Chap. I. SUMMARY OF VAEIATION. 51 



have once been formed in any country, their occasional inter- 

 crossing-, -with the aid of selection, has, no doubt, largely aided 

 in forming new sub-breeds ; but the importance of crossing 

 has been much exaggerated, both in regard to animals and 

 to those plants which are propagated by seed. With plants 

 which are temporarily propagated by cuttings, buds, etc, the 

 importance of crossing is immense ; for the cultivator may 

 here disregard the extreme variability both of hybrids and of 

 mongrels, and the frequent sterility of hybrids ; but plants 

 not propagated by seed are of little importance to us, for their 

 endurance is only temporary. Over all these causes of Change 

 the accumulative action of Selection, whether applied method- 

 ically and quickly, or unconsciously and slowly, but more efB- 

 ciently, is by far the predominant Power. 



