52 Instinct and Intelligence 



which must therefore contain elements capable 

 of transforming ordinary modes of energy act- 

 ing on them into purposive movements. We 

 hold that in the course of the evolution of the 

 ascending classes of animals purposive elements 

 such as those governing the behaviour of uni- 

 cellular beings have separated, or become dif- 

 ferentiated from the rest of the protoplasmic 

 elements, so as to constitute a very important 

 part of the living matter of the nerve cells of 

 more highly organised beings ; elements of this 

 description in the course of time have become 

 the instruments whereby energy derived from 

 external and internal sources is transmuted into 

 force, which becomes manifest in purposive or 

 instinctive movements of animals. 



The delicate structures forming the various 

 sensory organs of Medusoids are of a distinctly 

 higher order than those of lower orders of 

 beings; and we find a progressive evolution of 

 these tissues in advancing families of animals. 

 Doubtless the evolution of these structures, like 

 those of other parts of the body, is largely attri- 

 butable to natural selection; but these changes 

 of structure in a continuous series throughout 



