THE VITAL FABRIC OF DESCENT 323 



vital^ fabric is weakened by narrow segregation or selective 

 inbreeding, mutative degenerations and Mendelian disjunctions 

 appear. Variations thus induced afford examples of evolution- 

 ary motion, but in its aberrant and destructive form. Symbasic 

 evolution is a process of constructive integration ; it proceeds 

 the better when the diverse individuals of a species remain 

 together, not when they are kept apart. Variations of positive 

 evolutionary significance are prepotent ; they strengthen the 

 organism, and are shared and preserved by the vigorous, inter- 

 breeding members of the species. The conditions under which 

 a^species enjoys its greatest numerical prosperity are also the 

 most favorable for its evolutionary progress. 



