170 EXPERIMENTAL EVOLUTION LECT. 



dactyls afford good illustrations of the variability of 

 the group, and the number of fossil forms discovered 

 in the Tertiary strata is very considerable. Among 

 domestic animals there are very different degrees of 

 variability ; Cornevin gives the following list of 

 domestic birds and mammals, where the animals are 

 arranged according to the order of decreasing varia- 

 bility : 



Pigeon. Hogs. 



Fowl. Dogs. 



Duck. Oxen. 



Pheasant. Sheep. 



Goose. Rabbits. 



Guinea-fowl. Hares. 



Peacock. Donkeys. 



Swan. Camels. 



Turkey. Goats. 



Barbary Ducks. Guinea-pigs. 



Of these species, three which vary but very slightly 

 (turkey, Barbary ducks, and guinea-pigs) are recent 

 importations and will perhaps vary more in later 

 periods. At all events guinea-pigs have already 

 sufficiently varied to have become infertile with their 

 original stock. 



The modifying influence of domestication on ani- 

 mals has a parallel in the influence of cultivation on 

 plants, and to these we must now turn, to call atten- 

 tion to the important transformations which wild 



