26 Comparative Conceptions 



§ 4. Psychophysical Variations 

 Allusion has been made to the great biological topic of 

 variation as embodying a conception common to the two 

 series — mind and body. It is only recently that the 

 theory of selection from congenital variations has been 

 brought over into psychology. Formerly the idea of 

 hereditary transmission of the results of mxcntal education 

 was simply assumed. But the failure of that idea in biol- 

 ogy has led to the revision of the facts with an equally 

 pronounced verdict against it in psychology also. Begin- 

 ning with certain brilhant independent examinations of the 

 question, notably that of W. James/ the theory of mental 

 variations has come in to account for the evolution of mind 

 in strict correlation with that of the organism. We find not 

 only the correlation of inteUigence with plasticity, as 

 pointed out above, but also many other correlated details 

 which the psychophysical processes actually exhibit. This 

 means that natural selection has worked upon correlated 

 psychophysical variations — not upon organic variations 

 merely. In other words, it has been the psychophysical, 

 not the physical alone, nor the mental alone, which has been 

 the unit of selection i7t the main trend of evolution, and 

 Nature has done what we are now urging the science of 

 evolution to do — she has carried forward the two series 

 together, thus producing a single genetic movement. It 

 would have been impossible for mind to develop by selec- 

 tion with reference to utilities for which the necessary 

 organic variations were not present ; and so also it would 

 have been impossible for the organism to evolve in ways 

 which the consciousness of the same animal forms did not 



1 Principles of Psychology, II. Chap. XXVIII. 



