214 PIONEERS OF EVOLUTION PART 



luminous irritations, or of his own skin to pricking, 

 passes on to like enquiry into the numerical relations 

 between the energy of the stimuli of light, sound, 

 and so forth, and the energy of the sensations which 

 they arouse in the nerve-channels.' An excellent 

 summary, with references to the newest authorities 

 on the subject, is given by Prince Kropotkin in the 

 Nineteenth Century of August 1896. 



All this, to the superficial onlooker, seems rank 

 materialism. But we cannot think without a brain 

 any more than we can see without eyes, and any 

 enquiry into the operations of the organ of thought 

 must run on the same lines as enquiry into the 

 operations of any other organ of the body. And 

 the enquiry leaves us at the point whence we began 

 in so far as any light is thrown on the connection 

 between the molecular vibrations in nerve -tissue 

 and the mental processes of which they are the indis- 

 pensable accompaniment. Changes take place in 

 some of the thousands of millions of brain-cells in 

 every thought that we think, and in every emotion 

 that we feel, but the nexus remains an impenetrable 

 mystery. Nevertheless, if we may not say that the 

 brain secretes thought as we say that the liver 

 secretes bile, we may also not say that the mind is 

 detachable from the nervous system, and that it is 

 an entity independent of it. Were it this, not only 

 would it stand outside the ordinary conditions of 

 development, but it would also maintain the equi- 

 librium which a dose of narcotics or of alcohol, or 

 which starvation and gorging alike rapidly upset. 



In his posthumous essay On the Immortality of 

 the Soul, Hume says : ' Matter and spirit are at 



