50 SUMMAEY Oy SELECTION. [Chap. 1- 



to the increased use or disuse of parts. The final re- 

 sult 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 forma- 

 tion 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 cut- 

 tings, buds, &c., the importance of crossing is immense; 

 for the cultivator may here disregard the extreme varia- 

 bility both of hybrids and of mongrels, and the steril- 

 ity 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 ac- 

 cumulative action of Selection, whether applied method- 

 ically and quickly, or unconsciously and slowly but 

 more efiiciently seems to have been the predominant 

 Power. 



