iv MODERN E VOL UTION 2 1 3 



between the retina and the muscles undergoes a 

 series of changes, precisely analogous to those which, 

 in the man, give rise to sensation, a train of thought, 

 and volition.' This passage occurs in Huxley's 

 ' Reply to Mr. Darwin's Critics/ which appeared in 

 the Contemporary Review, 1871, and it may be 

 supplemented by a quotation from the chapter on 

 ' The Mental Phenomena of Animals ' in his Hume. 

 ( It seems hard to assign any good reason for deny- 

 ing to the higher animals any mental state or process 

 in which the employment of the vocal or visual 

 symbols of which language is composed is not 

 involved ; and comparative psychology confirms the 

 position in relation to the rest of the animal world 

 assigned to man by comparative anatomy. As com- 

 parative anatomy is easily able to show that, physi- 

 cally, man is but the last term of a long series of 

 forms, which lead, by slow gradations, from the highest 

 mammal to the almost formless speck of living 

 protoplasm, which lies on the shadowy boundary 

 between animal and vegetable life ; so, comparative 

 psychology, though but a young science, and far 

 short of her elder sister's growth, points to the same 

 conclusion.' 



Within recent years the psychologists are doing 

 remarkable work in attacking the problem of the 

 mechanics of mental operations, and already in 

 Europe and America some thirty laboratories have 

 been started for experimental work. The subject is 

 somewhat abstruse for detailed reference here, and it 

 must suffice to say that the psychologist, beginning 

 with observations upon himself, measuring, for ex- 

 ample, * the degree of sensibility of his own eye to 



