212 THE PRINCIPLES OF HEREDITY 



itself of the information supplied through the medium of 

 only those rays of light which lie within that part of the 

 spectrum visible to me. Slower or faster undulations tell 

 me nothing ; but there are grounds for believing that they 

 tell a good deal to some animals. All my information comes 

 to me through a very limited number of senses, but, con- 

 ceivably, I might have many more, in which case my in- 

 formation would be more complete. Some lower animals 

 appear to have senses of which I have no more conception 

 than a man born blind can have of sight Thus, apparently 

 through the exercise of senses unknown to me, a bat seems 

 able to avoid obstacles when flying in absolute darkness. 

 Various other animals do not appear to possess all, or many, 

 or any of the senses I possess. Clearly, therefore, I must be 

 very cautious when I try to infer the mental processes of 

 lower animals from my own mental processes. The most I 

 can do with a reasonable degree of safety is to infer (1) 

 sense-impressions, and therefore a stream of sensations and 

 some sort of a mind, when I perceive structures which are 

 probably organs of sense, (2) to infer sense-impressions some- 

 what like mine when the sense organs and the nervous 

 structures connected with them resemble mine, and (3) to 

 infer a mind somewhat like mine when the brain associated 

 with it is somewhat like mine. If the animal I am considering 

 belongs to a species that is closely related to my own, I can 

 of course draw my inferences with a proportionate degree of 

 confidence and safety. 



361. All my experience tells me that feeling, and therefore 

 mind, is associated only with nervous tissue. Other sorts of 

 tissue, so far as I know, fulfil quite different functions. I 

 have no right, therefore, to infer the presence of mind in an 

 individual unless I know that the species to which it belongs 

 possesses a nervous system, however rudimentary. Accord- 

 ingly I do not suppose that plants and the lowest sorts of 

 animals are capable of feeling, much less of thinking. On 

 the other hand, the presence of nervous tissue does not 

 necessarily imply the presence of mind. For example, I 

 have no reason to suppose that consciousness is associated 

 with any of the nerve ganglia of my body except my brain. 

 It appears, therefore, that only certain more or less highly 

 evolved masses of nervous tissue are associated with mind. 



362. In common with other structures, nervous tissue, and 

 therefore mind, has been evolved by Natural Selection. Like 

 the physical structures, the mental faculties adapt the in- 

 dividual to his surroundings. Mind is quite useless to 



