Fishes and amphibians supply examples proving that among vertebrates, too, such essential movements in the life of the animals as are concerned with the catching of prey still take place in a purely reflex manner. Many fishes and most amphibians react only to moving prey. Thus they make no distinction if in stead of real food one waves other objects of corresponding size to and fro before them. Sharks seize all small objects thrown over board from a ship, even when these are empty tins. The possibility of angling for many kinds of fish with artificial bait is due to automatic response of the animals to stimuli. A frog would starve before snapping at a motionless earthworm. It seizes it only when the worm moves. The frog reacts to a fly only when the latter cleans itself or moves from the spot in which it was standing. In deed, for the frog as for other animals a summation of stimuli is often necessary. On the first movement of the prey on coming into the field of vision, usually a mere turning of the head results. More movement causes farther turning, the body also participat ing. Additional stimuli then resuit in a raising or a lowering of the head in the direction of the prey, and finally snapping or ex tension of the tongue follows. If the prey is missed, as not infre quently happens, the frog only snaps again when the animal moves anew.
The Nature of Instincts.—Whereas generally speaking reflexes are made up of single movements of an animal or of its organs in response to a stimulus, instincts are those reactions which are composed of numerous movements (Lat. instinctus, impulse). A requisite condition is that the responses in question be shown by all normal individuals of the species in an almost identical manner. This type of behaviour is inherited from the ancestors of the animals in exactly the same way as any visible morphological character. Since this mode of behaviour is inborn, it has not to be learnt by the individuals of the species. Instincts are always adapted to the normal mode of life of the animals and for this reason they appear to be essentially purposeful in character. But, like reflexes, they are quite automatic and give a certain stamp to each different species of animal. We are concerned, then, with all behaviour and modes of action of which it can be said that the animals are impelled by a motive from within. Physiology rightly regards instincts as series of reflexes or so-called chain-reflexes (Loeb). This means that they can be analysed into a greater or lesser number of individual reflexes of which the occurrence of one is always the cause of the initiation of the next. Thus an animal disturbed during one instinctive action, so that the series of reac tions is interrupted, is often incapable of continuing the action where it has left off, but must restart the whole series from the beginning. This, however, is frequently impossible, since often the
first stimulus, which set the series going, is no longer present, or the physiological conditions of the body of the animal have become altered. An instinct, then, is a sum of reflexes, whose connection with one another is rigid and immutable like the reflexes them selves. Formerly all those complicated and purposeful actions which take place as inborn impulses, without any realization on the part of the animals as to their significance or object, were classed as instinctive behaviour. As just explained, however, the modern view is unconcerned with the presence or absence of con sciousness or of purpose in the behaviour under consideration.
In a later section, we shall return to this question. But it may be stated here that in most cases of instinctive behaviour no realiza tion exists on the part of the animals as to the purpose of their conduct. From the nature of the case, no such realization could exist. Ziegler (1920) has given a comprehensive account of the historical development of the concept of instinct. The often amaz ing purposefulness of instincts has given rise to numerous ex planatory hypotheses, none of which has supplied a satisfactory solution of the problem involved. Nothing therefore remains but to attribute the purposeful character of the conduct of the ani mals to the same factors which are responsible for the adaptive structure and functioning of their organs. Selection co-operates as a secondary but very important factor.
Instincts can be subdivided according to their objects into those of nutrition, migration, cleansing, protection, defence, metamor phosis, pairing, egg-laying, care of young, nest construction and social life. To the last-mentioned belong all the complex social instincts. We meet with instinctive actions in all divisions of the animal kingdom in which a central nervous system has been de veloped. Instincts are present from worms up to mammals, and many human actions are purely instinctive. Instinctively the new born seek the mother's breast and the drowning man clutches in stinctively to any solid object which offers. In point of fact, by far the greater number of the actions of invertebrate animals de pend upon such inherited inflexible instincts, as also do those of many lower vertebrates. Just as reflexes are found to be par ticularly characteristic of echinoderms, so the higher arthropods, in particular certain crustaceans and above all the insects, are in stinctive animals par excellence. A few examples only of the instinctive actions of animals will be cited here.