natural selection whether whitish longitudinal or oblique stripes should
result. In this case then the "useful variations" were actually "always
there," and we see that in the same group of Lepidoptera, e.g. species of
Sphingidae, evolution has occurred in both directions according to whether
the form lived among grass or on broad leaves with oblique lateral veins,
and we can observe even now that the species with oblique stripes have
longitudinal stripes when young, that is to say, while the stripes have no
biological significance. The white places in the skin which gave rise,
probably first as small spots, to this protective marking could be combined
in one way or another according to the requirements of the species. They
must therefore either have possessed selection-value from the first, or, if
this was not the case at their earliest occurrence, there must have been
SOME OTHER FACTORS which raised them to the point of selection-value. I
shall return to this in discussing germinal selection. But the case may be
followed still farther, and leads us to the same alternative on a still
more secure basis.

Many years ago I observed in caterpillars of Smerinthus populi (the poplar
hawk-moth), which also possess white oblique stripes, that certain
individuals showed RED SPOTS above these stripes; these spots occurred only
on certain segments, and never flowed together to form continuous stripes.
In another species (Smerinthus tiliae) similar blood-red spots unite to
form a line-like coloured seam in the last stage of larval life, while in
S. ocellata rust-red spots appear in individual caterpillars, but more
rarely than in S. Populi, and they show no tendency to flow together.

Thus we have here the origin of a new character, arising from small
beginnings, at least in S. tiliae, in which species the coloured stripes
are a normal specific character. In the other species, S. populi and S.
ocellata, we find the beginnings of the same variation, in one more rarely