36 DARWINISM AND EVOLUTION. 



The brain, which is thus a register of sensations, is able in the 

 same manner to record sensory changes that have not even been 

 consciously perceived. Moreover it not only contains a symbolic 

 representation of much of an animal's past environment ; it 

 likewise represents in some degree, by the law of heredity, the 

 experiences of the race. 



An organism whose inner relations thus answer to outer 

 relations, both present and past, may possess a sense of the 

 continuity of life and a feeling of personal identity. 



Waves of molecular motion set up by sensory changes con- 

 tinually pass through appropriate groups of storage-cells, increasing 

 their tension and augmenting the organism's potential recollection ; 

 or, spreading through wider groups, they may excite interaction 

 between cerebral centres, and occasion memories or trains of 

 thought. We hear the scream of a child ; we think of possible 

 peril and of possible succour. A wave of energy has passed 

 through the storage-cells that represent those ideas ; and perhaps 

 discharges some of their accumulated force in remedial effort. 



When local action takes place in the brain, and feeble currents 

 flow among groups of storage-cells, " unconscious cerebration " 

 and unremembered dreams may be the result. The passage of 

 more vigorous intracerebral waves coincides with reasoning 

 processes and with conscious mental operations. The "mechanical 

 equivalent" of thought has not yet been determined, but it is 

 indicated by the formation and excretion of a quantity of 

 phosphatic and nitrogenous waste. 



When the brain is exhausted by work, or when its nutrition is 

 retarded by anaemia or by age, memory is enfeebled and every 

 new association of ideas becomes difficult or impossible ; and 

 when its blood-supply is arrested for a single instant, consciousness 

 ceases. In fact, whatever impairs the integrity of cerebral 

 protoplasm — cold, chloroform, pressure, poison — puts an end to 

 cerebral function. Drugs introduced from without, waste-matter 

 from internal organs unduly retained, make ideation irregular and 

 delirious ; and when the nervous centres are perturbed or 

 interrupted by disease, there arise the delusions and hallucinations 

 of the insane. 



And now what divisions of psychological evolution remain to be 

 noticed ? Emotion ; Art ; The Moral Sense ; Religion ; Science. 

 Let us glance at them in order. 



