Page - 57 - in The Origin of Species

CIRCUMSTANCES FAVOURABLE TO SELECTION 57 rendered infinitely complex. In some cases the intercrossing of aboriginally distinct species appears to have played an im- portant part in the origin of our breeds. When several breeds have once been formed in any country, their occa- sional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds ; but the im- portance of crossing has been much exaggerated, both in re- gard 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 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, seems to have been the pre- dominant Power.