It is the "expressive movements" visible to the eye which per mit of the argument by analogy. There are the gestures of animals nearest related to ourselves, and added to these there are finer changes which can be shown physiologically. These agree fully with those changes in ourselves accompanying our sensations. If there is any mental correlative to the expressive movements of animals, it must be represented by the so-called emotional side of our own mind. Our sensations are those of pleasure or displeas ure, of excitement or calm, of tension or relaxation. The rise and fall of our feelings are accompanied by definite changes in the rhythm and in the intensity of respiration and pulse. Exactly simi lar changes can be observed in animals either directly or with the help of special apparatus. All small animals, such as rabbits, rats or birds, show this when seized or even when approached by the hand in their cage. Changes in the rate of respiration, on stimulation, have been found to occur not only in fishes but even in arthropods, particularly in insects. Are these purely physiological processes, or may we argue from analogy and attribute to these animals feel ings such as we ourselves know ? In men these respiratory and vasomotor phenomena occur when particularly strong and lasting emotions are concerned. It is just these, for example joy, sorrow, care, fear, anger, fury and others, which are normally accom panied by typical expressive movements. In us these movements consist of mimicry, the expression of the eyes, particular move ments of the limbs, the whole attitude of the body. They are involuntary and take place in a reflex manner. We find a number of these expressions repeated by our nearest animal relations.
Of greatest interest to us are the expressions of the emotions in apes, which in many ways resemble our own to a remarkable degree. Nevertheless, differences exist, as Kohler points out as a result of his experiences with the Teneriffe chimpanzees. A shake of the hand was a sign of displeasure. Greater discontent was expressed by striking the ground rapidly with the hand. Indigna tion was made known by a forceful beating with both fists on the ground and an excited jumping up and down. Animated nodding of the head was the sign of joyous expectation. Fits of anger in anthropoids, as has long been known from experience in zoological gardens, are wont to be of uncommon intensity and long duration. Travellers relate how the hunted gorilla makes the most enraged assaults on its enemies, accompanied by roaring and beating of the chest with its hands. Just as men do in similar circumstances, chimpanzees scratch their head or the side of their body as a sign of uncertainty, indecision or embarrassment. When very aston ished they leave their mouth wide open. They are even capable of a sort of weeping, although without tears, when afflicted with grief. And so, too, they laugh in a certain measure, as an expres sion of pleasure or joy. When tickled in the palm of the hand their face is drawn into a sort of grin, while they titter or squeal with apparent pleasure. In common with human beings apes can blush and grow pale in the face.
In birds, too, movements exist which may be interpreted as the expression of feelings. These animals seem likewise to be subject to moods and humours corresponding to physiological conditions. On the other hand, the interpretation of particular movements of invertebrates is most uncertain, although they too have been re garded as expressive movements. The bee-keeper believes he hears the moods and feelings of his wards from the note given out by the inhabitants of a hive. In cephalopods a lively play of colours of the chromatophores is observable which makes the whole body or separate parts of it appear now light, now dark. At the same time, particularly in Sepia, the arms are held in a peculiar manner. It appears certain that these changes reflect definite excitations in the nervous system. But whether or not they are the expressions of definite feelings can only be guessed at.
One of the most emphatic feelings in human beings is that of pain. It is usually accompanied by particularly marked expressive movements. The most characteristic signs are violent withdrawal, warding off of the stimulus which causes the pain, and other move ments, the use of the voice being a common accompaniment, rising on occasions to a shout. In the vertebrates we find quite similar expressive movements and sounds, following on stimuli which we can suppose, on the analogy of their effects on ourselves, to give rise to perceptions and feelings of pain. It is true that these ex pressions are much less intense in herbivores than in the more agile carnivores and more especially in the apes. In the lower verte brates, and even in birds, these signs of a sensation of pain are even less apparent. Must we then suppose that these animals have a duller sense of pain than the higher vertebrates? In invertebrates, the existence of a sense of pain becomes very questionable indeed. Certainly here too we of ten find violent defensive and other move ments following on corresponding stimuli. But we have no. means of testing whether these are the expression of a sensation of pain or merely the consequence of a very strong excitation of the nervous system. The following experiment is suggestive. If an earthworm is suddenly cut into two pieces with a spade, the front half creeps on apparently unaffected while the hinder end writhes violently. Yet the latter contains only the ventral nerve-cord, with its lower centres, whereas the cerebral ganglion or brain is in the front portion. But the brain being the seat of the highest centres must be regarded as the bearer of consciousness, including the feeling of pain. It is worthy of remark, too, that in the highest organized invertebrates, the insects, injuries and mutilations tend usually to have no externally apparent effects. These animals usually behave in a quite unconcerned manner in response to extremely powerful stimuli, from which we would, from our own experience, be in clined to assume a violent feeling of pain. An ant continued quietly to suck honey after first the antennae and then the whole abdomen had been cut off. When the whole lower portion of the head, including the mouth, was cut off in bees and wasps (leaving the antennae intact), the animals went back again to some honey which they had found. When a caterpillar is wounded in the posterior part of the body and the head is turned round to the wound, the animal will eat its own flesh. It is possible to place the hinder end of a dragon-fly's abdomen between its jaws, upon which it begins at once to consume its own body. This seems to indi cate that a sensation of pain is lacking in arthropods. We conclude that to all appearances a pain sensation is present in the higher vertebrates and that the lower we descend in the animal scale, the more doubtful it becomes whether or not pain exists, until in the lowest animals it is almost certainly absent.
In many of the higher animals, mammals and birds, expressive movements are to be observed which by comparison with similar movements in ourselves may be taken as signs of fear. In addition to the changes in respiration and pulse-rate referred to already, trembling of the body is seen, together with timid withdrawal or, in the case of animals living in herds, panic-stricken flight. Numerous analogies are found, too, in the animal kingdom, again particularly in the higher mammals, for the outward and visible expressions of our own feelings of pleasure. And while we cannot be certain in the lower animals that there are any expressions of pain, almost all animals, right down to the worms, show a violent excitement during pairing and in other actions connected with sexual life. One might be inclined to interpret these as the ex pression of greatly heightened feelings of pleasure, of the same nature as our sensations of sexual satisfaction.