226 PIONEERS OF EVOLUTION. 



nerve-cells, and nerve-fibres, of increasing complex- 

 ity as we ascend the scale of life. The entire nervous 

 system, with its connections; the brain and all the 

 subtle mechanism with which it controls the body; 

 the organs of the senses alike begin as sacs formed 

 by infoldings of the primitive outer skin." 



Biologists are agreed that a certain stage in the 

 organization of the nervous system — ^the germs of 

 which, we saw, are visible in the quivering of an 

 amoeba, and probably in plants as well as animals 

 — must be reached before consciousness is manifest. 

 Obscurity still hangs round the stage at which mere 

 irritability passes into sensibility, but so long as the 

 continuity of development* is clear, the gradations 

 are of lesser importance. And, for the present pur- 

 pose, there is no need to descend far in the life-scale ; 

 if the psychical connection between man and the 

 mammals immediately beneath him is proven, the 

 connection of the mammals with the lowest inver- 

 tebrate may be assumed as also established. Speak- 

 ing only of vertebrates, the brain being, whether in 

 fish or man, the organ of mental phenomena, how 

 far does its structure support or destroy the theory 

 of mental continuity? In Man's Place in Nature, 

 and its invaluable supplement, the second part of 

 the monograph on Hume, this subject is expounded 

 by Huxley with his usual clearness. In the older 

 book he traces the gradual modification of brain in 

 the series of backboned animals. He points out that 

 the brain of a fish is very small compared with the 



