The Evidence from Variation. 141 



ganisrn, bnt Darwin, after long and careful study of the 

 subject, reaches tlie conclusion that each variation is ex- 

 cited bv a chansre of some kind in the environment. It 

 is impossible to expose animals for any length of time to 

 absolutely uniform conditions, and we therefore find 

 that when careful attention is given to the subject, mi- 

 nute individual differences may be detected in animals 

 which are apparently most uniform. A shepherd easily 

 learns to recognize each sheep in a large flock, and ants 

 are able to perceive a difference betw^een the members 

 of their own community and those from another nest. 



It is impossible to show by direct proof that uniform 

 conditions of life would prevent variation; but it is 

 quite possible to approach the subject from the other 

 side, and to show that slight external changes cause 

 slight variability, and greater changes greater variabil- 

 ity. 



Wild animals and plants vary somewhat and have in- 

 dividual peculiarities, for each one is under slightly dif- 

 ferent relations to the external world from all the 

 others, but as compared with domesticated species their 

 conditions of life are very uniform. 



A wild animal has become habituated to the circum- 

 stances under which it lives, by exposure, for generations 

 after generations to the action of natural selection, and a 

 host of competing animals tend to keep it in its jDlace, 

 but domesticated animals are protected from their ene- 

 mies and competitors, they are removed from their nat- 

 ural conditions, and they are frequently carried from 

 their native land and are exposed in other countries to 

 unnatural food and climate. They are compelled to 

 change their habits, and they are never left long at 

 rest, or exposed for any considerable length of time to 

 closely similar conditions, but they are carried from dis- 



