6 VARIATION UNDER DOMESTICATION. [Chap. I. 



action, we must bear in mind that in every case, as Professor 

 Weismann has lately insisted, and as I have incidentally shown in 

 my work on 'Variation under Domestication,' there are two 

 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 which appear to be nearly 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 generations 

 aro 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, &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 invariably 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 dis- 

 tinguish 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 ol 

 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 as the indefinite effects of the conditions of life on each 

 individual organism, in nearly the same manner as a chill affects 

 different men in an indefinite manner, according to their stato 



