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 caterpillars, which to-day show
oblique stripes, possessed longitudinal stripes in Tertiary times? We can
read this fact from the history of their development, and I have before
attempted to show the biological significance of this change of colour.
("Studien zur Descendenz-Theorie" II., "Die Enstehung der Zeichnung bei den
Schmetterlings-raupen," Leipzig, 1876.)

For the present I need only draw the conclusion that one and the same
caterpillar may exhibit the initial stages of both, and that it depends on
the manner in which these marking elements are INTENSIFIED and COMBINED by