It is not merely that the observation of the actions of animals almost irresistibly suggests the attribution to them of mental states, such as those which accompany corresponding actions in men.

The minute comparison which has been instituted by anatomists and physiologists between the organs which we know to constitute the apparatus of thought in man, and the corresponding organs in brutes, has demonstrated the existence of the closest similarity between the two, not only in structure, as far as the microscope will carry us, but in function, as far as functions are determinable by experiment.

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.