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. 



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 1 . 



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 tins 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 



1 Studien zur Descendenz-Theorie n.,"Die Enstehung der Zeichnung bei den Schmetter- 

 lings-raupen," Leipzig, 1876. 



