Tlie Evidence from Variation. 143 



til an that which they would find for themselves in a 

 state of nature. 



Excess of food often causes variation, yet the turkey 

 and goose have been encouraged and tempted for gener- 

 ations to feed to excess, and they have varied but little. 



These examples show that the chai'acter of the change 

 is, unimportant, and that variability cannot be attribu- 

 ted to the exclusive influence of any particular class of 

 external conditions; that the exciting cause of variation 

 is change, but not any particular kind of change. 



Darwin quotes a number of cases to show how slight 

 a change may result in variability. 



Thus the wild horses of the pamjiasof South America 

 are of one of three colors, and the wild cattle are of one 

 color; but when the same horses and cattle are domesti- 

 cated, although they are not confined, but are allowed to 

 run at large like the wild forms, they entirely lose their 

 similarity of color, and display the greatest diversity in 

 this particular. In India several species of fresh-water 

 fishes are reared in great tanks as large as natural ponds, 

 and they are all very variable. Darwin quotes from Down- 

 ing the statement that varieties of the plum and peach 

 which breed truly by seed, lose this power, and like 

 other Avorked trees give variable seedlings when grafted 

 on another stock. 



Variability almost Exclusively Confined to Organism 

 Produced from Fertilized Ova. 



The only method open to us besides the study of hy- 

 brids for observing the influence of the sexes in heredity, 

 is by a comparison of sexual with asexual heredity. As 

 I shall show in another place, all the various forms of 

 asexual reproduction are so connected that we may pass 

 from fission, or the formation of two new organisms 



