percipient
portions of the organs of special sense, especially of optic organs, are
often formed from the same part of the primitive epidermis" which forms
the central nervous system.[59] Similarly is it with the organs for
smelling and hearing. These, too, begin as sacs formed by infoldings of
the epidermis; and while their parts are developing they are joined from
within by nervous structures which were themselves epidermic in origin.
How are we to interpret these strange transformations? Observing, as we
pass, how absurd from the point of view of the special-creationist,
would appear such a filiation of structures, and such a round-about
mode of embryonic development, we have here to remark that the process
is not one to have been anticipated as a result of natural selection.
After numbers of spontaneous variations had occurred, as the hypothesis
implies, in useless ways, the variation which primarily initiated a
nervous centre might reasonably have been expected to occur in some
internal part where it would be fitly located. Its initiation in a
dangerous place and subsequent migration to a safe place, would be
incomprehensible. Not so if we bear in mind the cardinal truth above set
forth, that the structures for holding converse with the medium and its
contents, arise in that completely superficial part which is directly
affected by the medium and its contents; and if we draw the inference
that the external actions themselves initiate the structures. These once
commenced, and furthered by natural selection where favourable to life,
would form the first term of a series ending in developed sense organs
and a developed nervous system.[60]

Though it would enforce the argument, I must, for brevity's sake, pass
over the analogous evolution of that introverted layer, or hypoblast,
out of which the alimentary canal and attached organs arise. It will
suffice to emphasize the fact that having been originally external, this
layer continues in its developed form to have a quasi-externality, alike
in its digesting part and in its respiratory part; since it continues to
deal with matters alien to the organism. I must also refrain from
dwelling at length on the fact already adverted to, that the
intermediate derived layer, or mesoblast, which was at the outset
completely internal, originates those structures which ever remain
completely internal, and have no communication with the environment save
through the structures develope