Chap. I.] 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 gen- 

 erations on many individuals, all probably would be 

 modified in the same manner. Such facts as the com- 

 plex and extraordinary out-growths which invariably fol- 

 low from the insertion of a minute drop of poison by a 

 gall-producing insect, show us what singular modifica- 

 tions 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 coun- 

 try 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 



