50 SUMMABY OF SELECTION. [Chap. I. 



be attributed to the increased use or disuse of parts. 

 The final result is thus rendered infinitely complex. 

 In some cases the intercrossing of aboriginally distinct 

 species appears to have played an important part in 

 the origin of our breeds. When several breeds have 

 once been formed in any country, their occasional 

 intercrossing, with the aid of selection, has, no doubt, 

 largely aided in the formation of 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, &c, the 

 importance of crossing is immense ; for the cultivator 

 may here disregard the extreme variability both of 

 hybrids and of mongrels, and the sterility of hybrids ; 

 but plants not propagated by seed are of little import- 

 ance to us, for their endurance is only temporary. 

 Over all these causes of Change, the accumulative 

 action of Selection, whether applied methodically and 

 quickly, or unconsciously and slowly but more 

 efficiently seems to have been the predominant Power. 



