Chap. L] VARIATION UNDER DOMESTICATION. 9 



induced. There can, however, be little doubt about many 

 slight changes, — such as size from the amount of food, 

 colour from the nature of the food, thickness of the skin 

 and hair from climate, &c. Each of the endless variations 

 which we see in the plumage of our fowls must have had 

 some efficient cause ; and if the same cause were to act 

 uniformly during a long series of generations on many 

 individuals, all probably would be modified in the same 

 manner. Such facts as the complex and extraordinary 

 out-growths which variably follow from the insertion 

 of a minute drop of poison by a gall-producing insect, 

 show us what singular modifications might result in 

 the case of plants from a chemical change in the nature 

 of the sap. 



Indefinite variability is a much more common result 

 of changed conditions than definite variability, and has 

 probably played a more important part in the forma- 

 tion of our domestic races. We see indefinite vari- 

 ability in the endless slight peculiarities which 

 distinguish the individuals of the same species, and 

 which cannot be accounted for by inheritance from 

 either parent or from some more remote ancestor. 

 Even strongly-marked differences occasionally appear 

 in the young of the same litter, and in seedlings from 

 the same seed-capsule. At long intervals of time, out 

 of millions of individuals reared in the same 

 country and fed on nearly the same food, deviations of 

 structure so strongly pronounced as to deserve to be 

 called monstrosities arise ; but monstrosities cannot be 

 separated by any distinct line from slighter variations. 

 All such changes of structure, whether extremely 

 slight or strongly marked, which appear amongst 

 many individuals living together, may be considered 



