Darwin s Artificial and Natural Selection 95 



with feathered feet have skin between the outer toes, and 

 those with short beaks have small feet, and vice versa. 



Another source of variation is that of reversion, or the 

 reappearance in the offspring of characters once possessed 

 by the ancestors. Finally, Darwin thinks that a source of 

 variation is to be found in modifications due to the influence 

 of a previous union with another male, or, as it is generally 

 called, telegony. As an example Darwin cites the famous 

 case of Lord Morton's mare. " A nearly purely bred Ara- 

 bian chestnut mare bore a hybrid to a quagga. She subse- 

 quently produced two colts by a black Arabian horse. These 

 colts were partially dun-colored and were striped on the legs 

 more plainly than the real hybrid or even than the quagga." 1 

 This case, however, is not above suspicion, since it is well 

 known that stripes [often appear on young horses, and the 

 careful analysis made later by Ewart, as well as his other 

 experiments on the possibility of the transmission of influ- 

 ences of this sort, puts the whole matter in a very dubious 

 light. 



These citations show that Darwin recognized quite a num- 

 ber of sources of variation, and, although he freely admits that 

 " our ignorance of the laws of variation is profound," yet 

 some at least of these sources of variation are very question- 

 able. Be this as it may, it is important to emphasize that 

 Darwin recognized two main sources of variation, — one of 

 which is the indefinite, or fluctuating, variability that appears 

 constantly in domesticated animals and plants, and the other, 

 definite variability, or a change in a definite direction, that can 

 often be traced to the direct action of the environment on 

 the parent or on its reproductive cells. It is the former, 

 i.e. the fluctuating variability, that, according to Darwin, has 

 been used by the breeder to produce most of our domestic 

 races. In regard to the other source of variation, the 

 definite kind, we must analyze the facts more closely. 

 1 " Animals and Plants under Domestication," Chap. IX. 



