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ways. They may either modify the body of the individual 
or the sexual elements. 
“ Some variations are induced by the direct action of the sur 
rounding conditions on the whole organisation, or on certain parts 
alone, and other variations are induced indirectly through the repro 
ductive system being affected in the same manner as is so common 
with organic beings when removed from their natural conditions. 
The causes which induce variability act on the mature organism, 
on the embryo, and, as we have good reason to believe, on both 
sexual elements before impregnation has been effected.”—{The 
Variation, vol. zz - ., p. 270.) 
Mr. Darwin argues from the fact that changed con 
ditions produce sterility ; that it is therefore probable 
that changed conditions will modify offspriñg. 
“ Slight changes in the conditions of life have a remarkable power 
in causing a greater or less degree of sterility. Hence it seems not 
improbable that beings generated through a system so easily affected 
should themselves be affected. . . . On the whole, it is probable 
that any cause affecting the organs of reproduction would likewise 
affect their product—that is, the offspring thus generated.”—{The 
Variation, vol. zV., p. 26S.) 
Mr. Darwin also contends that, if such cause acted at all, 
it would influence all subjected to its influence in the 
same definite manner. 
“ The following case, which has been carefully examined and 
communicated to me by Dr. William Ogle, is highly instructive. 
Two girls, born as twins, and in all respects extremely alike, had 
their little fingers on both hands crooked ; and in both children the 
second bicuspid tooth in the upper jaw, of the second dentition, was 
misplaced ; for these teeth, instead of standing in a line with the 
others, grew from the roof of the mouth behind the first bicuspids. 
Neither the parents nor any other member of the family had ex 
hibited any similar peculiarity. Now, as both these children were 
affected in exactly the same manner by both deviations of structure, 
the idea of accident is at once excluded ; and we are compelled 
to admit that there must have existed some precise and sufficient 
cause which, if it had occurred a hundred times, would have affected 
a hundred children.”—(The Variation, vol. ii., p. 2jj.)