ON COLOURS IN NATURE. 89 
The effects of certain food plants, about which the evidence 
was conflicting, were also further investigated, the periods in- 
quired into during which the larvee are most susceptible to 
the influence of the food plant, and the instances carefully 
watched for of individual variation among the larve from the 
same batch of eggs, and fed upon the same food. ‘The 
influence of the food plant, so far as observation has been 
made, is not uniform, and must act during a large proportion 
of the whole family life to produce an effect. It is probable 
that effects accumulate during successive generations. The 
effects are partially due to pigment which is proper to the 
larva, and which has no immediate relation to the food plant. 
The changes produced in derived pigments are more com- 
plicated, and due to the predominance of one or another of 
vegetable colouring matters in the tissues and blood. The food 
in the digestive tract is a probable first cause of larval colour, 
and the dark dorsal line of the larva a very early marking. 
Where the large-leaved varieties of the willow have been used 
as the food plant, the tendency in the larva has usually been 
towards a whitish hue, and where the long but narrow and 
small-leaved ditto, towards yellow. Towards yellow again 
when the dark green upper side of Salix viminalis has served 
as food, and, on the other hand, the tendency always towards 
white, when the feeding has taken place on the under-sides of 
the same leaves, the said under-sides being, of course, hoary, 
as in the case of the poplar, upon which tree as well as upon 
many varieties of willow, and upon the apple, and cultivated 
and wild crab, experiments have been made. Another kind 
of willow, Salix triandra, with whitish bloom, is evidently in 
the direction of yellow, but ditto without the white bloom, 
more strongly than the last in the direction of yellow. ‘This, 
of course, would indicate that the presence of white bloom 
on the leaf counteracts so far as it produces any effect, although 
it cannot counteract altogether and effectually, yellow in the 
larva. The parent larve experimented on of Smerinthus 
ocellatus were extreme white varieties, and belonged to a 
group which evidently inherited a very strong tendency in 
this direction, as was shown by the comparatively slight 
effect that followed the use of foods which most powerfully 
tend to produce white varieties. It is noteworthy to 
observe that the leaves of the crab produced most extreme 
white in the bred larva, as also in the case of the 
parents previously experimented on, but that Salix rubra 
and Babylonica produced much less effect than in the 
case of the parents. And Mr. Poulton’s argument, drawn 
