elaboration in two different directions: first, improvements in the ability to utilize animal substances which happened to stick to the leaves, and second, an increase in the probability of animals sticking to the leaves, and so becoming available. Thus there arose, on the one hand, dissolving and digestive juices, and arrangements for absorption; and, on the other hand, viscid slime, and traps of various kinds to secure the animals, as well as honey and bright colours to attract them. But it is not merely transformations in the form of the stems and leaves which have come about; there are also important physiological changes. The sensitiveness to stimulus of various parts of the leaf is greatly increased, to a certain extent in the butterwort, the edges of whose leaves turn inwards in response to stimulus, still more in the sundew, in which the stimulus is conveyed from the tentacles touched to all the others, but most wonderfully of all in the Venus fly-trap and _Aldrovandia_, whose sensitive hairs so transmit the stimulus that the whole leaf is affected by it, and is set in motion, in a manner quite comparable to the effects of a nerve-stimulus in animals. Thus the case of carnivorous or insectivorous plants shows us that, in the course of natural selection, quite new organs can be produced in a plant by a thoroughgoing transformation of old ones, as, for instance, the pitchers of _Nepenthes_, and that, furthermore, even the physiological capacities of the plant may be changed in the most far-reaching manner, increasing and varying until they come to resemble the functions of the animal body. LECTURE VIII THE INSTINCTS OF ANIMALS The robber-wasp--Statement of the problem--Material basis of instincts--Instincts are not 'inherited habits'--Instinct of self-preservation--Fugitive instinct: death-feigning--Masking of crabs--Nutritive instinct--Monophagous caterpillars--Diverse modes of acquiring food: May-flies, sea-cucumbers, fishes that snare--'Aberration' of instinct--Change of instinct during metamorphosis: Eristalis, Sitaris--Imperfection of adaptation points to origin through natural selection--Instinct and will--Instincts and protective coloration--Leisurely flight of Heliconiidæ--Rapid flight of Papilionidæ--Instincts which act only once in a lifetime--Pupation of butterflies--Pupation of the Longicorns--Pupation of the silk-moth--The emperor moth--The cocoons of Atlas--Oviposition of butterflies.