VARIATION UNDER DOMESTICATION. 7 



and indirectly by affecting the reproductive system. With 

 respect to the direct 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, dissimi- 

 lar 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 con- 

 ditions during several generations 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, 

 color 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 effi- 

 cient 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 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 proba- 

 bly 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 inherit- 

 ance 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-eapsule. 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 moustrosities arise; but monstrosities 

 cannot be separated by any distinct line from slighter varia- 

 tions. All such changes of structure, whether extremely 



