Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/170

158 IN.STINCT conditions of life. Under changed conditions of life it is at least possible that slight modifications of instinct might be profitable to a species ; and if it can be shown that instincts do vary ever so little, then I can see no diffi culty in natural selection preserving and continually accumulating variations of instinct to any extent that was profitable. It is thus, I believe, that all the most complex and wonderful instincts have originated." But here it is of importance to note that there is no reason why instincts should be restricted to one or other of these two modes of origin. On the contrary, there seems to be every reason to suppose that many instincts may have had, as it were, a double root intelligent adjustment and natural selection blending their influences to a joint production. For example, the grouse of North America display the curious instinct of burrowing a tunnel just below the surface of the snow. In the end of this tunnel they sleep securely ; for, when any four-footed enemy approaches the mouth of the tunnel, the bird, in order to escape, has only to fly up through the thin covering of snow. Now in this case the grouse probably began to burrow for the sake of protection, or concealment, or both, and if so, thus far the burrowing was probably an act of intelligence. But the longer the tunnel the better would it have served the purposes of escape, and therefore natural selection would almost certainly have tended to preserve the birds which made the longest tunnels, until the utmost benefit that length of tunnel could give had been attained. And similarly the origin of many other instincts may be satisfactorily explained by thus supposing the combined operation of two causes intelligent adjustment arid natural selection where there is a difficulty in explaining their origin as due to either cause alone. And if even in fully formed instincts we often find " a little dose of judgment or reason," we can have no cause to doubt that in the formation of instincts by natural selection such small admixtures of judgment or reason may often greatly assist the process, while, conversely, it is even more evident that an instinct which is developing from the habitual perform ance of an intelligent action might be greatly assisted by natural selection favouring the individuals which most frequently or most promptly performed that action. Varia- it is necessary to the above interpretation of the origin bihty. O f i ns ti nc ts that the latter should not be immutably fixed. A few words may therefore be added to show that the view commonly entertained as to the unalterable character of instincts is erroneous. As a matter of fact, instincts are eminently variable, and therefore admit of being modified as modifying circumstances may require ; their variability gives them plasticity whereby they may be moulded always to fit an environment, however continuously the latter may be subject to gradual change. For the sake of brevity we may confine our attention to a single instinct, and for the sake of procuring a good test we may again take as our example the instinct of incuba tion. This affords a good test because it must be regarded, .not merely as one of the most important, but also as one of the oldest of instincts, and therefore one which for both these reasons we should deem least likely to exhibit variability. Yet we- find it to exhibit variability in every imaginable direction. Thus the complicated effects of domestication and artificial selection on some of our breeds of poultry appear to have almost completely destroyed this instinct, while in other breeds it remains intact, if indeed it has not even been intensified. Among the latter breeds experiment shows that the natural period of incubation may be indefinitely prolonged by substituting " dummies " for eggs, while the following experiment, which we owe to Mr Spalding, shows " how far the time of sitting may be interfered with in the opposite direction. Two hens," he says, "became broody on the same day, and I set them on dummies. On the third day, I put two chicks a day old to one of these two hens; she pecked at them once or twice, seemed rather fidgety, then took to them, called them to her, and entered on all the cares of a mother. The other hen was similarly tried, but with a very different result ; she pecked at the chickens viciously, and both that day and the next stub bornly refused to have anything to do with them." Similarly the period of maternal supervision after the chickens have been hatched admits of being greatly modified, as is proved by some experiments made and published several years ago by the present writer. In one of these experiments there was given to a Brahma hen a pea-fowl s egg to hatch ; the hen was an old one, and had previously reared several broods of ordinary chickens. A pea-chicken requires a much longer period of maternal care than does an ordinary chicken, and for the wonderfully long period of eighteen months the old Brahma hen continued to pay unremitting attention to her supposed offspring. Through all this time she never laid any eggs, and eventually the separation seemed to take place from the side of the peacock. In other cases, however, where the conditions of the experi ment were exactly parallel, the pea-chickens were abandoned by their Brahma mothers at the time when the latter ordi narily abandon their chickens. But not only will a hen thus take to a brood of birds so unlike her natural chickens as are pea-fowl, and adapt her instincts to their peculiar needs ; she may even take to young animals belonging to a different class, and adapt her instincts to their still more peculiar needs. Thus the writer gave to a hen, which for several weeks had been sitting on dummies, three newly- born ferrets ; she took to them almost immediately, and remained with them for more than a fortnight, when they were taken away from her. During the whole of this time she had to sit upon the nest, for of course the young ferrets were not able to follow her about as young chickens would have done. Two or three times a day she would fly off her nest, calling upon her brood to follow ; but, on hearing their cries of distress from cold, she always returned im mediately, and sat with patience for six or seven hours more. She only took one day to learn the meaning of these cries, and after that she would always run in an agitated manner to any place where the crying ferrets were concealed. Yet it would not be possible to conceive a greater contrast than that between the shrill piping note of a young chicken and the hoarse growling noise of a young ferret. It is of importance to add that the hen very soon learnt to accommodate herself to the entirely novel mode of feeding that her young ones required ; for, although at first she showed much uneasiness when the ferrets were taken from her to be fed, before long she used to cluck when she saw the milk brought, and surveyed the feeding with satisfaction. But she never became accustomed to the attempt of the ferrets at sucking, and to the last used now and then to fly off the nest with a cackle when nipped by the young mammals in their search for the teats. Enough then has been said on the variability of instinct Her to show that there is supplied to natural selection abundant tar J opportunity for the development of new and more highly u wrought instincts from previously formed and less elabor ated instincts. But in order to show that this opportunity has been utilized it is not enough to show that hereditary instinct may be modified by individual experience ; it must also be shown that such a modification when successively repeated through a number of generations itself becomes inherited. Now, although the evidence on this point is necessarily scanty, it is .sufficient for the purpose here required. The evidence is scanty because there are only a very few cases in which human observation has, as it were, the opportunity of watching the continuance of effects of