306 O. F. COOK 



variation " only when the change is of a degenerative nature, and 

 not truly symbasic and constructive. 



If the two geographical halves of a species become separated 

 they will also become different, but this only shows that evolu- 

 tionary motion is everywhere taking place; it does not prove 

 that either of the new species has travelled farther than the 

 undivided group would have gone, or that segregation has 

 served as an agency of evolution. Evolution has very little to 

 do with the origination or subdivision of species ; this is almost 

 entirely a matter of segregation, geographical or otherwise, and 

 is a mere incident of the process of change. That separated 

 groups of organisms so universally and so promptly become 

 different, affords the strongest possible testimony that evolu- 

 tionary motion is not determinate or limited to one direction, 

 but it gives no warrant for looking upon isolation as contribut- 

 ing to evolutionary progress. 



As general evolutionary factors, natural selection and geo- 

 graphical isolation are negative and restrictive ; they influence, 

 but do not actuate, the progress of species. 



To say that isolation causes species-formation because it 

 brings the separated groups under different environmental or 

 selective conditions is only to confuse the issue. Segregated 

 groups become different, even in the same environment, and in 

 characters having no relation to environmental differences which 

 may exist. Unsegregated groups can remain relatively uniform 

 in very different environments. No evidence has been found 

 that any action of the environment can produce evolution, either 

 by direct transformation or by the indirect influence of selec- 

 tion and segregation. All nature abounds, on the other hand, 

 with evidence that evolution can take place without environ- 

 mental differences, without selection, without isolation. Evolu- 

 tion takes place without any external cause or compulsion, and 

 is capable of no explanation which does not recognize the fact 

 that specific groups or organisms, no less than sidereal systems, 

 are in motion. 



