(> VARIATION UNDER DOMESTICATION. [CHAP. I. 



factors : namely, the nature of the organism, and the nature of the 

 conditions. The former seems to be much the more important ; 

 for nearly similar variations sometimes arise under, as far as we 

 can judge, dissimilar conditions ; and, on the other hand, dissimilar 

 variations arise under conditions whichappear to be n early uniform. 

 The effects on the offspring are either definite or indefinite. They 

 may be considered as definite when all or nearly all the offspring 

 of individuals exposed to certain conditions during several genera- 

 tions are modified in the same manner. It is extremely difficult to 

 come to any conclusion in regard to the extent of the changes 

 which have been thus definitely 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, etc. 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 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 differences occa 

 sionally 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. Al] 

 such changes of structure, whether extremely slight or strongly 

 marked, which appear amongst 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 

 chill affects different men in an indefinite manner, according to 

 their state of body or constitution, causing coughs or colds, rheu 

 matism, or inflammation of various organs. 



With respect to what I have called the indirect action of changed 

 conditions, namely, through the reproductive system of being 

 affected, we may infer that variability is thus induced, partly from 

 the fact of this system being extremely sensitive to any change in 





