30 MENTAL EVOLUTION IN ANIMALS. 



results ; vre can, as it were, play upon this beautifully- 

 adjusted mechanism, so as to produce at will the balancing oi 

 this stimulus against that one — the results, as expressed in 

 the movements of the animal, being so many exemplifica- 

 tions of the mechanical principle of the parallelogram of 

 forces.* 



As we proceed through the animal series we find nervous 

 systems becoming more and more integrated ; nerve-centres 

 multiply, become larger, and serve to innervate more numerous 

 and more complex groups of muscles. It is, however, need- 

 less for me to devote space to describing this advance of 

 structure, because the subject is one belonging to compara- 

 tive anatomy. It is enough to say that everywhere the 

 nervous machinery is so arranged that, owing to the ana- 

 tomical plan of a nerve-centre with its attached nerves, there 

 is no alternative of action presented to the nerve-centre 

 other than that of co-ordinating the group of muscles over 

 the combined contraction of which it presides. The next 

 question, therefore, which arises is — 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 particular channels required ? 

 The following is the theory whereby Mr. Herbert Spencer 

 seeks to answer this question, and in order fully to under- 

 stand it we must begin by noticing the effects of stimulation 

 upon undifferentiated protoplasm. A stimulus, then, applied 

 to homogeneous protoplasm, which is everywhere contractile 

 and nowliere presents nerves, has the effect of giving rise to 

 a visible wave of contraction, which spreads in all directions 

 from the seat of stimulation as from a centre. A nerve, on 

 the other hand, conducts a stimulus without undergoing any 

 contraction, or change of shape. Nerves, then, are func- 

 tionally distinguished from undifferentiated protoplasm by 

 the property of conducting invisible or molecular waves of 

 stimulation from one part of an organism to another, so 

 establishing physiological continuity between such parts 

 without the necessary passage of visible waves of contraction. 



Now, beginning with the case of undifferentiated proto- 

 plasm, Mr. Spencer starts from the fact that every portion of 

 the colloidal mass is equally excitable and equally contrac- 



* For a full account of tliese experiments, see Croonian Lecture, Phil. 

 Trans., 1883. 



