The Influence of External Conditions 23 
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. 11. 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 For somewhat similar experiments see Romanes, “Darwin and After Dar- 
win,” II, pp. 217-218. 
D 
