What, then, shall we understand by Spence’s phrase that instinct involves the performance of “certain actions”? And how far shall we accept it? We shall take it as implying so much definiteness of behaviour as renders instinctive acts susceptible of scientific investigation, and in this sense shall accept it with some modification of phraseology. We shall freely admit, however, the existence of variations of instinctive behaviour analogous to variations in animal structure. It is the occurrence of such variations that renders the natural selection of instinctive modes of behaviour conceivable. We shall also admit some, nay much, variation in detail. Take, for example, two of the cases which Mr. Marshall cites—nest-building and migration. Both involve, not merely a simple response to a given stimulus, but a complex sequence of actions. In detail there may be much variation even among members of the same species. And yet, can it be questioned that the behaviour as a whole is in each case relatively definite? May we not even say that it is remarkably definite? May we not even go further, and assert that only on the assumption that any given instinctive act is relatively definite, can we regard it as a subject for scientific investigation, and can we hope to distinguish it from other modes of behaviour?
The next point for consideration in Spence’s definition, which we have taken as our text, is his characterization of instinctive acts as “tending to the well-being of the individual and the preservation of the species.” Here we have Mr. Marshall with us, for he too lays stress on the fact that instinctive behaviour has reference to a definite biological end. But in saying that the biological end is the objective mark of an instinct,[28] he seems to be in error. Because, in the first place, there are other “objective marks,” and because, in the second place, this objective mark is not restricted to instinctive behaviour. According to Spence, a further characteristic of instinctive acts is that they are independent of instruction or experience; and this serves to differentiate them from other modes of behaviour which are also subservient to a biological end. Intelligent behaviour, not less than that which we term instinctive, has reference to a biological end. Many intelligent acts, for example, have for their object the well-being of the individual; many subserve race preservation; these bear, every whit as much as instinctive acts, the “objective mark” which Mr. Marshall regards as characteristic of instinct. And if we turn to his subjective criterion—the absence of any conception of the biological end which the behaviour subserves—Mr. Marshall’s position is equally untenable. There are thousands of acquired modes of behaviour, dependent on instruction or experience, in which there is, on the subjective side, so far as we can judge, no conception of the biological end to be attained. What can the animal in the early stages of intelligence know of biological ends? Mr. Marshall’s subjective criterion applies just as much to a wide range of intelligent behaviour as it does to instinctive actions.
In accepting, therefore, Spence’s statement that when animals behave instinctively they perform, without a knowledge of the end in view, certain actions tending to their own well-being and the preservation of the species, we must take it in connection with the preceding limitation, remembering that they are also performed without instruction and experience.
A further point for very brief consideration is suggested by the phrase in which Spence says that animals are all alike impelled to the performance of certain actions. As it stands it is too sweeping and general. Still, we do require some explicit statement of the facts which he had in mind when he wrote the words “all alike.” And we find it with sufficient exactness in Dr. Peckham’s definition, where he comprises under the category of instinctive behaviour “all complex acts which are performed previous to experience, and in a similar manner by all members of the same sex and race.” This places congenital behaviour in line with morphological structure as a subject for comparative treatment.
One more question remains. What shall we understand by “complex acts”? In the first place, it is well to restrict the term instinctive to co-ordinated actions; and this implies the presence of nerve-centres by which the co-ordination is effected. We thus exclude the organic behaviour of plants, since there is no evidence in the vegetable kingdom of co-ordinating centres. In the second place, the co-ordination is, as we have seen, congenital, and not acquired in the course of individual experience. Young water-birds, and indeed young chicks, as soon as they are born, and have recovered from the shock of birth, can swim with definite co-ordination of leg movements. Here the definiteness is not only congenital, but connate, if we use the latter term for an instinctive activity which is performed at or very shortly after birth. On the other hand, young swallows cannot fly at birth; they are then too immature, and their wings are not sufficiently developed. But when they are some three weeks old, and the wings have attained functional size and power, little swallows can fly with considerable if not perfect skill. The co-ordination is congenital, for it is not acquired in the course of individual experience; but it is not connate, since it is not exhibited at or shortly after birth. The term deferred may be applied to such congenital activities as are thus carried out when the animal has undergone a certain amount of further development after birth.
In the third place, it is customary to distinguish between such reflex actions as have already been briefly exemplified,[29] and instinctive behaviour. It is, however, by no means easy, if indeed it be possible, to draw any sharp and decisive line of demarcation. Instinct has indeed been well described by Mr. Herbert Spencer as compound reflex action; hence the distinction between instinctive and reflex behaviour turns in large degree on their relative complexity. It would seem, however, that whereas a reflex act—such as the withdrawal of the foot of a sleeping child when the sole is tickled—is a restricted and localized response, involving a particular organ or a definite group of muscles, and is initiated by a more or less specialized external stimulus; instinctive behaviour is a response of the animal as a whole, and involves the co-operation of several organs and of many groups of muscles. Partly initiated by an external stimulus or group of stimuli, it is also, seemingly, determined in part, in a greater degree than reflex action, by internal factors which cause uneasiness or distress, more or less marked, if they do not find their normal instinctive satisfaction. This point, however, may be more profitably discussed in connection with the conscious aspect of instinct. If, then, we say that reflex acts are local responses of the congenital type due to specialized stimuli, while instinctive activities are matters of more general behaviour, usually involving a larger measure of central (as opposed to local or ganglionic) co-ordination, and due to the more widely-spread effects of stimuli in which both external and internal factors co-operate, we shall probably get as near as is possible to the distinction of which we are in search. But it must be remembered that there are cases in which the distinction can hardly be maintained.
We are now in a position to define instinctive behaviour as comprising those complex groups of co-ordinated acts which are, on their first occurrence, independent of experience; which tend to the well-being of the individual and the preservation of the race; which are due to the co-operation of external and internal stimuli; which are similarly performed by all the members of the same more or less restricted group of animals; but which are subject to variation, and to subsequent modification under the guidance of experience.
II.—Instinctive Behaviour in Insects
Since instinctive behaviour is, by definition, independent of experience, and since the animals which act instinctively are also, in many cases, able to act intelligently, it is clear that, apart from hereditary variations, we must expect to find acquired modifications of instinct. As Huber said of bees, their instinctive procedure often indicates “a little dose of judgment.” It is, indeed, exceedingly difficult, as a matter of observation, to distinguish between hereditary variation and acquired modification. For the rôle played by these two factors in any given behaviour can only be determined if the whole life-history of the individual be known, and if there be opportunities for comparing it with the complete life-histories of other members of its race. And this is seldom possible.
These considerations must be borne in mind as we proceed to a brief study of some of the instinctive modes of behaviour in insects.