g Outline of Genetics 



from the parent realm that free crossing between the 

 colonists and the stay-at-homes does not take place. 

 Any chance variations that may have existed in the few 

 original emigrants will now have an opportunity to 

 pe^etuate themselves instead of being ''swamped out" 

 through free crossing with the other members of the 

 parent species, as would have happened if there had 

 been no isolation. Thus the various populations of a 

 species that we see today somewhat isolated from one 

 another have had a chance to express and later augment 

 chance differences to such a degree that we may now 

 regard them as different varieties. With time the diver- 

 gence of characteristics between the isolated groups 

 will become still greater. 



The isolation theory, as outlined above, does not by 

 itself provide a ''complete" explanation of evolution. 

 It is best to be regarded as a sort of a coroUary to the 

 Darwinian or DeVriesian schemes. In either event, 

 it is the principle of natural selection that brings about 

 progressive or adaptive evolution, while isolation either 

 serves to multiply species "on the same level," through 

 giving chance variations an opportunity to express 

 themselves and become augmented, or else it serves to 

 enlarge the scope of natural selection by thrusting repre- 

 sentatives of the species into a somewhat new environ- 

 ment or by presenting natural selection with a slightly 

 new population from which to make the choices. 



Another matter that should be mentioned is ortho- 

 genesis. Orthogenesis may be regarded either as an 

 "explanation" of evolution or merely as the name of a 

 phenomenon. There is considerable evidence support- 

 ing the beUef that the variations (or at least many of 



