40 ON THE ORIGIN OF SPECIES 



their occasional intercrossing, with the aid of selection, 

 has, no doubt, largely aided in the formation of new 

 sub-breeds ; but the importance of the crossing of 

 varieties has, I believe, been greatly exaggerated, both 

 in regard to animals and to those plants which are 

 propagated by seed. In plants which are temporarily 

 propagated by cuttings, buds, etc., the importance of 

 the crossing both of distinct species and of varieties is 

 immense ; for the cultivator here quite disregards the 

 extreme variability both of hybrids and mongrels, and 

 the frequent sterility of hybrids ; but the cases of 

 plants not propagated by seed are of little importance 

 to us, for their endurance is only temporary. Over all 

 these causes of Change I am convinced that the 

 accumulative action of Selection, whether applied 

 methodically and more quickly, or unconsciously and 

 more slowly, but more efficiently, is by far the pre- 

 dominant Power. 



