Before I enter upon this question it is necessary to consider for a little the physiological basis of instinct. We can distinguish three kinds of actions: purely reflex, purely instinctive, and purely conscious actions. In the case of the first, we see most clearly that they depend on an existing mechanism, for they follow of necessity on a particular stimulus, and cannot always be suppressed. Bright light striking our eye makes the pupil narrower by a contraction of the iris, and in the same way our eyelids close if a finger be thrust suddenly towards them. We know, too, the principle of these reflex mechanisms; they depend on nerve connexions. Sensory nerves are so connected in the nerve-centres with motor nerves, that a stimulus affecting the former at the periphery of the body, as at the eye, is carried to certain nerve-cells of the brain, and from these it excites to activity certain motor centres, so that definite movements are set up. It is rarely only one muscle that is thus excited to activity, there are usually several, and here we have the transition to instinctive action, which consists in a longer or shorter series of actions, that is, of motor combinations. These, too, are originally, at least, set a-going by a sense impression, an external stimulus which affects a sensory nerve exactly in the same way as in the reflex mechanism, and this stimulus is carried to a particular group of sensory nerve-cells in the central nervous organ, and from these transmitted by very fine inter-connexions to motor centres. There are extraordinarily complex instinctive actions, and in these the completion of one action is obviously the stimulus to the second, the completion of the second to the third, and so on, until the entire chain of inter-dependent movements which make up the whole performance has been completed.
Instincts have thus a material basis in the cells and fibres of the nervous system, and through variations in the connexions and irritability of these nervous parts they too can be modified, like any of the other characters of the body, such as form and colour.
Conscious actions depend directly on the will, and they have a close connexion with instinctive actions in as far as these also can be controlled by the will, that is, can be set a-going or inhibited, and also, on the other hand, in as far as purely voluntary actions may become instinctive through frequent repetition. The first case is illustrated, for instance, when the suckling of a child at the mother's breast is continued into the second year of life, as not infrequently happens in the southern countries of Europe. Such a child knows exactly why it wants the breast, and its action is a conscious one, while the newborn child seeks about with the mouth instinctively, and when it has found what it sought performs the somewhat complex sucking movements automatically. The second case is illustrated, when, for instance, we have made a habit of winding up a watch on going to bed, and do it when we happen to change our clothes through the day, although it is then purposeless and would have been omitted if the action had required a conscious effort of will. One can often observe on oneself in how short a time a conscious action may become instinctive. I once sent my keyless watch to a watchmaker for repairs, and received from him for the time an ordinary watch, which had to be wound with a key, which key I kept for safety in my purse. At the end of eight days I got back my own watch, and on undressing the first night I found myself 'instinctively' taking my purse from my pocket in order to get the key, which, as I very well knew, I no longer needed. And that a long series of complex movements, originally performed only consciously, may be gone through instinctively, is shown by the fact that pieces of music which have been learnt by heart can often be played without mistake from beginning to end while the player is thinking of quite other things. The complex instinctive actions of animals are performed in quite a similar manner.
There is thus no sharp boundary line between reflex and instinctive actions, nor between instinctive and conscious actions, but one passes over into the other, and the thought suggests itself, that in the phyletic development also transitions from one kind of action to the other must have taken place. As long as one believes the Lamarckian principle to be really operative one can suppose that actions, which were originally dependent on the will, when they were often repeated, became instinctive, or, in other words, that instincts, many of them at least, are inherited habits.
I shall endeavour to show later on that this assumption, plausible as it seems at first sight, cannot be correct; but in the meantime I must confine myself to saying that there are a great number of instincts which must be referred to the process of selection, and that the rest can be similarly interpreted in their essentials at least.
The instinct of self-preservation is universally distributed, and it is exhibited in many animals by flight from their enemies. The hare flees from the fox and from men; the bird flies away at the approach of the cat; the butterfly flies from even the shadow of the net spread to catch it. These might be regarded as purely conscious actions, and in the case of the hare and the bird experience and will have undoubtedly some part in them, but even in these the basis of the action is an organic impulse; this, and not reflection, causes the animal to flee at sight of an enemy. In the butterfly, indeed, this must be purely instinctive, since it is done with the same precision immediately on leaving the pupa state, before the animal has had any experience. But even in the case of the hare and the bird, taking to flight would in most cases come too late if reflection were necessary first; if it is to be effective it must take place as instantaneously as the shutting of the lids when danger threatens the eye.
The hermit-crab ([Fig. 34], p. 163), which conceals its soft abdomen in an empty mollusc shell, and drags that about with it on the floor of the sea, withdraws with lightning-like rapidity into its house as soon as any suspicious movement catches its eye, and it is very difficult to grip one of its legs with the forceps in time to draw it out of its shell. The same is the case with the so-called Serpulids, worms of the genus Serpula, and its allies; it is not easy to seize them, because, however quick one is with the forceps, their instinct of fugitive self-preservation acts more quickly still, and they shoot back into their protecting tubes before one has had time to grasp them. But this impulse to flee from enemies, though it seems almost a matter of course, is by no means common to all animals, for in quite a large number the instinct of self-preservation finds expression in an exactly contrary manner, namely, in the so-called 'death-feigning,' that is, remaining absolutely motionless in a definite position precisely prescribed to the animal by its instinct. In speaking of protective colouring, I drew attention to the 'wood-moth' (Xylina), which resembles a broken fragment of half-decayed wood so deceptively, and I pointed out that the colour-resemblance to wood would be in itself of but little use to the insect if it were not combined with the instinct to remain motionless in danger, to 'feign death.' The antennæ and legs are drawn close to the body, so that they rather heighten the disguise, and, instead of running away, the insect does not move a muscle until the danger is past. This instinct must have evolved hand in hand with the resemblance to a piece of wood, and, just as we sought to interpret the latter from the fact that the moths which most resembled the wood had always the best chance of surviving, so we maintain that those moths would profit most by their resemblance which drew in their legs and antennæ closely and lay most perfectly still. Thus the brain-mechanism, which effected the keeping still whenever the senses announced danger, would be more and more firmly established and perfected in the course of selection.
Even nearly related animals may have quite different instincts which secure them against danger. Thus in the group of pocket crabs (Notopoda) there are some species which run away when danger threatens, but others which anticipate the risk of discovery by masking themselves to a certain extent. With the last pair of legs they hold over themselves a large piece of sponge, which then grows till it often leaves only the limbs and frontal region uncovered. Of course there can be no question of consciousness in what the crab does, as is proved by the fact that these crabs will, in case of necessity, take a transparent piece of glass instead of the sponge; but the impulse to cover themselves with something is strong in them, and finds expression not only when they see a really protective substance, but even when they see one which is transparent and therefore wholly useless for the purpose. Crabs from which the sponge has been taken away wander about until they find another; the impulse is thus set up not only by the sight of the sponge or of a stone, but also by the feeling that their back is uncovered. The large spider-crab of the Mediterranean (Maja squinado) effects its disguise in a somewhat different manner. It has peculiar hooked bristles on the back, and on these it hooks little bunches of seaweed, often many of them, so that it is entirely covered and looks like a bunch of wrack rather than like an animal. Here again a bodily variation has gone hand in hand with the development of the instinct to cover itself: the bristles of the back have become hooked. Many instincts are accompanied by structural modifications, and in the crabs which cover themselves with sponge or stone this is the case, for the last pair of thoracic legs is turned towards the back, instead of being set at the side of the body, as is usual among crabs. They are thus enabled to hold the sponge much better and more permanently, and as this is advantageous we may well ascribe the change to natural selection.
Let us now turn our attention to another category of instincts, the most common and most indispensable of all, those which lead to the seeking and devouring of food.
The chicken just emerged from the egg picks up the seeds thrown to it with no experience of what eating is, or what can be made to serve it as food; its instinct for food expresses itself in picking up, and it is awakened or stimulated to action by sight of the seeds. As Lloyd Morgan in his excellent book on Habit and Instinct well says, 'It does not pick at the seeds because instinct says to it that this is something to be picked up and tested, but because it cannot do anything else.'