32 MENTAL EVOLUTION IN ANIMALS. 



Such being the manner in which Mr. Spencer supposes 

 nerve-fibres to be evolved, he further supposes nerve-cells to 

 arise in positions where a crossing or confluence of fibres 

 gives rise to a conflict of molecular disturbances ; but it is 

 unnecessary for our present purposes to enter upon this more 

 elaborate and less satisfactory part of his theory.* All I 

 desire now to point out is the cb priori probability that 

 nervous channels become developed where they are required 

 simply from the fact of their being required — that is by use. 



And this a priori probability derives so much confirma- 

 tion from facts that it is scarcely possible to refrain from 

 accepting it as an answer to the question above propounded, 

 namely, How are we to explain the fact that the anatomical 

 plan of a ganglion with its attached nerves comes to be that 

 which is needed to direct the nervous tremours into the par- 

 ticular channels required ? It is a matter of daily observa- 

 tion that " practice makes perfect," and this only means 

 that the co-ordinations of muscular movement which are 

 presided over by this or that nerve-centre admit of more 

 ready performance the more frequently they have been pre- 

 viously performed — which, in turn, only means that the dis- 

 charges taking place in the nerve-centre travel more and 

 more readily through the channels or nerve-fibres which are 

 being rendered more and more permeable by use. So much, 

 indeed, is this the case, that when an associated muscular 



theory by my own work on the physiology of nerves in Medusae. For a full 

 account of this, I may refer to a lecture published in the Proceedings of the 

 Boyal Institution for 1877, on " Evolution of Nerves." The principal facts 

 are that when physiological continuity of a sheet of neuro-muscular tissue is 

 interrupted by overlapping or spiral sections, so that the passage both of 

 visible or muscular waves of contraction and invisible or molecular waves of 

 stimulation are blocked, after a long succession of contraction waves are 

 allowed to break upon the shore of the physiological interruption, they at 

 last begin to force a passage, and very soon this passage becomes perfectly 

 free, so that neither the waves of contraction nor those of stimulation are 

 any longer hindered. Whether in such a case a definite nerve-fibre is de- 

 veloped, or only a " line of discharge," I cannot say ; but most probably the 

 passage is effected through previously existing fibres of the plexus which 

 become more functionally developed by their increase of activity. 



* Less satisfactory, not only because more speculative, but because the 

 whole weight of embryological and histological evidence appears to me to be 

 opposed to the speculation. For the whole weight of this evidence goes to 

 show that nerve-cells are the result of the specialization of epithelial or epi- 

 dermal cells — that is, that they arise, not out of undifferentiated protoplasm, 

 but by way of a further differentiation of a particular kind of already dif- 

 ferentiated tissue, where this is exposed to particular kinds of stimulation. 



