6 THE ORIGIN OF SPECIES 



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

 mann 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 condi- 

 tions. 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 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, 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 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 occasion- 

 ally 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 sep- 

 arated by any distinct line from slighter variations. All such 

 changes of structure, whether extremely slight or strongly marked, 

 which appear among many individuals living together, may be con- 

 sidered 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 



