THEORIES OF HEREDITY AND VARIATION. 49 



new organ or character ; and second, a change, more or less marked, 

 in some organ or character already in existence. It is in this second 

 sense in which the word will be principally used, and it will be 

 variation of existing characters of which the following pages will 

 treat. 



"The characters which are inherited, and which are present at 

 birth, are termed congenital, while those that appear in the body 

 under the influence of extreme stimuli are termed acquired." 1 If 

 the character which appears at birth differs from the character of 

 the parent, then we have a congenital variation. An acquired 

 variation we can understand because we can see it occurring as the 

 direct results of causes with which we are familiar, but congenital 

 variations occur out of sight, and we can arrive at the cause of 

 them only by a process of reasoning or experiment. 



One of the causes assigned for congenital variations is the 

 result of environment or circumstances under which the ancestors 

 have lived. Dogs taken from England to India degenerate in a 

 few generations; sheep taken from one place to another change 

 in their form and in the quality of their wool; and plants moved 

 from their natural habitat acquire new characteristics which are 

 inherited. 



Another cause assigned for variations that become hereditary 

 is the result of use and disuse. Darwin found that tame ducks 

 have their legs larger and wings smaller than wild ducks. This 

 is assumed to be an inherited effect arising from the fact that tame 

 ducks walk more and fly less. Some naturalists think that the 

 large hind legs of a kangaroo are due to his habit of jumping, 

 while others maintain that his habit of jumping is due tr *he fact 

 that he has large and powerful legs. 



(i) Cope, Primary Factors of Origin of Evolution, p. 399. 



