CHAP. XXIV.] NATURAL SELECTION. 309 



circumstances of life and by mixture of races, when these 

 do not go further than what is good for the health and 

 vigour of the species ; and that some species are more 

 variable than others. It has also been shown that when 

 any particular character in a race once begins to vary, it is 

 apt to acquire a habit of varying ; and it is obvious how 

 this fact must facilitate the accumulation of variations 

 through descent in particular directions. I mean that 

 when any one character becomes variable, such as the 

 form of the beak in the domestic pigeon, it will be com- 

 paratively easy to obtain by selection new breeds that 

 differ fi'om the parent ones in that point. 



Domestication is a great change in the circumstances of 

 the life of both animals and vegetables, and cross-breeding, 

 so as to produce slight mixtures of race, is systematically 

 practised with many of the domestic races of animals ; 

 and these two causes are amply sufficient to account for 

 the great variability which is common, though perhaps Causes of 

 not universal, among domesticated races, vegetable as well ^^(10™°^ 

 as animal. It is, I think, an unsettled question, whether tic races. 

 a perceptible degree of variation ever takes place, at least 

 among the higher animals and vegetables, without the 

 stimulus either of change of circumstances or of mixture of 

 race ; and it will not be easy to decide the question, for 

 evidence is difficult to obtain concerning wild races, and 

 the variability of the domestic ones is probably due to the 

 change in the conditions of their life effected by their first 

 domestication, and to subsequent crossings of the breed. 



But it is certain that changes in the circumstances of Changes 

 life must, throughout geological time, be constantly occur- stances ^i 

 ring, and stimulating variation. Geological, and I may geological 

 add astronomical,^ revolutions alter the climate and *™^" 

 physical geography of whole continents ; and quite inde- 

 pendently of this, geological revolutions effect vast changes 

 in the circumstances of the life of species, by either forming 



1 See Mr. Croll's Papers on the Glacial Climate, and kindred subjects, 

 in the Philosophical Magazine. I do not agree with all his results, hut 

 I think he has done great service to science by showing that the changes 

 in the earth's orbit must produce climatic changes. 



