The Influence of External Conditions 35 
It is not difficult to starve caterpillars, but it is difficult to 
make them take more food than they do normally; yet by 
indirect means this can be done by giving them food containing 
an excessive amount of nutritive substances. 
Reviewing the results of the effects produced by changes in 
food, Pictet points out that, in general, the variations produced 
are either in the direction of lighter (albinistic) or darker (mel- 
anistic) shades. If the kinds of food that produce the former 
effect are compared with those that produce the latter effect, it 
will be seen that the lighter colors result from feeding on plants 
which, owing to their anatomical structure, present obstacles to 
mastication —either as a result of a thick epidermis or of the pres- 
ence of crystals or of hairs, etc. In consequence of the imperfect 
nutrition of the caterpillar, the butterfly is less pigmented. On 
the other hand, the melanistic or black variations result from 
those plants that offer no obstacles to mastication or nutrition. 
The caterpillars develop rapidly, acquire a size greater than the 
‘normal, and the butterfly shows a greater development of pig- 
ment. 
The influence of different food plants in causing an albinistic 
or melanistic change in the moths can be traced, according to 
Pictet, to the length of the pupal stage, and this in turn to that 
of the caterpillar stage, which again is due to the amount of 
nourishment received. Thus, those food plants that give in- 
sufficient nourishment prolong the caterpillar stage; but this 
leads to a shortening of the pupal stage, and the albinistic effect 
is produced, because the dark pigments do not have time to 
develop extensively. Conversely, an abundant nourishment 
shortens the caterpillar stage; but this causes a prolongation of 
the pupal stage during which the pigments have time to develop 
even more fully than normally, and the melanistic variation is 
the result. 
Not only the colors of the moth or butterfly are affected by 
the character of the food, but the colors of the caterpillars them- 
selves may sometimes be affected, although to a less degree. 
Pictet finds, in fact, that there is a correlation between the pig- 
