Some years ago, with a success which delighted the observer that I am, I changed the diet of various carnivorous larvæ. To those which lived on Weevils I gave Locusts; to those which lived on Locusts I gave Flies. My nurslings unhesitatingly accepted the food unknown to their race and were none the worse for it; but I would not undertake to rear a caterpillar with the first sort of leaves that came to hand: it would starve sooner than touch them. Animal matter having undergone a more thorough refinement than vegetable substances, enables the stomach to pass from one dish to another without gradually becoming accustomed to each, whereas vegetable food, being comparatively refractory, calls for an apprenticeship on the part of the consumer. To turn Sheep’s flesh into Wolf’s flesh is an easy matter: a few minor transmutations are enough; but to make mutton out of grass is a complicated process of digestive chemistry, for which the ruminant’s four stomachs are none too many. The carnivorous insect is able to vary its diet, all sorts of game being of equal value. Vegetable food involves other conditions. With its starches, oils, essences and spices and often with its poisons, each plant tried would be a perilous innovation, to which the insect, repelled by the first mouthfuls, would never consent. How greatly preferable to these dangerous novelties is the invariable dish consecrated by ancient custom! This, no doubt, is why the vegetarian insect is faithful to its plant. How is this division of the earth’s abundance among its consumers effected? We can hardly hope to understand the problem; it is too far beyond our methods of research. The most that we can do is, by experimental methods, to explore this corner of the unknown a little, to seek to discover how far the insect’s diet is fixed and to note its variations, if any. This will give us data which the future will employ to carry the problem farther. Towards the end of the autumn, I had placed in the vivarium two couples of the Stercoraceous Geotrupes, with an ample heap of provender obtained from the Mules. I had no plans as regards my captives; I had put them there because it was an old habit of mine never to lose an opportunity. Chance had set them within my reach; chance would do the rest. With the sumptuous provision which I had bestowed upon them, the Geotrupes had had plenty wherewith to attend to their domestic affairs. They were overlooked all the winter, without any further intervention on my part. On the approach of spring, curiosity impelled me, in a leisure moment, to inspect them. It had been raining as hard through the sides of the cage, which consisted of a metal trellis, as it had in the streets; and, as the water could not trickle away through the wooden floor, the soil in the vivarium had turned to mud.