THE EVOLUTION OF INTELLIGENCE 113 



a corresponding conscious phase. Needless to say, we must 

 eschew for present purposes any discussion of this larger issue. 



There is, second, the view that consciousness appears in the 

 evolution of the nervous system at that point at which there is 

 demonstrable evidence that creatures can learn to improve 

 their reactions on the basis of experience. From this point of 

 view, the frog, among the more familiar of our animals, 

 would represent perhaps the stage at which the first gleams of 

 intelligence are discernible. Practically, this test is a very diffi- 

 cult one to apply, because it would be precarious to allege with 

 confidence that, granted sufficient repetitions of a given situa- 

 tion, any animals would fail to show some modification in 

 response. But purely physical and chemical reactions, for 

 example the formation of crystals in minerals, exhibit a similar 

 slow modification. And yet, in this case, one would hardly be 

 disposed to allege the presence of consciousness. Furthermore, 

 where animals low in the scale apparently display adaptive 

 reactions, it is difficult to be certain that the chemical and 

 physical stimuli affecting them are all under control, so that 

 the seeming adaptation may be accepted as bona fide. 



A third view holds that no animals are conscious, but that 

 all their reactions are essentially of the mechanical and tropistic 

 type. One must, of course, admit that we have no direct access 

 to animal consciousness, if such exists, but the same thing is 

 true of the consciousness of one's human neighbor. 



It has seemed to me desirable to eliminate this whole issue 

 from the present discussion, because we are primarily con- 

 cerned with the demonstrable evidence of actions such as we 

 can properly designate intelligent without regard to any par- 

 ticular psychic mould in which they may be cast. This is not 

 because the question is devoid of interest far from it but 

 because it does not lend itself to profitable discussion within 

 the limits of this article, if we are also to make headway on 

 the general problem of the evolution of intelligence. 



