90 ISOLATION AS A FACTOR 



life, we know, are split up into species, with 

 adaptation to external conditions traceable in 

 every structure. We know of no way in which, 

 organisms can become adapted to special condi-l 

 tions except by the progressive failures of those 

 not adaptable. Hence we know of no organisnoi 

 which has escaped or can escape from the influ- 

 ence of selection. In like manner, as the world 

 is covered with physical barriers, no organism 

 can escape the form of evolutionary friction 

 which prevents uniformity in breeding. There 

 must be some degree of " raumhche Sonderung," 

 even in a drop of water. 



To admit these facts, and yet to say that selec- 

 tion and isolation are not factors in evolution, 

 would appear to make the matter a mere question 

 of words. If by evolution we mean the theoret- 

 ical progress of life, in vacuo, the effects solely 

 of forces intrinsic in organisms, then extrinsic 

 forces or extrinsic obstacles are of course not fac- 

 tors in such evolution. If we mean by evolution, 

 the actual life movements of actual organisms, on 

 this actual earth, then forces and obstacles are 

 alike factors in modifying change, and both spe- 

 ciation and adaptation as well necessary parts of 

 the process. 



We admit the primary necessity of variation 

 and of heredity, but we can conceive of no case 

 of actual animal or plant in the forming of which 

 selection and isolation have not played each a 

 large and persistent part. Among the factors 



