There is no question in the mind of anyone 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.