Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/171

INSTINCT 159 recently acquired or altered experiences over a number of generations ; but in the few cases in which we have this opportunity we find good evidence that new or changed experience, when continued over a number of generations, is bequeathed to future generations as a legacy of intuitive knowledge, and that any newly acquired adjustive actions may in time be similarly transmitted as instinctive actions which no longer require to be separately learnt by each individual. Perhaps the best instance that can be quoted is that of the many species of birds, and some mammals, which when first found by man on oceanic islands were perfectly tame, but whose descendants now show a dread of man which is, in the most rigorous sense of the word, instinctive. The only other instances in which we have an oppor tunity of actually observing the transmission of newly acquired mental habits are those in which such habits have been artificially taught to domesticated animals. It so happens that these instances are very few in number, but it is not too much to say that, in all the cases where such habits have been taught for a long series of generations, some tokens of their hereditary transmission may now be observed. Thus, to quote Mr Darwin, whose accuracy on such a subject is not likely to be disputed by any one, " it cannot be doubted that young pointers I have myself seen a striking instance will sometimes point and even back other dogs the very first time they are taken out ; retriev ing is certainly in some degree inherited by retrievers ; and a tendency to run round instead of at a flock of sheep by shepherd dogs. If we were to behold one kind of wolf when young and without any training, as soon as it scented its prey, stand motionless like a statue, and then slowly crawl forward with a peculiar gait, and another kind of wolf rushing round instead of at a herd of deer, and driv ing them to a distant point, we should assuredly call these actions instinctive. Domestic instincts, as they may be called, are certainly much less fixed than natural instincts, but they have been acted on by far less rigorous selection, and have been transmitted for an incomparably shorter period, under less fixed conditions." Now these three habits, or mental attainments, are the only ones that have been systematically taught to any animals for a number of generations, and the fact that they all show a marked tendency to become intuitive may be taken as lending a greater amount of confirmation to the present theory of the origin of instincts than we might on a priori grounds be led to expect. The only other facts bearing upon this point are those which are thus tersely rendered by Mr Darwin. " How strongly these domestic instincts, habits, and dispositions are inherited, and how curiously they become mingled, is well shown when different breeds of dogs are crossed. Thus it is well known that a cross with a bull-dog has affected for many generations the courage and obstinacy of greyhounds, and a cross with a greyhound has given to a whole family of shepherd dogs a tendency to hunt hares. These domestic instincts resemble natural instincts, which in like manner become curiously blended together, and for a long time exhibit traces of the instincts of either parent." The above doctrine as to the nature, origin, and development of instincts serves very satisfactorily to explain nearly all the enor mous number of instincts with which we are acquainted. There, are, however, several special cases where there is still some difficulty in applying the above doctrine as a full and satisfactory explanation of the observed facts. This article may therefore fitly conclude with a brief enumeration of these cases. 1. The so-called " migratory instinct " is one that is still shrouded in much obscurity. The main difficulty with regard to it is to account for the "sense of direction," whereby the animals are guided to their destinations. Thus, for instance, many migratory birds fly at night, when it wonld seem impossible that they can be guided on their way by the sight and memory of landmarks. More over, it is asserted on good authority that among some species it is the habit for the young brood to fly separately by themselves, or apart from the older birds, and therefore to travel over enormous tracts of land and sea without either guidance or previous experi ence of the way ; such is unquestionably the case with the young of the cuckow. Lastly, it is certain that several species fly across immense tracts of ocean, where it is impossible that they can bo guided on their way by landmarks. Several theories have been pro pounded to account for these facts ; but, as none of them are satis factory, we need not here occupy space with their enumeration. 2. Closely allied to, if not identical with, this so-called "sense of direction" as manifested in migration, is the faculty whereby various species of animals which may not be migratory in their habits are enabled to find their way over greater or less distances. This has been called the "homing" faculty, and is chiefly mani fested by various species of domesticated mammals. It is very generally believed that it is also manifested by carrier-pigeons ; as a matter of fact, however, there is no trustworthy instance on record of one of these birds having found its way back over a tract of country with which it was previously unacquainted. In order that a carrier-pigeon should find its way home, it is necessary first to teach the animal, by flying it at a series of points along the route, the landmarks of the country which it is afterwards to traverse. But, although the " sense of direction " may be a figment as regards the carrier-pigeon, there can be no doubt that it is a fact as regards many species of our domesticated mammals. Thus the evidence is unequivocal with regard at least to dogs, cats, horses, sheep, pigs, and cattle. Judging from hitherto unpublished correspondence received from Australia and South America, there seems to be practically no limit to the distance over which these animals may be able to return ; and, what is of more importance, there can bo no doubt that these animals, when finding their way home, do not require to traverse the exact routes by which they came ; on the contrary, they generally seem to select the shortest or the straightest course, however circuitous the way may have been by which they were taken ; or, if their outward journey is over two sides of a tri angle, their homeward journey will probably be taken over the third side. The sense by which they are guided therefore cannot be, as has been suggested by more than one eminent naturalist, the sense of smell ; and for the same reasons it cannot be either the sense of sight or that of hearing. More plausible is the hypothesis that the faculty consists in an automatic process of "brain registration," every change of direction in the outgoing journey leaving behind it a record in the cerebral nervous system, and therefore in the mind of the animal, so that as a total result the general direction of the starting place is retained in the memory, just as we are ourselves able in a smaller degree to preserve our general sense of direction when winding through the streets of a town. One great difficulty attaching to this view appears to be that the animals in question are able to find their way home over land even when they have made their outward journey by sea, for it is evident that the difficulties of "brain registration " must in such cases be indefinitely increased, not only by the many meaningless movements of a vessel at sea, but still more by the fact that the changes of direction made by the vessel, being made in long and easy curves, and without muscular effort on the part of the animals, arc movements which we can scarcely suppose to be appreciated by the cerebral organization of the animals. On the whole, therefore, with regard to the faculty of "homing," as with regard to the analogous if not identical faculty exhibited in migration, it can only be said that further investigation- is required in order to explain that which, in the present state of our knowledge, must properly be regarded as inexplicable. 3. Mr Darwin has pointed out a serious difficulty lying against his theory of the origin of instincts by natural selection, and one which, as he justly remarks, it is surprising that no one should have hitherto advanced against the well-known doctrine of inherited habit, as taught by Lamarck. The difficulty is that among various species of social insects, such as bees and ants, there occur "neuter " or asexual individuals, which manifest entirely different instincts from the other or sexual individuals, and as the neuters cannot breed it is difficult to understand how their peculiar and distinctive instincts can be formed by natural selection, which, as we have seen, requires for its operation the transmission of mental faculties by heredity. The only possible way in which this difficulty can be met is the way in which it has been met by Mr Darwin, viz., by supposing "that selection may be applied to the family as to the individual." "Such faith may be placed in the power of selection that a breed of cattle always yielding oxen with extraordinarily long horns could, it is probable, be formed by carefully watching which individual bulls and cows, when matched, produced oxen with the longest horns ; and yet no one ox would ever have propa gated its kind " ; and similarly, of course, with regard to the instincts of neuters. As Mr Darwin has argued out this difficulty at length, it seems unnecessary to say more with regard to it than that he has shown it to be not so formidable as to exclude his doc trine as fully explanatory of such cases, when we have already accepted his doctrine as explanatory of other cases. 4. There arc two or three other special instincts of minor import-