286 THE ORIGIN OF SPECIES 



Now let lis turn to the other side of the argument. 

 Under domestication we see much variability, caused, or 

 at least excited, by changed conditions of life; but often 

 \n so obscure a manner that we are tempted to consider 

 he variations as spontaneous. Variability is governed—by 

 many complex laws — by correlated growth, compensation, 

 the increased use and disuse of parts, and the definite 

 action of the surrounding conditions. There is much 

 difficulty in ascertaining how largely our domestic pro- 

 ductions have been modified; but we may safely infer 

 that the amount has been large, and that modifications 

 can be inherited for long periods. As long as the condi- 

 tions of life remain the same, we have reason to believe 

 that a modification, which has already been inherited for 

 many generations, may continue to be inherited for an 

 almost infinite number of generations. On the other 

 hand, we have evidence that variability, when it has 

 once come into play, does not cease under domestication 

 for a very long period; nor do we know that it evej 

 ceases, for new varieties are still occasionally produced 

 by our oldest domesticated productions. 



Variability is not actually caused by man; he only 

 unintentionally exposes organic beings to new conditions 

 of life, and then nature acts on the organization and 

 causes it to vary. But man can and does select the 

 variations given to him by nature, and thus accumulates 

 them in any desired manner. He thus adapts animals 

 and plants for his own benefit or pleasure. He may do 

 this methodically, or he may do it unconsciously by 

 preserving the individuals most useful or pleasing to him 

 without any intention of altering the breed. It is certain 

 that he can largely influence the character of a breed by 



