nearly ten degrees. Among insects, the range is from two to ten
degrees above that of the air: the excess varying according to their
activity. The heat of reptiles is from four to fifteen degrees more than
that of their medium. While mammals and birds maintain a heat which
continues almost unaffected by external variations, and is often greater
than that of the air by seventy, eighty, ninety, and even a hundred
degrees.

Once more, in greater _self-mobility_ a progressive differentiation is
traceable. Dead m