e in accordance with the variations
that present themselves. This proposition is undeniably true, but so
also is another, which apparently negatives it: the variation required
has in the majority of cases actually presented itself. Selection cannot
solve this contradiction; it does not call forth the useful variation,
but simply works upon it. The ultimate reason why one and the same
insect should occur in green and in brown, as often happens in
caterpillars and locusts, lies in the fact that variations towards brown
presented themselves, and so also did variations towards green: THE
KERNEL OF THE RIDDLE LIES IN THE VARYING, and for the present we
can only say, that small variations in different directions present
themselves in every species. Otherwise so many different kinds of
variations could not have arisen. I have endeavoured to explain this
remarkable fact by means of the intimate processes that must take place
within the germ-plasm, and I shall return to the problem when dealing
with "germinal selection."

We have, however, to make still greater demands on variation, for it
is not enough that the necessary variation should occur in isolated
individuals, because in that case there would be small prospect of its
being preserved, notwithstanding its utility. Darwin at first believed,
that even single variations might lead to transformation of the species,
but later he became convinced that this was impossible, at least
without the cooperation of other factors, such as isolation and sexual
selection.

In the case of the GREEN CATERPILLARS WITH BRIGHT LONGITUDINAL STRIPES,
numerous individuals exhibiting this useful variation must have been
produced to start with. In all higher, that is, multicellular organisms,
the germ-substance is the source of all transmissible variations, and
this germ-plasm is not a simple substance but is made up of many primary
constituents. The question can therefore be more precisely stated thus:
How does it come about that in so many cases the useful variations
present themselves in numbers just where they are required, the white
oblique lines in the leaf-caterpillar on the under surface of the body,
the accompanying coloured stripes just above them? And, further, how has
it come about that in grass caterpillars, not oblique but longitudinal
stripes, which are more effective for concealment among grass and
plants, have been evolved? And finally, how is it that the same
Hawk-moth caterpill