tion, so there appears to be a
connexion between integration of parts and sameness of function.

* * * * *

Closely related to the general truth that the evolution of all organisms
is carried on by combined differentiations and integrations, is another
general truth, which physiologists appear not to have recognized. When
we look at the organic world as a whole, we may observe that, on passing
from lower to higher forms, we pass to forms which are not only
characterized by a greater differentiation of parts, but are at the same
time more completely differentiated from the surrounding medium. This
truth may be contemplated under various aspects.

In the first place it is illustrated in _structure_. The advance from
the homogeneous to the heterogeneous itself involves an increasing
distinction from the inorganic world. In the lowest _Protozoa_, as some
of the Rhizopods, we have a homogeneity approaching to that of air,
water, or earth; and the ascent to organisms of greater and greater
complexity of structure, is an ascent to organisms which are in that
respect more strongly contrasted with the relatively structureless
masses in the environment.

In _form_ again we see the same truth. A general characteristic of
inorganic matter is its indefiniteness of form, and this is also a
characteristic of the lower organisms, as compared with the higher.
Speaking generally, plants are less definite than animals, both in shape
and size--admit of greater modifications from variations of position and
nutrition. Among animals, the _Amoeba_ and its allies are not only
almost structureless, but are amorphous; and the irregular form is
constantly changing. Of the organisms resulting from the aggregation of
amoeba-like creatures, we find that while some assume a certain
definiteness of form, in their compound shells at least, others, as the
Sponges, are irregular. In the Zoophytes and in the _Polyzoa_, we see
compound organisms, most of which have modes of growth not more
determinate than those of plants. But among the higher animals, we find
not only that the mature shape of each species is quite definite, but
that the individuals of each species differ very little in size.

A parallel increase of contrast is seen in _chemical composition_. With
but few exceptions, and those only partial ones, the lowest animal and
vegetal forms are inhabitants of the water; and water is almost their
sole constituent. D