own
motions or the motions of other things. It was the part to receive the
sound-vibrations occasionally propagated through the water; the part to
be affected more strongly than any other by those variations in the
amounts of light caused by the passing of small bodies close to it; and
the part which met those diffused molecules constituting odours. That is
to say, from the beginning the surface was the part on which there fell
the various influences pervading the environment, the part by which
there was received those impressions from the environment serving for
the guidance of actions, and the part which had to bear the mechanical
re-actions consequent upon such actions. Necessarily, therefore, the
surface was the part in which were initiated the various
instrumentalities for carrying on intercourse with the environment. To
suppose otherwise is to suppose that such instrumentalities arose
internally where they could neither be operated on by surrounding
agencies nor operate on them,--where the differentiating forces did not
come into play, and the differentiated structures had nothing to do; and
it is to suppose that meanwhile the parts directly exposed to the
differentiating forces remained unchanged. Clearly, then, organization
could not but begin on the surface; and having thus begun, its
subsequent course could not but be determined by its superficial origin.
And hence these remarkable facts showing us that individual evolution is
accomplished by successive in-foldings and in-growings. Doubtless
natural selection soon came into action, as, for example, in the removal
of the rudimentary nervous centres from the surface; since an
individual in which they were a little more deeply seated would be less
likely to be incapacitated by injury of them. And so in multitudinous
other ways. But nevertheless, as we here see, natural selection could
operate only under subjection. It could do no more than take advantage
of those structural changes which the medium and its contents initiated.

See, then, how large has been the part played by this primordial factor.
Had it done no more than give to _Protozoa_ and _Protophyta_ that
cell-form which characterizes them--had it done no more than entail the
cellular composition which is so remarkable a trait of _Metazoa_ and
_Metaphyta_--had it done no more than cause the repetition in all
visible animals and plants of that primary differentiation of outer from
inner which it first wro