48 MENTAL EVOLUTION IN ANIMALS. 



being, and if, as we have taken for granted, every change of 

 Mind is associated with some change of Body, it follows that 

 this distinctive peculiarity ought to admit of being trans- 

 lated into some physiological equivalent. Further, if there 

 is any such physiological equivalent to be found, we should 

 expect to find it much lower down in the scale of physio- 

 logical development than in the functions of the human 

 brain. For not only do the lower animals manifest, in a 

 long descending scale, powers of choice which gradually fade 

 away into greater and greater simplicity ; but we should be 

 led & priori to expect, if there is a physiological principle 

 which constitutes the objective basis of the psychological 

 principle, that the former should manifest itself more early 

 in the course of evolution than the latter. For, whatever 

 views we may entertain concerning the relation of Body 

 and Mind, there can be no question, on the basis of the 

 evolution theory which I assume, that, as a matter of his- 

 torical sequence, the principles of physiology were prior to 

 those of psychology ; and therefore, if in accordance with 

 our original agreement we allow that the latter have a phy- 

 sical basis in the former, it follows that the principles of 

 physiology, which now constitute the objective basis of 

 choice, whatever they may be, probably came into operation 

 long before they were sufficiently evolved thus to constitute 

 the foundation of psychology. 



Now I think that the a priori expectation thus briefly 

 sketched is fully realized in the occurrence of a physiological 

 principle, which first appears very low down in the world of 

 life, and which, in its relation to psychology, has not yet 

 received the attention which it deserves. I may best state 

 the principle by giving an example. I have observed that if 

 a sea-anemone is placed in an aquarium tank, and allowed 

 to fasten upon one side of the tank near the surface of the 

 water, and if a jet of sea water is made to play continuously 

 and forcibly upon the anemone from above, the result of 

 course is that the animal becomes surrounded with a turmoil 

 of water and air-bubbles, Yet, after a short time, it becomes 

 so accustomed to this turmoil that it will expand its tentacles 

 in search of food, just as it does when placed in calm water. 

 If now one of the expanded tentacles is gently touched with 

 a solid body, all the others close around that body, in just 

 the same way as they would were they expanded in calm 



