Many more cases of animal-catching plants might be adduced, but it is far from my intention to try to describe all the existing contrivances; those already mentioned may suffice to give an idea of the diversity and of the detailed effectiveness of these adaptations. They amplify—so it seems to me—our conception of the scope of natural selection, by showing us that adaptations may arise which are quite foreign to the original mode of life of the organism in question, and stand, indeed, in apparent contradiction to its fundamental physiological processes. It is hardly necessary to enter into a special argument to show that they can only have been brought about in the course of natural selection, since every other interpretation of their occurrence fails. Neither climatic nor any other external direct influence could have effected these modifications of the parts of plants, which are all so different, yet all so well suited to their purpose; they are different even in plants growing quite close together, like the sundew and the butterwort. The Lamarckian principle of use and disuse hardly enters into the question at all, since plants do not possess a will, and we can hardly speak of 'chance' where we have to do with such complex and diversely combined transformations. A process of selection actually operative in each of these cases can easily be thought out, and I shall leave it to my readers themselves to do this, and shall only indicate that we have to do with increasing 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.
We have hitherto considered animals with especial regard to the variation and re-adaptation of morphological characters, e.g. modifications of form and colour; and we have now to ask whether their behaviour also is to be referred as to its origin, in whole or in part, to the principle of selection. All around us we can see that animals know how to use their parts or organs in a purposeful manner: the duckling swims at once upon the water; the chicken which has just been hatched from the egg pecks at the seeds lying on the ground; the butterfly but newly emerged from the pupa, as soon as its wings have dried and hardened, knows how to use them in flight; and the predatory wasp requires no instruction to recognize her victim, a particular caterpillar, a grasshopper, or some other definite insect; she knows how to attack it, to paralyse it by stings, and then hesitates not a moment as to what she has to do next; she drags it to her nest, deposits it in one of the cells already prepared for her future brood, lays a single egg upon it, and roofs the cell carefully over. It is only because all these complex acts are so precisely performed, as precisely as if the wasp knew why she performed them, that the species is able to maintain its existence, for only thus can the rearing of the next generation be secured. Out of the egg there slips a little larva, which at once makes for the paralysed victim, feeds upon it, and grows thereby, then, within the shelter of the closed cell, passes through the pupa stage and is transformed into a perfect wasp. Many species of these predatory wasps do not lay the egg directly beside or upon their prey, but lest its movements should endanger their offspring, they hang the egg above it by a silken thread. It is thus in security, and the young larva, too, when it appears, can withdraw to its safely swinging resting-place as soon as danger threatens from the convulsive struggles of the unfortunate victim at whose body it is gnawing.
Every animal has a great many such 'instincts,' which lead it, indeed force it, to act appropriately towards an end, without having any consciousness of that end. For how should the butterfly know what flying is, or that it possessed the power of flight at all, or who could have shown the predatory wasp, when she wakened from the pupa sleep to quite a new kind of life, all that she had to do in order to procure food for herself and to secure shelter and nourishment for the brood which was still enclosed within her ovary? Since species have developed from other species, these regulators of the body, the instincts, cannot have been the same in earlier times; they must have evolved out of the instincts of ancestors, and the questions we have to ask are: By what factors? In what way? Has the principle of selection been operative here too, or can we refer instincts to the inherited effects of use and disuse?