30 VARIATION UNDER NATURE. [CHAP. iL 



under all circumstances. The greater or less force of inheritance 

 and reversion determine whether variations shall endure. Varia- 

 bility 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 attri- 

 buted 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 several 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 

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

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

 gated by cuttings, buds, &c., the importance of crossing is im- 

 mense ; 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 predominant Power. 



CHAPTER II. 



VARIATION UNDER NATURE. 



Variability Individual differences Doubtful species Wide ranging, much 

 diffused, and common species, vary most Species of the larger genera in 

 each country vary more frequently than the species of the smaller genera 

 Many of the species of the larger genera resemble varieties in being 

 very closely, but unequally, related to each other, and in having restricted 

 ranges. 



BEFORE applying the principles arrived at in the last chapter to 

 organic beings in a state of nature, we must briefly discuss whether 

 these latter are subject to any variation. To treat this subject 

 properly, a long catalogue of dry facts ought to be given ; but these 

 I shall reserve for a future work. Nor shall I here discuss the 

 various definitions which have been given of the term species. No 

 one definition has satisfied all naturalists ; yet every naturalist 

 knows vaguely what he means when he speaks of a species. Gene- 

 rally the term includes the unknown element of a distinct act of 

 creation. The term "variety" is almost equally difficult to 



