There is
no question in the mind of any one acquainted with the facts that, so
far as observation and experiment can take us, the structure and the
functions of the nervous system are fundamentally the same in an ape, or
in a dog, and in a man. And the suggestion that we must stop at the
exact point at which direct proof fails us; and refuse to believe that
the similarity which extends so far stretches yet further, is no better
than a quibble. Robinson Crusoe did not feel bound to conclude, from the
single human footprint which he saw in the sand, that the maker of the
impression had only one leg.
Structure for structure, down to the minutest microscopical details, the
eye, the ear, the olfactory organs, the nerves, the spinal cord, the
brain of an ape, or of a dog, correspond with the same organs in the
human subject. Cut a nerve, and the evidence of paralysis, or of
insensibility, is the same in the two cases; apply pressure to the
brain, or administer a narcotic, and the signs of intelligence disappear
in the one as in the other. Whatever reason we have for believing that
the changes which take place in the normal cerebral substance of man
give rise to states of consciousness, the same reason exists for the
belief that the modes of motion of the cerebral substance of an ape, or
of a dog, produce like effects.
A dog acts as if he had all the different kinds of impressions of
sensation of which each of us is cognisant. Moreover, he governs his
movements exactly as if he had the feelings of distance, form,
succession, likeness, and unlikeness, with which we are familiar, or as
if the impressions of relation were generated in his mind as they are in
our own.
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