82 SUMMARY OF SELECTION. [Chap. I 



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 selec- 

 tion, 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 pro- 

 pagated 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 importance 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, soems to 

 have been the predominant Power. 



