64 MENTAL EVOLUTION IN ANIMALS. 



to render it higlily improbable that this diagrammatic repre- 

 sentation of it will, in the future, be altered in any of its 

 main features by any advances that science may be destined 

 to make. 



From the groundwork of Excitability, or the distinguish- 

 ing peculiarity of living matter, I represent the structure of 

 mind as arising by a double root — Conductility and Discrimi- 

 nation. To what has already been said on these topics it is 

 needless to add more. We have seen that the distinguishing 

 property of nerve-fibre is that of transmitting stimuli by a 

 propagation of molecular disturbance irrespective of the pas- 

 sage of a contraction wave ; and this property, laying as it 

 does the basis for all subsequent co-ordination of protoplasmic 

 (muscular) movements, as well as of the physical aspect of 

 all mental operations, deserves to be marked off in our map 

 as a distinct and important principle of development ; it is 

 the principle which renders possible the executive faculty of 

 appropriately responding to stimuli. Not less deserving of 

 similar treatment is the cognate principle of Discrimination, 

 which, as we have seen, is destined to become the most 

 important of the functions subsequently distinctive of nerve- 

 cells and ganglia. But we have also seen that both Conduc- 

 tility and Discrimination first appear as manifested by the 

 cellular tissues of plants, if not even in some forms of 

 apparently undifferentiated protoplasm. It is, however, on]y 

 when these two principles are united within the limits of the 

 same structural elements that we first obtain optical evidence 

 of that differentiation of tissue which the histologist recognizes 

 as nervous ; therefore I have represented the function of 

 nerve-tissue in its widest sense, Neurility, as formed by a 

 confluence of these two root-principles. Neurility then 

 passes into Keflex Action and Volition, which I have repre- 

 sented as occupying the axis or stem of the psychological 

 tree. On each side of this tree I have represented the out- 

 growth of branches, and for the sake of distinctness I have 

 confined the branches which stand for the faculties of Intellect 

 on one side, while placing those which represent the Emotions 

 upon the other. The level to which any branch attains re- 

 presents the degree of elaboration which the faculty named 

 thereon presents ; so that, for instance, when the branch 

 Sensation, taking origin from Neurility, proceeds to a certain 

 level of development, it gives off the commencement of Per- 



