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 indefinite 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- 
