VARIATION UNDER DOMESTICATION 73 



highest importance in causing variability, both by acting 

 directly on the organization, and indirectly by affecting 

 the reproductive system. It is not probable that vari- 

 ability is an inherent and necessary contingent, under all 

 circumstances. The greater or less force of inheritance 

 and reversion determine whether variations shall endure. 

 Variability is governed by many unknown laws, of which 

 correlated growth is probably the most important. Some- 

 thing, 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 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 sev- 

 eral 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 exagger- 

 ated, both in regard to animals and to those plants which 

 are propagated by seed. With plants which are tempo- 

 rarily propagated by cuttings, buds, etc., the importance 

 of crossing is immense; for the cultivator may here dis- 

 regard 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. 



— Science — 1 



