VARIATION. 25 



In such cases as these the common origin of 

 the varieties is frequently a matter of observation. 

 But the same sort of varieties occurs in a wild state, 

 although here it is seldom possible directly to ob- 

 serve their common origin. It is probable also that 

 they are not so numerous or so great as in the case 

 of domestic animals. Domestic animals are under 

 such abnormal conditions, they are so free from the 

 struggle for existence, so frequently manipulated 

 by the breeder, as to make them very much more 

 liable to variation than are wild species. Our breeders 

 can, by careful selection, produce changes much more 

 rapidly than would take place naturally. All of this 

 is of course absent in animals under nature, and the 

 variation will consequently be rather less numerous. 

 That animals in a state of nature do vary is, how- 

 ever, well known. Individual differences are every- 

 where found, for no two animals are alike. Nor are 

 these variations confined to isolated individuals. 

 They are inherited and transmitted. Changes in 

 the environment produce very great effect upon or- 

 ganisms, changes which gradually increase by inheri- 

 tance. Changes in food affect the color of animals, 

 changes in temperature, climate, moisture, or dry- 

 ness, winds, all have been found to produce marked 

 effects upon organisms. Some rats escaping from a 

 ship upon the island of Formosa became wild, and in 

 a very few generations had become so much modi- 

 fied as to be no longer recognizable as European 

 species. Allen has shown that variations in latitude 

 produce in American birds changes in color, length 

 of bill, claws, and tails, while variations in longitude 



