124 HUME v 



demonstrated the existence of the closest simi- 

 larity between the two, not only in structure, as 

 far as the microscope will carry us, but in func- 

 tion, 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. Eobinson Crusoe did not feel 

 bound to conclude, from the single human foot- 

 print 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 ol- 

 factory 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 conscious- 

 ness, the same reason exists for the belief that 



