ANIMAL AND RATIONAL INTELLIGENCE 227 



Evolution. Continuity which had been demonstrated 

 in the history of organism, has not been traced in the 

 relations of animal to human intelligence. This 

 is only another way of expressing the conclusion, 

 otherwise reached, that sensory impression cannot 

 cause thought. There is not in the natural history 

 of intelligent life any evidence pointing definitely 

 in the opposite direction. There is a mass of con 

 vincing evidence for organic evolution under action of 

 environment ; there is none for evolution of human 

 intelligence from that of animals. For test of this 

 difference, nothing better can be done than to bring 

 familiar examples of evolution into comparison with the 

 ordinary facts of intellectual life. Take a single case 

 of structural modification, such as the foot of the horse; 

 take variations such as those seen in the several breeds 

 of dogs, or of pigeons. With these examples in view, 

 it is apparent in what perplexity we are involved, even 

 in attempting to make out analogies. In the one 

 case, there is a structural basis ; advance can be traced 

 through successive stages. In the other, we pass from 

 the sensori-motor system, with brain structure, without 

 being able to explain the appearance of animal intelli 

 gence, and in it we find no rudimentary phases of 

 thought. The utmost found is, transference into action 

 of the sensory impressions made by signs which we 

 employ. Functions are now severed from physical 

 structure. Or, lest this should seem too much, the facts 

 recognised are not included within observations of 

 structure, and its functions. In mode of observation, 

 we are now separated from the whole field of natural 

 history. This break as a simple fact must be deliberately 

 stated as a condition of scientific advance. Scientific 



