If a brief dehnition of instinct, from the purely biological point of view be required, that given in the Dictionary of Philo-Demm sophy and Psychology may be accepted: “ An inherited 0, , s reaction of the sensori-motor type, relatively complex and markedly adaptive in character, and common to a group of individuals.” Instinctive behaviour thus depends solely on how the nervous system has been built through heredity; while intelligent behaviour depends also on those characters of the nervous system which have been acquired under the modifying influence of individual relation to the environment. Such definitions, however, are not universally accepted. Wasmann, for example, divides instinctive actions under two groups: (1) those which immediately spring from the inherited dispositions; (2) those which indeed proceed from the same inherited dispositions but through the medium of sense experience. The first group, which he regards as instinctive in the strict acceptance of the term, seem exactly to correspond to those which fall under the definition given above. The second group, which he regards as instinctive in the wider acceptance of the term, nearly, if not quite, correspond to those above spoken of as intelligent-though he regards this term as falsely applied (see INTELLIGENCE or ANIMALS). By using the term instinctive in both its strict and its wider significance, Wasmann includes under it the whole range of animal behaviour. It will be seen that from the biological standpoint there fall under the stricter definition those hereditary modes of behaviour which are analogous to hereditary forms of structure; and that a sharp line of distinction is drawn between the behaviour which is thus rendered definite through heredity, and the behaviour the distinguishing characteristics of which are acquired in the course of individual life. What in popular usage are spoken of IS the instincts of animals, for example, the hunting of prey by foxes and wolves, or the procedure of ants in their nests, are generally joint products of hereditary and acquired factors. Wasmann's comprehensive definition so far accords with popular usage. But it tends to minimize the importance of the distinction of that which is prior to individual experience and that which results therefrom. It is the business of scientific interpretation to disentangle the factors which contribute to the joint-products. It is indeed by no means easy to distinguish between what is dependent on individual experience, and what is not. Only the careful observation of organisms throughout the earlier phases of their life-history can the closely related factors be distinguished with any approach to scientific accuracy. By the patient study of the behaviour of precocious young birds, such as chicks, pheasants, ducklings and moorhens, it can be readily ascertained that such modes of activity as running, swimming, diving, preening the down, scratching the ground, pecking at small objects, With the characteristic attitudes expressive of fear and anger, are so far instinctive as to be definite on their first occurrence?-they do not require to be learnt. No doubt they are subsequently guided to higher excellence and effectiveness with the experience gained in their oft-repeated performance. Indeed it may be said that only on the occasion of their initial performance are they purely instinctive; all subsequent performance being in some degree modified by the experience afforded by previous behaviour of like nature and the results it affords. It should be remembered that such comparatively simple activities, though there is little about them to arrest popular attention, are just the raw material out of which the normal active life of such organisms is elaborated, and that for scientific treatment they are therefore not less important than those more conspicuous performances which seem at first sight to call for special treatment, or even to demand a supplementary explanation. The instincts of nest-building, incubation and the rearing of young, though they occur later in life than those concerned in locomotion and the obtaining of food, are none the less founded on a hereditary basis, and in some respects are less rather than more liable to modification by the experience gained by the carrying out of hereditarily dehnite modes of procedure. Here the instinctive factor probably predominates over that which is experiential. Examples
But in the “ homing ” of pigeons there is little question that the experiential factor predominates. The habit results mainly from the modification of the higher nerve-centres through individual and intelligent use.. In the migration of birds we are still uncertain as to the exact nature and proportional value of the instinctive and intelligent factors. The impulse to migrate, that is to say, the calling forth of specific activities by climatal or other presentations, appears to be instinctive; whether the direction of migration is in like manner instinctive is a matter of uncertainty; and, if it be instinctive, the nature of the stimuli and the manner in which they are hereditarily linked with responsive acts is unexplained. To say that it is due to hereditary experience is generally regarded as inadmissible. For modern interpretation hereditary modes of behaviour afford experience; in no other sense can it be said that experience is inherited. A good example of the methods of recent investigation is to be found in Dr G. W. and Mrs Peckham's minute observations on the habits and instincts of the solitary wasps. They enumerate the following primary types of instinctive € ';";"P'° behaviour: the manner of attacking and capturing a msec; gh, particular kind of prey which alone affords the requisite presentation to sense; the manner of conveying the prey to the nest; the general style and locality of the nest; the method and order of procedure in stocking the nest with food for the unseen young. It is noteworthy, however, that although the manner in which the prey is stung (for example) is on the whole similar in the case of the members of any given species-that is to say, all the wasps of the species behave in very much the same manner yet there are minor variations in detail. This outcome of prolonged and careful observation is of importance. It affords- a point of departure for the interpretation of the genesis of existing instincts. Furthermore, the observations on American wasps render it probable that the earlier accotmts of the instinctive behaviour of such wasps are exaggerated. Romanes thought that the manner of stinging and paralysing their prey might be justly deemed the most remarkable instinct in the world. Spiders, caterpillars and grasshoppers are, he said, stung in their chief nerve-centres, in consequence of which the victims are not killed outright, but rendered motionless and continue to live in this paralysed condition for several weeks, being thus available as food for the larvae when these are hatched. Of course, he adds, the extraordinary fact which stands to be explained is that of the precise anatomical, not to say the physiological, knowf ledge which appears to be displayed by the insect in stinging only the nerve-centres of its prey. But the Peckhams' careful observations and experiments show that, with the American wasps, the victims stored in the nestsare quite as often dead as alive; that those which are only paralysed live for a varying number of days, some more, some less; that wasp larvae thrive just as well on dead victims, sometimes dried up, sometimes undergoing decomposition, as on living and paralysed prey; that the nerve-centres are not stung with the supposed uniformity; and that in some cases paralysis, in others death, follows when the victims are stung in parts far removed from any nerve-centre. It would seem then that by the stinging of insects or spiders their powers of resistance are overcome and their escape prevented; that some are killed outright and some paralysed is merely an incidental result.
Granted that instinctive modes of behaviour are hereditary and definite within the limits of congenital variation, the question of their manner of genesis is narrowed to a clear issue., Do they originate through the natural selection of Tggzf those variations which are the more adaptive; or do ' . they originate through the inheritance of those acquired modifications Which are impressed on the nervous system in the course of individual and intelligent use P Romanes, taking up the inquiry where Darwin left it, came to the conclusion that some instinctive modes of behaviour which he termed “primary” are due to the operation of natural selection alone; that others, which he termed “ secondary, ” and of which he could give few examples, were due to the inheritance of acquired modifications
from which, in the phrase of G. H. Lewes, the intelligence had