ke positions
with respect to modifying forces, that the originally-like units of a
living mass become unlike. We must not therefore expect any established
differentiation of parts in creatures which exhibit no established
differences of position in their parts.

This negative evidence is borne out by abundant positive evidence. When
we turn from these ever-changing specks of living jelly to organisms
having unchanging distributions of substance, we find differences of
tissue corresponding to differences of relative position. In all the
higher _Protozoa_, as also in the _Protophyta_, we meet with a
fundamental differentiation into cell-membrane and cell-contents,
answering to that fundamental contrast of conditions implied by the
words outside and inside. And on passing from what are roughly classed
as unicellular organisms to the lowest of those which consist of
aggregated cells, we equally observe the connexion between structural
differences and differences of circumstance. In the sponge, permeated
throughout by currents of sea-water, the absence of definite
organization corresponds with the absence of definite unlikeness of
conditions. In the _Thalassicolla_ of Professor Huxley--a transparent,
colourless body, found floating passively at the surface of the sea, and
consisting essentially of "a mass of cells united by jelly"--there is
displayed a rude structure obviously subordinated to the primary
relations of centre and surface: in all of its many and important
varieties, the parts exhibit a more or less concentric arrangement.

After this primary modification, by which the outer tissues are
differentiated from the inner, the next in order of constancy and
importance is that by which some part of the outer tissues is
differentiated from the rest; and this corresponds with the almost
universal fact that some part of the outer tissues is more directly
exposed to certain environing influences than the rest. Here, as before,
the apparent exceptions are extremely significant. Some of the lowest
vegetable organisms, as the _Hematococci_ and _Protococci_, evenly
imbedded in a mass of mucus, or dispersed through the Arctic snow,
display no differentiations of surface: the several parts of the surface
being subjected to no definite contrasts of conditions. The
_Thalassicolla_ above mentioned, unfixed, and rolled about by the waves,
presents all its sides successively to the same agencies; and all its
sides are alike. A cili