156 Mr. E. B. Poulton. Colour-relation between the [Feb. 4, 



a much greater uniformity prevails when the larvae are bred from 

 the same batch of eggs. The former argument is enforced by the 

 fact that the captured divergent larvae are sometimes of different 

 age (and therefore probably of different parentage) from the normal 

 larvae upon the same tree. It must be clearly understood that in 

 speaking of these extreme divergences in the field, I am not alluding to 

 such instances as Mr. Boscher's eighteen yellow larvae upon 8. viminalis, 

 or my own instances of yellow larvse upon crab. I believe that these 

 are to be interpreted in another way which will be explained later. 

 There are altogether three factors which determine by their relative 

 predominance the colour of these larvae : (1) the tendency produced 

 by the food-plant ; (2) the hereditary larval tendency ; (3) individual 

 variation. (It does not signify for the present purpose whether the 

 third factor is a definite and independent tendency, or merely a vari- 

 able disturbance of a normal equilibrium between the first and second 

 factors, or an irregular recurrence to the influences of earlier genera- 

 tions.) Of these three factors the third has been shown to be com- 

 paratively unimportant, while many extreme exceptions are explicable 

 by the second. But I shall show later that the first factor may also 

 produce variable results in the case of the same food-plant, and it is 

 to such a cause that we must refer the interpretation of the conflicting 

 testimony concerning the effects of S. viminalis, &c. At the outset it 

 would be unlikely that the other two factors could have produced the 

 exceptions (upon 8. viminalis, &c), because of their number and 

 uniformity upon certain varieties of the food-plant. (See Mr. Meldola's 

 account of Mr. Boscher's captures, pages 241 and 306 of the English 

 translation of Weismann's " Studies in the Theory of Descent," Part II.) 

 I was very interested to find that two of the bred larvae possessed the 

 red spots. In my last paper I pointed out (p. 309) that the occur- 

 rence of the spots upon the yellowish variety only was an "argument 

 against the conclusion that these effects are in any way due to the 

 food- plants." It was, therefore, very satisfactory to find a spotted 

 larva which did not advance beyond the intermediate variety, and 

 which at an earlier stage was even whiter. (Series III, No. 17.) The 

 other instance was in accordance with the observation (which was 

 universal until the above recorded instance appeared) that the 

 spots are always found upon yellow varieties, for out of about a 

 hundred bred larvae in 1884 and 1885, there was only one yellow 

 variety, and this, with one exception, was the only red spotted larva. 

 But if the spots were always necessarily connected with one variety, 

 this would not prove that there could be no larval colour modifica- 

 tions, depending on the colour of the food- plant (in fact nothing can 

 do away with this conclusion now that it has so firm a basis of experi- 

 mental proof). There are many reasons for thinking that the ancestral 

 form of the larva was yellow, brightly spotted, and ornamented 



