is common to both man
and animal gives their reasoning processes a fundamental unity.
But Descartes's position is open to very serious objections, if the
evidence that animals feel is insufficient to prove that they really
do so. What is the value of the evidence which leads one to believe
that one's fellow-man feels? The only evidence in this argument of
analogy, is the similarity of his structure and of his actions to
one's own. And if that is good enough to prove that one's fellow-man
feels, surely it is good enough to prove that an ape feels. For the
differences of structure and function between men and apes are utterly
insufficient to warrant the assumption, that while men have those
states of consciousness we call sensations, apes have nothing of the
kind. Moreover, we have as good evidence that apes are capable of
emotion and volition as we have that men other than ourselves are. But
if apes possess three out of the four kinds of states of consciousness
which we discover in ourselves, what possible reason is there for
denying them the fourth? If they are capable of sensation, emotion,
and volition, why are they to be denied thought (in the sense of
predication)?

No answer has ever been given to these questions. And as the law of
continuity is as much opposed, as is the common sense of mankind, to
the notion that all animals are unconscious machines, it may safely be
assumed that no sufficient answer ever will be given to them.

There is every reason to believe that consciousness is a function
of nervous matter, when, that nervous matter has attained a certain
degree of organization, just as we know the other "actions to which
the nervous system ministers," such as reflex action and the like, to
be. As I have ventured to state my view of the matter elsewhere, "our
thoughts are the expression of molecular changes in that matter of
life which is the source of our other vital phenomena."

Mr. Wallace objects to this statement in the following terms:--

"Not having been able to find any clue in Professor Huxley's
writings to the steps by which he passes from those vital
phenomena, which consist only, in their last analysis, of
movements by particles of matter, to those other phenomena
which we term thought, sensation, or consciousness; but,
knowing that so positive an expression of opinion from him
will have great weight with many persons, I shall endeavour
to show, with as much b