Chap. I. UNDEK DOMESTICATION. 43 



To sum up on the origin of our Domestic Races of 

 animals and plants. I believe that the conditions of 

 life, from their action on the reproductive system, are 

 so far of the highest importance as causing variability. 

 I do not believe that variability is an inherent and 

 necessary contingency, under all circumstances, with all 

 organic beings, as some authors have thought. The 

 effects of variability are modified by various degrees of 

 inheritance and of reversion. Variability is governed 

 by many unknown laws, more especially by that of cor- 

 relation of growth. Something may be attributed to 

 the direct action of the conditions of life. Something 

 must be attributed to use and disuse. The final result 

 is thus rendered infinitely complex. In some cases, I 

 do not doubt that the intercrossing of species, aborigin- 

 ally distinct, has played an important part in the origin 

 of our domestic productions. When in any country 

 several domestic breeds have once been established, 

 their occasional intercrossing, with the aid of selection, 

 has, no doubt, largely aided in the formation of new 

 sub-breeds ; but the importance of the crossing of 

 varieties has, I believe, been greatly exaggerated, both 

 in regard to animals and to those plants which are pro- 

 pagated by seed. In plants which are temporarily pro- 

 pagated by cuttings, buds, &c, the importance of the 

 crossing both of distinct species and of varieties is im- 

 mense ; for the cultivator here quite disregards the ex- 

 treme variability both of hybrids and mongrels, and the 

 frequent sterility of hybrids ; but the cases of plants not 

 propagated by seed are of little importance to us, for 

 their endurance is only temporary. Over all these 

 causes of Change I am convinced that the accumu- 

 lative action of Selection, whether applied methodically 

 and more quickly, or unconsciously and more slowly, 

 but more efficiently, is by far the predominant Power. 



