THE ORIGIN OF NERVES. 289 



and to impressions derived therefrom. And the influence 

 of use and habit can be well understood and appreciated, 

 when it is alleged that these exposed parts of the body 

 will become more sensitive than the non-exposed portions, 

 and impressions will thus come to select, or to be directed 

 in, certain lines or paths in preference to others. These 

 stimulated parts will become the seat of constantly recur- 

 ring molecular changes and movements inducing the forma- 

 tion of definite contractile tissues or muscles ; whilst the 

 lines along which the impulses have passed will ultimately 

 represent the primitive nerve-tracts or nerve-fibres, such, 

 indeed, as are seen in varying degrees of perfection in the 

 jelly-fishes. Mr. Spencer's own comparison of the develop- 

 ment of nerve-tracts to the formation of water-channels is a 

 perfectly just simile. Constantly recurring molecular waves 

 define the primitive lines of discharge in living tissues, just 

 as continuous currents of water deepen the shallow and 

 ill-defined channel along which the first waters of the river 

 ran. Once established, nerve-actions and impulses will 

 continue to flow and to become better defined ; and with 

 the necessity and demand for sensory apparatus of still 

 higher kind, the same inevitable law of use and habit will 

 supply an increased and more perfect nervous system. 



Such, briefly told, is the history of the evolution of 

 nerves. If we pass a little higher in the scale of animal 

 life from the jelly-fishes, we find that nerve-fibres and 

 nerve-cells the elements found in the highest nervous 

 systems become distinctly developed; although, indeed,, 

 the beginnings of these elements are to be discerned in 

 these graceful organisms themselves. The arrangement of 

 nerve-systems in animals follows the inevitable law of 

 necessity, in that their nerve-fibres and cells are placed so as 

 most perfectly to control and correlate bodily actions with 

 the impressions which are received from the outer world. 

 Organs of sense, specialised * parts of the nervous system, 

 adapted to receive one kind of impression alone, may be 

 regarded as having arisen in obedience to the same law of 



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