﻿1916J GATES— PAIRS OF SPECIES 183 



of these changes could not have arisen through a single mutation, 

 so it becomes necessary to postulate a common ancestor for the 

 two species. Such an ancestor we may suppose threw off a series of 



mutations which again continued to mutate in new directions, as 

 we know to happen in other forms from genetic experiments. The 

 surviving forms which we now know as C. borealis and C. umbellu- 

 lata might easily represent a differential of not more than three or 



