306 Mr. E. B. Poulton. Essential Nature of the [Apr. 23 ; 



Babylonica is also strong evidence, if we assnme (as there is some 

 reason for doing) that the six intermediate larvae had tended towards 

 the whitish variety. 8. Triandra and 8. Viminalis remain a difficulty, 

 "but there is some confliction of testimony. The cases of S. Smithiana 

 and Ferruginea are favourable to my view of the effects of S. Viminalis 

 and of 8. Triandra, although indirectly, for the leaves of the former 

 species are intermediate between those of S. Caprea and 8. Viminalis 

 (8. Smithiana may be a hybrid between these two, and 8. Ferruginea 

 a variety of 8. Smithiana or a hybrid between it and 8. Ginerea. The 

 leaves of the 8. Ferruginea spoken of exactly resembled those of 

 S. Smithiana, except that they were smaller.) Hence if 8. Viminalis 

 tends towards yellowish, it would be expected that S. Ferruginea 

 would tend at least as strongly in the same direction, for its leaves 

 resemble those of 8. Ginerea as well as 8. Viminalis, and 8. Ginerea 

 produces yellowish larvae. And so with S. Smithiana, which resembles 

 S. Viminalis and 8. Caprea (which probably produces, yellow larvae 

 from its resemblance to a large S. Cinerea). But both these interme- 

 diate sallows produced, as a rule, intermediate varieties — that is to 

 say, their effects were between 8. Viminalis and S. Cinerea (or ? S. 

 Caprea), just as their leaves combine the characters of these forms. 

 And, further, in the intermediate varieties produced, the whitish form 

 predominated, just as the characters of 8. Viminalis predominate on 

 the underside of the leaves. Hence the observations upon these two 

 food-plants support my view as to the effect of S. Viminalis. In the 

 case of 8. Triandra I have some little experimental evidence, but I 

 have never found a larva on this tree in the field. I feel strongly 

 that its true tendency is, like that of 8. Rubra, towards a protective 

 (i.e., a yellowish) variety. This year I hope to breed a large number 

 of larvae from the egg upon this tree, and upon S. Viminalis. Omit- 

 ting these two trees, the whole list is very favourable to the theory 

 that the results are protective, and that the tendencies can be ascer- 

 tained by looking at the undersides of the leaves.* 



19. The Complex Nature of the Influence of the Food-plant upon the 

 Larvae of S. Ocellatus. 



The simplest view of this matter would be to suppose that the 

 colour of the leaf was itself the cause of the colour in the larva, show- 

 ing through the transparent skin, and that the change of diet was 

 therefore the direct cause of the change of colour. But it is quite 

 clear that the influence cannot be of this simple kind, for the amount 

 of substance which forms the underside of the leaf is a very small 

 proportion of the whole substance that is eaten. 



* I hare to thank Mr. Gr. C. Druce for much kind help in the difficult task of 

 naming the sallows. 



