ISOLATION 295 



Isolation, geographical and physiological, undoubt- 

 edly plays an important part in species differentiation, 

 but it is not an independent factor; much less can it 

 be regarded as the exclusive factor. From a theoret- 

 ical point of view it will ever remain an auxiliary fac- 

 tor, furnishing a new basis for natural selection or 

 introducing a noticeable change into environmental 

 conditions. 



