VARIATION UNDER DOMESTICATION 33 



climate, etc. Each of the endless variations which we 

 see in the plumage of our fowls must have liad some 

 efficient cause; and if the same cause were to act uni- 

 formly 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 

 outgrowths 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 formation 

 of our domestic races. We see indefinite variability 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 differ- 

 ences 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 de- 

 serve to be called monstrosities arise; but monstrosities-^ 

 cannot be separated by any distinct line frornslighter \ 

 variaiIonsr^'"ATr'Wch changes^oT^structurey' whether ex- 

 tremely slight or strongly marked, which appear among / 

 many individuals living together, may be considered as 

 the indefinite effects of the conditions of life on each 

 individual organism, in nearly the same manner as the 

 chili affects different men in an indefinite manner, ac- 

 cording to their state of body or constitution, causing 



V 



