The Making of Species 
think it more probable that all species throw off 
at greater or less intervals discontinuous varia- 
tions, and that it is upon these that natural selec- 
tion acts. 
We further hope that we have succeeded in 
making clear what we believe to be the very 
sharp distinction between continuous and dis- 
continuous variations, even when the latter are 
inconsiderable, as frequently happens. 
Before leaving the subject of variation it is 
necessary to notice the distinction, which Weis- 
mann was the first to emphasise, between somatic 
and germinal variations. 
Every adult organism must be regarded as the 
result of two sets of forces ; inherited tendencies 
or internal forces, and the action of environment 
or external forces. The differences which the 
various members of a family show are due in 
part to the initial differences in the germinal 
material of which they are composed, and in part 
to the differences of their environment. The 
former differences are the result of what we may 
call germinal variations, and the latter the result 
of somatic variations. It is scarcely ever possible 
to say of any particular variation that it is a 
germinal or a somatic one, because even before 
birth a developing organism has been subjected 
to environmental influences. One of a litter may 
have received more nourishment than the others. 
Nevertheless, any marked variation which appears 
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