THE INSTINCTS OF ANIMALS 147 



The answer to this will be found in the following facts. On the 

 Belladonna plant there lives a little beetle whose feeding instinct is 

 aroused by this plant alone. But as Atrojxc belladonna is avoided 

 entirely by other animals on account of its poisonousness, this beetle 

 is, so to speak, sole proprietor of the Belladonna ; no other species 

 disputes its food, and in this there must assuredly be a great 

 advantage, as soon as the other instincts, above all that of egg-laying, 

 are so regulated as to secure that the larva shall have access to its 

 food-plant; and this is the case. The monojjhagy of many cater- 

 pillars is to be understood in the same way : it is an adaptation to 

 a plant otherwise little sought after, and it is combined with a more 

 or less complete loss of sensitiveness to the stimulus of other plants. 

 The establishment of such a specialized food-instinct depends on its 

 utility, and has resulted from the preference given, through natural 

 selection, to those individuals in which the food-instinct responded to 

 the stimulus of the smallest possible number of plants, and at the 

 same time to those 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 Sphingidas, like Deilepltila eivp^torhioi, but whole groups 

 of tropical Paj^ilionidffi, Danaides, Acrseides, and Heliconiidas. 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 Ephemeridte or Day-flies. All their larv^ live 

 by preying on other animals, but those of one family, represented by 

 the genus Chloeon, seek to secure their victims by agility and 

 speed, while the larvae of the second family, with the typical 



