The Influence of External Conditions 33 



tained similar results in other experiments. It is not entirely 

 clear from his figure that the darker form in the fourth gener- 

 ation is really a return to the ancestral type at least the 

 possibility of a different interpretation must be kept open. 



Another experiment showed that when the first generation 

 was fed on walnut, and the second and the third on oak, the 

 effects of the walnut were still apparent in the last generations. 



When the first generation was fed on walnut, the second on 

 the oak, and the third on flowers of different kinds (rose and 

 peony), the last seemed to accentuate the effects of the walnut 

 and "tend to cause to disappear those of the normal food. To 

 the same extent as the walnut the flowers appear to be a 

 poor alimentation." The caterpillars of Ocneria were also fed 

 on other plants, the results being in some respects like the pre- 

 ceding cases, in others different. 



There are two known aberrant forms of the moth Psilura 

 monacha. The typical male form is shown in Fig. 10, and the 

 female in Fig. u. The aberrant form nigra, male and female, 

 is shown in Figs. 12 and 13, and the form eremita, male and 

 female, in Figs. 14 and 15. The caterpillars are found on oak, 

 birch, and conifers. 



Caterpillars fed on oak and on birch produced moths of all 

 three types, in the proportion of 58 per cent type-form ; 24 per 

 cent ab. nigra, and 18 per cent ab. eremita; fed on walnut the 

 proportions were 38 per cent type-form; 23 per cent ab. nigra; 

 30 per cent ab. eremita. The moths were smaller than the aver- 

 age. The proportion of the aberrant forms is greater from the 

 walnut than from the normal food. In the second generation, 

 nourished on the walnut, the females of the type form assume 

 the characters of the typical males. 



In all, Pictet's experiments with feeding included 21 species, 

 comprising 4695 individuals. In nearly all cases some effects 

 of a change from the normal type could be directly traced 

 to the food. 1 



1 For somewhat similar experiments see Romanes, "Darwin and After Dar- 

 win," II, pp. 217-218. 

 D 



