various aspects of consciousness one at a time, and we must
unavoidably turn our backs temporarily on most of the proc-
esses, apart from the special one we are for the moment
engaged in studying. We must bear contantly in mind, there-
fore, this partial and tentative mode of procedure, remember-
ing that the mind, which we thus analyse piecemeal, is in
point of fact a real unit.

It will assist us to gain a proper appreciation of the sig-
nificance of sensation to remark at the outset certain facts
about the evolution of sense organs.

The Evolution of Sense Organs.-- That it may be put into
the most delicate and complete accord with the world in which
it is placed, the organism must be capable of responding to
the various objects found therein. To this end we find the
sense organs so devised that they may give information about
the most widely differing kinds of physical existence. Each
form of sensation which we possess is apparently connected
with the activity of a specially constituted receiving ap-
paratus. In some cases this apparatus is extremely complex,
as in the eye for example: in other instances it is very simple,
as in the case of the so-called pain nerves of which we shall
speak in a moment. These end-organs are connected with
special parts of the cerebral cortex which they incite to
activity. These cortical connections we examined in Chap-
ter II.

There seems to be no doubt that even very simple forms of
organism are sensitive in a rude way to most, if not all, of the
types of sensory stimuli to which human beings respond, e. g.,
light, sound, mechanical impact, etc. This is simply another
way of saying that protoplasm itself is sensitive to these
modes of stimulation. But so far as concerns the develop-
ment of definitely differentiated sense organs, specially de-
vised to receive particular modes of sensory stimuli, the facts
seem to indicate great irregularity and wide variation among
different organic forms. The kinds of sensitivity which are

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