32 THE ORIGIN OF SPECIES 



version determine whether variations shall endure. Variability is 

 governed by many unknown laws, of which correlated growth is 

 probably the most important. Something, but how much we do not 

 know, may be attributed to the definite action of the conditions 

 of life. Some, perhaps a great, effect may be attributed to the in- 

 creased 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 propagated 

 by seed. With plants which are temporarily propagated by cut- 

 tings, buds, etc., the importance of crossing is immense; for the 

 cultivator may here disregard the extreme variability both of hy- 

 brids and of mongrels, and the sterility of hybrids; but plants not 

 propagated by seed are of little importance to us, for their endur- 

 ance is only temporary. Over all these causes of change, the accu- 

 mulative action of selection, whether applied methodically and 

 quickly, or unconsciously and slowly, but more efficiently, seems 

 to have been the predominant power. 



