ANIMAL AND RATIONAL INTELLIGENCE 235 



acuteness of hearing can yield no more than increased 

 susceptibility in this one form. If we next introduce 

 combination of the simpler ones/ either we are dealing 

 with mere association, or we have introduced some 

 higher power, effecting the combination. In the 

 latter case, the hypothesis of evolution is not sus 

 tained. Rational power is essential for production of 

 the results contemplated, and is assumed to be in 

 operation. 



Accepting the guidance of the most ardent Evolu 

 tionists, we do not escape a sense of the weakness of 

 the argument. To prove evolution of mind, we must 

 open a road from sensory impressions to ideas of objects, 

 and from these to general abstract ideas; and this must 

 be such a road as the higher mammals could find 

 for themselves, before man s appearance on the earth. 

 Here is the essential test of an all-embracing scheme 

 of Evolution : to account for interpretation of sensory 

 experience. Beyond this we seek a natural history 

 of human thought, constructed on the observational 

 method, and worked out on conditions supplied by 

 antecedent existence, accepting the best results of 

 animal intelligence. 



This problem separates us from much that has been 

 already assured in natural history, strongly favouring 

 evolution. The facts are quite distinct from those 

 bearing on structural development. An organ being 

 given, we trace the history of its modifications through 

 varying phases of adaptation to circumstances. In 

 such a case, continuity is demonstrated on observa 

 tional lines. Again, the facts are quite different from 

 those illustrating continuity of stimulation along nerve 

 fibres, onwards through the brain, and onwards still 



