iv MODERN E VOL UTION 209 



* But what excited that modification ? The all- 

 surrounding medium, without which no life had been, 

 which determined its limits, and touches it at every 

 point with its throbs and vibrations. In the be- 

 ginnings of a primitive layer or skin manifested by 

 creatures a stage above the lowest, unlikenesses 

 would arise, and certain parts, by reason of their 

 finer structure, would be the more readily stimulated 

 by, and the more quickly responsive to, the ceaseless 

 action of the surroundings, the result being that 

 an extra sensitiveness along the lines of least re- 

 sistance would be set up in those more delicate 

 parts. These, developing, like all things else, by use, 

 would become more and more the selected paths of 

 the impulses, leading, as the molecular waves thrilled 

 them, to structural changes or modification into 

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

 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 

 organisation of the nervous system the germs of 

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

 amceba, 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 grada- 

 tions are of lesser importance. And, for the present 

 purpose, there is no need to descend far in the life- 

 scale ; if the psychical connection between man and 



Q 



