Thus, sundry definite varieties of the butterfly _Euprepia caja_ can be reared according to the different nourishment which is supplied to the caterpillar; and other butterflies are also known on whose colouring and markings the food of the caterpillar has great influence[114].
[114] See especially, Koch, _Die Raupen und Schmetterling der Wetterau_, and _Die Schmetterling des Südwestlichen Deutschlands_, whose very remarkable results of numerous and varied experiments are epitomized by Eimer, _Organic Evolution_, Eng. Trans. pp. 147-153; also Poulton, _Trans. Entom. Soc._ 1893.
Again, I may mention the remarkable case communicated to Darwin by Moritz Wagner, of a species of _Saturnia_, some pupae of which were transported from Texas to Switzerland in 1870. The moths which emerged in the following year were like the normal type in Texas. Their young were supplied with leaves of _Juglans regia_, instead of their natural food, _J. nigra_; and the moths into which these caterpillars changed were so different from their parents, both in form and colour, "that they were reckoned by entomologists as a distinct species[115]."
With regard to mollusks, M. Costa tells us that English oysters, when turned down in the Mediterranean, "_rapidly_ became like the true Mediterranean oyster, altered their manner of growth, and formed prominent diverging rays." This is most probably due to some change of food. So likewise may be the even more remarkable case of _Helix nemoralis_, which was introduced from Europe to Virginia a few years ago. Under the new conditions it varied to such an extent that up to last year no less than 125 varieties had been discovered. Of these 67, or more than half, are new--that is, unknown in the native continent of the species[116].
[116] Cockerell, _Nature_, vol. xli. p. 393.
In the case of Birds, the Brazilian parrot _Chrysotis festiva_ changes the green in its feathers to red or yellow, if fed on the fat of certain fishes; and the Indian Lori has its splendid colouring preserved by a peculiar kind of food (Wallace). The Bullfinch is well known to turn black when fed on hemp seeds, and the Canary to become red when fed on cayenne pepper (Darwin). Starting from these facts, Dr. Sauermann has recently investigated the subject experimentally; and finds that not only finches, but likewise other birds, such as fowls, and pigeons, are subject to similar variations of colour when fed on cayenne pepper; but in all cases the effect is produced only if the pepper is given to the young birds before their first moult. Moreover, he finds that a moist atmosphere facilitates the change of colour, and that the ruddy hue is discharged under the influence either of sunlight or of cold. Lastly, he has observed that sundry other materials such as glycerine and aniline dyes, produce the same results; so there can be no doubt that organic compounds probably occur in nature which are capable of directly affecting the colours of plumage when eaten by birds. Therefore the presence of such materials in the food-stuffs of birds occupying different areas may very well in many cases determine differences of colouration, which are constant or stable so long as the conditions of their production are maintained.