308 Mr. E. B. Poulton. Essential Nature of the [Apr. 23, 



the yellowish side in the case of apple. Far less can it account for 

 an almost typical yellow variety upon this latter plant. 



Tt is clear from the experiments upon the captured larvae that some 

 effect may "be produced in an intermediate variety by feeding it for 

 some considerable time (such as during the last stage), upon a food- 

 plant known to have strong tendencies. But this treatment does not 

 affect a larva which possesses a well-marked colouration — that is, a 

 larva with strong tendencies itself, and which coincide with those of 

 the food-plant. Nevertheless the experiments with larvae bred from 

 the egg prove that the variety towards which the larva is strongly 

 tending may be much modified by the food-plant given it during the 

 whole of its life. When such a change has been produced and is 

 manifest, if the food be appropriately shifted at the beginning of the 

 fifth stage, some considerable effect may be produced in the larva, in 

 the direction of the original tendency. 



20. A Suggested Explanation of the Larval Tendency towards Certain 

 Colours Independently of the Food-plant. 



The most probable explanation of these larval tendencies — which 

 differ so much in strength and direction, and which are independent 

 of the food-plant though generally modifiable by it — is that phyto- 

 phagic effects are hereditary, and thus stronger tendencies accumulate 

 as the larvae of successive generations feed upon the same food-plant 

 or on those which produce the same effects. In this way all shades 

 of colour may be occasionally found upon any tree, and the same irre- 

 gularity may be produced by interbreeding between individuals with 

 different tendencies. This explanation accounts for all the facts 

 brought out by this year's experiment and observation, and it is also 

 in harmony with previously recorded facts. Thus the unexpected 

 results of the breeding experiments would be explained by supposing 

 that during the previous generation (or perhaps generations), the 

 larvae had fed upon apple or some other food-plant tending to produce 

 whitish varieties. The yellowish larva found upon apple must, 

 according to this theory, have descended from a long line fed upon 

 8. Rubra, or some plant with leaves having green glabrous under- 

 sides. In this case I believe that there must have been several 

 generations accumulating yellowish tendencies by their food-plants, 

 for the effects of apple are so very strong in the other direction, and 

 yet so little change was produced. So also with regard to the two 

 larvae of different varieties upon the same tree, it may be supposed 

 that there had been different tendencies due to the food-plants of past 

 generations. In one of the cases quoted, there is some slight evidence 

 that this reasoning is correct. Of the seven larvae found upon 

 8. Babylonica, one was yellowish, and the others intermediate. 



