?,8 CIRCUMSTANCES FAVORABLE TO SELECTION; 



the sterility of hybirds; but plants not propagated by seed 

 are of little importance to ns, for their endurance is only 

 temporary. Over all these causes of change, the accumu- 

 lative action of selection, whether applied methodically 

 and quickly, or unconsciously and slowly, but more 

 efficiently, seems to have been the predominant power. 



