28 The Selection Theory 
prospect of its being preserved, notwithstanding its utility. Darwin 
at first believed, that even single variations might lead to trans- 
formation 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. ee 
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’. 
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 natural selection whether whitish longi- 
tudinal 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, evolu- 
tion 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 require- 
ments 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 
} Studien zur Descendenz-Theorie u.,‘‘Die Enstehung der Zeichnung bei den Schmetter- 
lings-raupen,” Leipzig, 1876. 
