Animal Behaviour

by C. Lloyd Morgan

What, then, has the alternative hypothesis of natural selection to advance in explanation of these facts? On this hypothesis instinctive acts have biological value in such degree that they have become congenital through the preservation of adaptive variations. But if this be so, why does not the chick respond instinctively to the sight of that which is so essential to its existence as water to drink? In reply to this question it may be suggested that, under natural conditions, the hen teaches all her chickens to peck at the water, and thus shields them from the eliminating influence which gives rise to natural selection, in the absence of which the habit of drinking in response to the sight of water, though acquired by each succeeding generation of birds, has not become instinctive and congenital. Or, to put the matter from a slightly different point of view, the maternal instincts of the hen protect her chicks from any elimination in this respect; and in the absence of such elimination the habit has not been inherited as instinct. But though the hen can lead her young to peck at the water, she cannot teach them how to perform the complex movements of the mouth, throat, and head in actual drinking. In this matter, therefore, her own instinctive procedure does not shield them from the incidence of that elimination which leads to survival under natural selection. Those chicks would be eliminated which, on pecking the water, failed to respond to the stimulus by the complex behaviour involved in drinking, leaving those to survive in which the response had been congenitally established. Thus it would seem that, when natural selection is excluded, the habit has not become congenitally linked with a visual stimulus; but when natural selection is in operation, the response has been thus linked with the stimulus of water in the bill. Whence we may infer that the co-operation of natural selection is an essential factor in the evolution of instinctive behaviour.

There are, however, cases of instinctive behaviour which may seem too trivial and unimportant to be subject to the sway of natural selection. There are numberless little idiosyncracies of behaviour which seem to be truly instinctive, which are readily recognizable as distinctive traits, but which can hardly be regarded as of sufficient biological value to determine whether the creatures in which they are developed should survive or be eliminated in the struggle for existence. In many cases, however, these serve rather to distinguish the detailed manner of behaviour than its biological end or purpose. In different species natural selection may determine the survival of those whose instinctive behaviour meets a biological need. The relatively unimportant details, differing slightly in each species, are mere adjuncts; and since natural selection deals with each species or inter-generating group separately, the essential behaviour may in each case carry with it the associated differences of manner. We must remember, too, that, as in the matter of structure so in that of behaviour, it is the animal as a whole that is selected for survival; and so long as the whole is adapted to the circumstances of life, the associated differences of form or manner may share in, without doing much to determine, survival. In any case these little instinctive traits, if they are so trivial as to seem of small value from the biological point of view, appear to be too unimportant to have been intelligently acquired as habits.

Let us now consider one or two cases of instinctive behaviour which would fall under Romanes’s category of instincts of blended origin partly due to natural selection, partly to the inheritance of acquired habit. It is the custom of the house martin to build beneath the eaves. Forsaking the ancestral rocky haunts, it has been led to utilize the houses that man has built. This has all the appearance of being due to an intelligent modification of the ancestral instinct; but how far the modification has become through heredity a congenital variation, we do not know. The intelligence which is said to have enabled the martin of the past to adopt this method of nidification is still operative. The nestlings brought up under the eaves would have opportunities for acquiring experience which might lead them to build under similar circumstances. Nest and eaves would be associated in the conscious situation. Nor would the effects of natural selection be necessarily excluded. One may suppose that in the open country, far from rock-shelters, those martins in which there was a congenital tendency to build in house-shelters would bring up their broods and transmit this tendency; while those in which it was absent would either go elsewhere or fail to bring up broods at all. In the absence of fuller knowledge as to the truly instinctive nature of the behaviour, and as to its mode of genesis, we are in large degree at the mercy of conjecture. But in any case the incidence of elimination is not necessarily excluded, and there are, therefore, no grounds for denying that natural selection has been a co-operating factor in the evolution of the instinctive behaviour, if such it be.

It is well known that the lapwing will apparently simulate the actions of a wounded bird, with the object, as it seems, of drawing intruders away from her nest. And such tactics are not restricted to this bird, nor even to one or two species. They are common, no doubt with diversities of detail, to such different birds as grouse, pigeons, plovers, rails, avocets, ducks, pipits, buntings, and warblers. Granting that the behaviour is truly instinctive, it forms a very pretty subject for transmissionists and their critics to quarrel over. “If we seek, as an example,” the transmissionist may exclaim, “an instinct which bears the marks of its intelligent, and therefore acquired origin, this of feigning wounded provides all that we can possibly demand.” “What mode of instinctive behaviour,” the selectionist may ask, “can be adduced which is more obviously useful to the species? Is not this just the kind of procedure which natural selection, if it be a factor at all, must fix upon and perpetuate through the elimination of failures? Those birds which, through congenital variation of behaviour, acted in this way would certainly enable their offspring to escape destruction by enemies, and these would survive to perpetuate the instinct.”

Let us expand the transmissionist position a little further. An extremist, of the type presented by Eimer, would perhaps urge that the lapwing reasons thus: “If I pretend to be wounded, trail my wing, and flutter along the ground, instead of flying off, I shall draw upon myself the intruder’s attention, and lead him to suppose that I shall be easily caught; and if I thus entice him away, my little ones will be saved, and my end gained.” Thus, it may be said, might the bird argue, and then give practical effect to its reasoning. But are we not here attributing to the lapwing powers of ratiocination beyond the capacity of the most intelligent of birds? Are we not assuming a histrionic power, and a realization of the effects on others of its display, which many a human actor might well covet?

“But may not the bird,” it may be urged in reply, “have found by experience, without any elaborate process of abstract reasoning, that the trick is effectual?” In any case it would be experience perilously acquired. Granting that the bird has the wit to try the trick, a little over-acting, a little too much lameness of wing, and she is herself seized and killed; a little under-acting, and the trick fails—her brood is found and destroyed. Does it not seem probable that such experience would be dearly bought, that failure would mean either death to the parent or death to the offspring? And is it not clear that natural selection is thus introduced in any case? And may not the selectionist pertinently ask: “If natural selection is thus introduced as a factor, why halt midway between two hypotheses? Why not take the further step—one by which all the difficulties attending the intelligent acquisition and the biological transmission are alike avoided—of allowing that natural selection exercises, throughout, its influence on congenital variations, and not on acquired modifications of behaviour?”

There is, however, a way in which, when natural selection is operative, intelligence may serve to foster congenital variations of the required nature and direction. We must remember that acquired habits on the one hand, and congenital variations of instinctive behaviour on the other hand, are both working, in their different spheres, towards the same end, that of adjustment to the conditions of life. If, then, acquired accommodation and congenital adaptation reach this end by different methods, survival may be best secured by their co-operation. And the more thorough-going the co-operation the better the chance of survival. There would be a distinct advantage in the struggle for existence when inherited tendencies of independent origin coincided in direction with acquired modifications of behaviour; a distinct disadvantage when such inherited tendencies were of such a character as to thwart or divert the action of intelligence. Thus any hereditary variations which coincide in direction with modifications of behaviour due to acquired habit would be favoured and fostered; while such variations as occurred on other and divergent lines would tend to be weeded out. Professor Mark Baldwin,[46] who has independently suggested such relation between modification and variation, has applied to the process the term “Organic Selection;” but it may also be described as the natural selection of coincident variations.

It may be urged, therefore, that if natural selection be accepted as a potent factor in organic evolution, and unless good cases can be adduced in which natural selection can play no part and yet habit has become instinctive, we may adopt some such view as the foregoing. While still believing that there is some connection between habit and instinct, we may regard the connection as indirect and permissive rather than direct and transmissive. We may look upon some habits as the acquired modifications which foster those variations which are coincident in direction, and which go to the making of instinct.

The net result of a study of instinctive behaviour is to lead us to the conclusion that its evolution runs parallel with the evolution of animal structure. This is perhaps best seen in the case of those insects in which typical instinctive acts are performed by larvæ of wholly different form and structure, though they are stages in the development of the same species. This is exemplified in the cases of Sitaris, Argyromœba, and Leucopsis which have been briefly described. It is probable that in all cases of instinctive action natural selection has been a co-operating factor. Without going so far as to assert with Professor Weismann the “all-sufficiency of natural selection,” we may echo the words of Professor Groos,[47] and say: “Nevertheless, we know no principle except that of selection, and we must go as far as that will take us. Absolute knowledge of such phenomena is unattainable.” And in this conclusion we have the support of Dr. Peckham, who says,[48] “We have found them [the instinctive acts of solitary wasps] in all stages of their development, and are convinced that they have passed through many degrees from the simple to the complex, by the action of natural selection. Indeed, we find in them beautiful examples of the survival of the fittest.”