Darwin's Artificial and Natural Selection 93 



states that an organism that has once begun to vary con- 

 tinues to vary for many generations, that this continuous 

 variation is always in the same direction, but only that 

 new combinations, scattering in all directions, continue to 

 appear. 



The nature of the organism seemed to Darwin to be a more 

 important factor in the origin of new variations than the 

 external conditions, " for nearly similar variations sometimes 

 arise under, as far as we can judge, dissimilar conditions ; 

 and, on the other hand, dissimilar variations arise under con- 

 ditions which appear to be nearly uniform." The following 

 statement is important in connection with the origin of 

 " definite " variations. " Each of the endless variations which 

 we see in the plumage of our fowls must have had some 

 efficient cause ; and if the same causes were to act uniformly 

 during a long series of generations on many individuals, all 

 probably would be modified in the same direction." Here 

 we find an explicit statement in regard to the accumulation of 

 variation in a given direction as the result of an external 

 agent, but Darwin hastens to add: "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 mdefinite vari- 

 ability 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 seed- 

 lings 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." 



Another cause of variation, Darwin believes, is in the in- 



