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SCIENCE 



[N. S. Vol. XXXVI. No. 923 



tain number of these specialized cells of the 

 external layer sank below the general sur- 

 face, partly perhaps for protection, partly 

 for better nutrition: they became nerve- 

 cells. They remained connected with the 

 surface by a prolongation which became an 

 afferent or sensory nerve-fiber, and through 

 its termination between the cells of the 

 general surface continued to receive the 

 effects of external impressions; on the 

 other hand, they continued to transmit 

 these impressions to other, more distant 

 cells by their efferent prolongations. In 

 the further course of evolution the nervous 

 system thus laid down became differenti- 

 ated into distinct afferent, efferent and 

 intermediary portions. Once established, 

 such a nervous system, however simple, 

 must dominate the organism, since it 

 would furnish a mechanism whereby the 

 individual cells would work together more 

 effectually for the mutual benefit of the 

 whole. 



It is the development of the nervous 

 system, although not proceeding in all 

 classes along exactly the same lines, which 

 is the most prominent feature of the evo- 

 lution of the metazoa. By and through it 

 all impressions reaching the organism 

 from the outside are translated into con- 

 traction or some other form of cell-activity. 

 Its formation has been the means of caus- 

 ing the complete divergence of the world 

 of animals from the world of plants, none 

 of which possess any trace of a nervous 

 system. Plants react, it is true, to external 

 impressions, and these impressions pro- 

 duce profound changes and even compara- 

 tively rapid and energetic movements in 

 parts distant from the point of application 

 of the stimulus — as in the well-known in- 

 stance of the sensitive plant. But the im- 

 pressions are in all cases propagated di- 

 rectly from cell to cell — not through the 

 agency of nerve-fibers ; and in the absence 



of anything corresponding to a nervous 

 system it is not possible to suppose that 

 any plant can ever acquire the least glim- 

 mer of intelligence. In animals, on the 

 other hand, from a slight original modifi- 

 cation of certain cells has directly pro- 

 ceeded in the course of evolution the elab- 

 orate structure of the nervous system with 

 all its varied and complex functions, which 

 reach their culmination in the workings of 

 the human intellect. "What a piece of 

 work is a man! How noble in reason! 

 How infinite in faculty! In form and 

 moving how express and admirable! In 

 action how like an angel! In apprehen- 

 sion how like a god!" But lest he be 

 elated with his psychical achievements, let 

 him remember that they are but the result 

 of the acquisition by a few cells in a remote 

 ancestor of a slightly greater tendency to 

 react to an external stimulus, so that these 

 cells were brought into closer touch with 

 the outer world; while on the other hand, 

 by extending beyond the circumscribed 

 area to which their neighbors remained re- 

 stricted, they gradually acquired a domi- 

 nating influence over the rest. These domi- 

 nating cells became nerve-cells; and now 

 not only furnish the means for transmis- 

 sion of impressions from one part of the 

 organism to another, but in the progress 

 of time have become the seat of perception 

 and conscious sensation, of the formation 

 and association of ideas, of memory, voli- 

 tion and all the manifestations of the mind! 

 The most conspicuous part played by 

 the nervous system in the phenomena of 

 life is that which produces and regulates 

 the general movements of the body — move- 

 ments brought about by the so-called vol- 

 untary muscles. These movements are 

 actually the result of impressions imparted 

 to sensory or afferent nerves at the pe- 

 riphery — e. g., in the skin or in the several 

 organs of special sense; the effect of these 



