Although the animals which especially display these exceptional or contingent instincts are those which are fitted for the use and comfort of man and may be domesticated, it is doubtless true that many other species are in some degree provided with them, and that they thus have a plasticity in their nature which enables them to exercise, under particular circumstances, unlooked-for attention, foresight, and caution. And besides, it is only in analogy with the laws of the physical world that instinct should admit of a slightly diversified application.
It is to be noticed in this connection that many animals are gifted with a wonderful sensibility of the senses,the action of which is sometimes mistaken not only for the action of instinct, but for that of reason also. The acuteness of the sense of smell in the dog, which enables him to trace the steps of his master for miles through crowded streets by the infinitesimal odor which his footsteps left upon the pavement, is quite beyond our conception. Equally incomprehensible to us are the keenness of sight and wide range of vision of the eagle, which enable him to discover the rabbit nipping the clover amid the thick grass at a distance at which a like object would be to us altogether imperceptible. The chameleon is enabled to seize the little insects upon which it feeds by darting forth its wonderfully constructed tongue with such rapidity and with such delicacy of perception that "wonder-loving sages" have told us that it feeds upon the air.
It has been the belief of some observers that some animals have senses by which they are enabled to take cognizance of things which are not revealed directly to our senses. It is easy enough to conceive of beings endowed with a more perfect perception of the external world, both in its condition and the number of objects it presents, than we have, by means of other organs of outward perception. Voltaire, in one of his philosophical romances, represents an inhabitant of one of the planets of the Dog-Star as inquiring of the Secretary of the Academy of Sciences in the planet of Saturn, at which he had recently arrived in a journey through the heavens, how many senses the men of his globe had; and when the Academician answered, that they had seventy-two, and were every day complaining of the smallness of the number, he of the Dog-Star replied, that in his globe they had very near one thousand senses, and yet with all these they felt continually a sort of listless inquietude and vague desire which told them how very imperfect they were. But we shall not travel so far as this for our illustrations. We have all seen in the fields and about our houses birds and insects which seem to take cognizance of the electric state of the atmosphere; and we have learnt to feel quite sure, when, early in the morning of a summer's day, we see fresh piles of sand around the holes of the ants, that a storm is approaching, although the sky may as yet be cloudless and the air perfectly serene. In like manner birds perceive the approach of rain, and are all busy oiling and smoothing their feathers in preparation for it; and then, before the clouds break away, they come out from their retreats and joyfully hail the return of fair weather. So, by some analogous sense, the birds of passage are informed of the approach of winter and the return of spring.
It is doubtless true that in some animals the senses are immediately connected with instincts which assist and extend their operation. Metaphysicians and physiologists are agreed that the perception of distance is an acquired knowledge. The sense of sight by itself principally makes us conversant with extension only. The painting upon the retina of the eye presents all external things with flat surfaces and at the same distance. Before we can have any correct ideas of distance, we must be able to compare the result of the sense of sight with the result of the sense of feeling. By experience we in time come to judge something of distance by the size of the image which an object makes upon the retina, but more by our acquired knowledge of the form and color of external things. It is true that the eyes of many animals are constructed like those of man; but they do not learn to judge of distance by the same slow process. It is known from experiment that some animals have a perfect conception of distance at the moment of their birth; and the young of the greater part of animals possess some instinctive perception of this kind. "A flycatcher, for example, just come out of its shell, has been seen to peck at an insect with an aim as perfect as if it had been all its life engaged in learning the art." And so when the hen takes her chickens out into the field for the first time to feed, they seem to perceive very distinctly the relative distance of all objects about them, and will run by the straightest course when she calls them to pick up the little grains which she points out to them. Without this instinctive power of determining the relative distance and figure of objects, the young of most animals would perish before their sense of sight could be perfected, as ours is, by experience.
We have now noticed the chief characteristics of instinct: its existence prior to all experience or instruction; its incapacity of improvement, except within the narrow sphere of domestication; its limitation to a few objects, and the certainty of its action within these limits; the distinctness and permanence of its character for each species; and its constant hereditary nature. In regard to the uniformity of instinct throughout each species, it may be further remarked, that this seems to be very constantly preserved in the lowest divisions of the animal kingdom. Among the Articulates, also, instinct appears almost unvarying; and it is in this department among the insect tribes that the most striking manifestations of instinct are to be met with. When we arrive among the higher orders of the Vertebrates, we find in some species that each individual is capable of some modification of its actions, according to the particular circumstances in which it finds itself placed. But throughout the long series of animals, from the polype to man, there is instinctive action more or less in amount in every species, with, perhaps, the exception of man alone. The variety of that endowment, which is adapted to definite objects, means, and results, in each particular one of the five hundred thousand species estimated to be now living, may well call forth our admiration and astonishment at the magnitude and extent of the prospective contrivance of the Creator. How various the relations of all these animals to each other and to the inanimate world about them! and yet how admirable the adjustments of that immaterial principle which regulates their lives, so as to secure the well-being of each and the symmetry of the general plan!
There has been much diversity of opinion as to the existence of instincts in the human species,some making the whole mind of man nothing but a bundle of instincts, and others wholly denying him any endowment of this nature, while others still have given him a complex mental nature, and have, moreover, declared that intellect and instinct in him are so interwoven that it is impossible to tell where the one begins and the other ends. But we believe, with the author of "Ancient Metaphysics," that in Nature, however intimately things are blended together and run into each other like different shades of the same color, the species of things are absolutely distinct, and that there are certain fixed boundaries which separate them, however difficult it may be for us to find them out. In regard to intelligence and instinct, the two principles seem to us to be not more distinctly and widely separated in their nature than in the provinces of their operation.
Sir Henry Holland, who believes that intelligence and instinct are blended in man, admits that instincts, properly so called, form the minimum in relation to reason, and are difficult of definition from their connection with his higher mental functions, but that, wherever we can truly distinguish them, they are the same in principle and manner of operation as those of other animals. He makes one distinction, however, between the instincts of man and those of lower animals,that in the former they have more of individual character, are far less numerous and definite in relation to the physical conditions of life, and more various and extensive in regard to his moral nature. But, on the other hand, Sir B.C. Brodie seems to be of opinion that the majority of instincts belonging to man resemble those of the inferior animals, inasmuch as they relate to the preservation of the individual and the continuation of the species; and that when man first began to exist, and for some generations afterwards, the range of his instincts was much more extensive than it is at the present time. When authorities so eminent as these differ so widely upon the question, to what human instincts relate, we see at least that it is very difficult to define and distinguish these instincts, and we may be led to doubt their existence at all. Of that marvellous endowment which guides the bee to fabricate its cells according to laws of the most rigid mathematical exactness, and guides the swallow in its long flight to its winter home, we agree with Professor Bowen, that there is no trace whatever in human nature. The actions of man which have been loosely described as instinctive belong for the most part to those classes of actions which we have already shown to be in no proper sense of the word instinctive, that is, those concerned in the appetites and in the functions of organic life. There are also numerous automatic and habitual actions which are liable to be mistaken for instincts. Some have included in the category of instincts those intuitive perceptions and primary beliefs which are a part of our constitution, and are the foundation of all our knowledge. But these propensities of thought and feeling are of a higher nature than mere instincts; they are immutable laws of the human mind, which time and physical changes cannot reach: they do not seem to depend upon the physical organization, but to be inherent in the soul itself. If these are instincts, then, why are not all the ways in which the mind exerts itself instincts also, and reason itself an instinct?