Periodic Mutations 705 



perhaps of still more universal occurrence. Pro- 

 gression and regression are seen to go hand in 

 hand everywhere. No large group and proba- 

 bly even no genus or large species has been 

 evolved without the joint agency of these two 

 great principles. In the mutation-period of the 

 evening-primroses the observed facts give direct 

 support to this conclusion, since some of the 

 new species proved, on closer inspection, to be 

 retrograde varieties, while others manifestly 

 owe their origin to progressive steps. Such 

 steps may be small and in a wrong direction; 

 notwithstanding this they may be due to the 

 acquisition of a wholly new character and there- 

 fore belong to the process of progression at 

 large. 



Between them however, there is a definite 

 contrast, which possibly is in intimate connec- 

 tion with the question of periodic and stray mu- 

 tations. Obviously each progressive change is 

 dependent upon the production of a new charac- 

 ter, for whenever this is lacking, no such muta- 

 tion is possible. Retrograde changes, on the 

 other hand, do not require such elaborate pre- 

 liminary work. Each character may be con- 

 verted into the latent condition, and for all we 

 know, a special preparation for this purpose is 

 not at all necessary. It is readily granted that 

 such special preparation may occur, because the 



