12 Psychophysical Evolution 



on all other occasions of the occurrence of either of 

 them. 



This formulation is, it is easy to see, absolutely neces- 

 sary to any science of psychophysics at all. The theory 

 of the localization of functions in the brain, for example, 

 assumes it. For if vision has its seat in the occipital 

 region at one time, it may be assumed that at another time 

 a lesion of that centre will interfere with vision, or that 

 certain troubles of vision may be taken as evidence of 

 lesion in that centre. If these expectations be not fulfilled, 

 the only alternative left open to the investigator is to be- 

 lieve that the first determination of the seat of vision was 

 erroneous. 



This requirement alone the demand for uniformity 

 in the facts with which psychophysics has to deal is 

 itself sufficient to justify the acceptance of psychophysical 

 parallelism as against all the theoretical objections that 

 may be and have been urged. As a rule of scientific 

 procedure, it is a necessary assumption. To deny it is to 

 say antecedently to the actual examination of the facts 

 which it claims to formulate, that a natural science of the 

 individual as a whole is impossible ; for any formulation 

 of the facts must proceed upon the assumption that they 

 have sufficient regularity and uniformity to allow of 

 formulation. 



Third, the principle must be a universal one, if it be 

 valid at all; which means, that wherever we find a series 

 of phenomena which are psychophysical, this principle 

 of parallelism has its application. This leads to the 

 necessity of extending the application of parallelism from 

 the sphere of development to that of evolution, as those 

 terms are distinguished on an earlier page. This may now 



