which showed themselves best adapted to a plant especially favourable to their kind, whose food-instinct was not only most strongly excited by this one plant, but also whose stomach and general metabolism made the best use of it. So we understand why so many caterpillars live on poisonous plants, not only some of our indigenous Sphingidæ, like _Deilephila euphorbiæ_, but whole groups of tropical Papilionidæ, Danaides, Acræides, and Heliconiidæ. With this again is connected the poisonousness or nauseousness of these butterflies. How diversely the instinct to procure food may be developed in one and the same group of animals is shown by the fact that not infrequently plant-eating, saphrophytic, and flesh-eating animals occur in a single group of organisms, as, for instance, in the order of water-fleas or Daphnidae, or in the class of Infusorians. Many species find their food by making an eddy in the water, which brings a stream towards the mouth, and with it all sorts of vegetable or dead particles; others live by preying upon other animals like themselves. But even when the food-instinct in all the species of a group is directed towards living prey, the procuring of it may be achieved by means of quite different instincts. Such finer graduations of the food-instinct are found not infrequently within quite small groups of animals, as in the Ephemeridæ or Day-flies. All their larvæ live by preying on other animals, but those of one family, represented by the genus _Chloëon_, seek to secure their victims by agility and speed, while the larvæ of the second family, with the typical genus _Baëtis_, have the instinct to press their smooth broad bodies, with large-eyed head, close to the brook pebbles on which they sit. They are exactly like these in colour, and thus they lurk almost invisible, until a victim comes within their reach, when they throw themselves upon it with a bound. The third group, with the typical genus _Ephemera_, follows its instinct to dig deep tubes in the mud at the bottom of the water, and to lurk in these for their prey. We have thus within this small group of Day-flies three distinct modifications of the food-instinct, which differ essentially from one another, are made up of quite different combinations of actions, and, consequently, must have their foundation in essentially different directive brain-mechanisms. All these cases have only one feature in common; the animals all throw themselves upon their prey as soon as they are near enough. [Illustration: FIG. 31. Sea-cucumber (_Cucumaria_), with expanded tentacles (_a_), and protruded tube-feet (_b_); after Ludwig.]