PSYCHICAL EVOLUTION iii 



2. Closely allied to the foregoing rcaGon for a 

 belief in the evolution of mind is that derived from 

 a comparative survey of the nervous s}stem in 

 man and other animals. In man, mind is closely 

 associated with a certain tissue or system of 

 tissues — nerve tissue or the nervous sjstun. That 

 mind is correlated with nerve structure, and that 

 mental anatomy may be learned from a study of 

 the anatomy of the nervous system, especially of 

 the brain, is the basic postulate of the science of 

 physiological psychology. Now, nerve cells exist 

 in all animals above the sponge, and a compara- 

 tively well-developed nervous system is found even 

 among many of the invertebrates, as the higher 

 worms, crustaceans, insects, and mollusks. The 

 nervous system of invertebrates, though composed 

 of the same kind of tissue, is constructed accord- 

 ing to a somewhat different plan of architecture 

 from that of the vertebrates. But in all of the 

 great family of backboned animals the nervous 

 system is built on the same general plan as in 

 man, with a cerebro-spinal trunk extending from 

 the head along the back and motory and sensory 

 nerves ramifying to all parts of the body. There 

 is also a sympathetic nervous system in all animals 

 down as far as the insects. The brain, which is 

 the most important part of the nervous system, and 

 which has been called the 'organ of consciousness,' 

 presents throughout the animal kingdom, from 

 its beginning in the worms to man, a graduated 

 series of increasing compHcation proceeding out 

 of the same fundamental type. This is especially 



