Popular Science Monthly/Volume 3/June 1873/Instinct in Insects II
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Instinct in Insects II
By Georges Pouchet
|A Scientific Home Missionary→|
Last in series
LET us now dwell a little on two grand facts presented to us by the animated world, these two properties of living beings equally undeniable and unintelligible in their essence—habit, and hereditary tendency; and let us see how, in Darwin's theory, they will combine with intelligence. As the theory is well known, we need not state it. Cuvier believed in the unchangeableness of the animal forms placed on the globe by the Creator after each of the great convulsions through which, as he held, our planet has passed. Modern geology questions these violent commotions, and Darwin, taking up in his turn Lamarck's ideas, after fifty years of scientific progress, maintains, by almost irresistible arguments, that animal forms, instead of being unchangeable, as Cuvier supposed, are slowly modified, under the control of time, of circumstances, and of the energies with which each individual and each race "fight the battle of existence." That individual which brings into life a slight yet advantageous modification of its organs will succeed better in life than another. It will have every chance, then, of leaving a more numerous posterity. If the advantageous modification is transmitted, which may occur through hereditary tendency, the descendants of this individual will have, in their turn, the chance of succeeding better than their contemporaries. The modification, then, in all probability, will go on becoming more general, by the same law of fatality that causes a strong people to absorb a weak one: so that, after a longer or shorter time, the whole race will end by presenting the modification which was only individual at the outset. And since there was no reason why the same phenomenon, so natural and so simple, should not be repeated indefinitely, with all imaginable variations, we understand how it may result, in the infinite lapse of time, in that multiplicity of forms and characters which distinguishes animal species to our eyes.
Darwin says, in those pages in which he treats of instinct, that, if it were possible to prove that a habit might become hereditary, all distinction between habit and instinct would absolutely vanish. Darwin's literary procedure is that of always urging his reader further than he seems to go himself. He suggests the best arguments in the world with a doubtful air, and one is every moment surprised to find one's self so strongly convinced when the author seems convinced so little. And, in fact, we cannot deny that young puppies often come to a point the very first time they are sent out hunting, and that even better than others after long training. The habit of saving life is hereditary in some breeds, just as the shepherd's dog has the habit of walking around the flock. All these acts are performed, without the aid of experience, by the young as well as the old, and certainly apart from any notion of the object—at the first time, at least. The objection is idle that only those habits imposed by men on brutes are transmitted in this way. More than one instance, taken from wild animals, proves the contrary. The best is perhaps that which we see done by a bird of our own country, the oriole. It has a very peculiar cradle-shaped nest, hung from the fork of a branch, sewed at the edges with flexible grass, and always with bits of string, shreds, or packthread. There is no oriole's nest without some fastening worked by man's hand. If this is a habit, it is hereditary; if it is an instinct, it will be admitted at least that it does not go back to the beginning of the world.
From birth, one individual, or several individuals of the same species, placed in similar conditions, have had some habit. One of two things: this habit is injurious, or it is useful; it is either good or bad, from the point of view of the preservation of the individual, and consequently of the species. If it is injurious, it necessarily tends to disappear, either with the individual which has taken it on, or with the descendants which will inherit from it. If the habit is favorable, it has the chance of transmitting itself under the form of an instinct. This instinct, at first confined to a few individuals of the same blood, tends to become general, since it is advantageous, and we thus fall back into a particular case of the great principle of natural selection formulated by Darwin. Let us go on. Thus far this instinct is but little complicated, since it has only the significance of a habit that one individual may have been able to take up with its share of intelligence. Now that it is seen rooted under the form of instinct, each individual in its turn, with its own share of intelligence, may be able to add something to it of its own accord. If that addition is still favorable, and again gets transmitted, it will tend in the same way to become general; the acquired instinct will grow so much the more complex; and, exactly as organic modifications scarcely perceptible, but accumulated successively, to a sufficient number, have been able to multiply animal forms infinitely, so instinct, by almost imperceptible but continuous additions, may be able to end by reaching that state of perfection in which philosophers had supposed they saw the convincing proof of a preestablished harmony.
Some naturalists even now are not, very fortunately, inspired when they attempt to prove to us that the corporeal organization of every animal is conceived and framed with regard to its instincts. We need not go far to learn, as indeed we might expect from what has gone before, that instinct is in many cases independent of external forms. All birds, whether they are masons, like the swallow; weavers, like the warbler; carpenters, like the crow; mound-builders, like the mégapode—have the same beak, the same claws, and forms almost the same. The European beaver, inhabiting the affluents of the Rhone and the Danube, is scarcely to be distinguished from the American beaver, yet he has quite a different kind of work to do. The American beaver, on his lakes and great, lonely rivers, builds the famous houses so well known; the European beaver burrows long galleries underground in the manner of moles. If he has always done so, what becomes of that supposed necessary correlation between the organs and the instinct of a burrowing animal on one continent, a building animal on the other, with the same members for two objects so different? If the European beaver did once build huts, where shall we find more decisive testimony in favor of the theory of mutability in instincts? Pursued for his warm covering and his flesh, he has changed his instincts, before invading civilization, more rapidly than his external form. It is a point well established at this day that the contact with man has had a decisive effect on the instinct of many animals. It is thus that in inhabited countries large birds take flight at his approach, while they still allow him to come close to them in countries visited by travellers for the first time. Wherever they have been hunted like a prey that is worth the trouble of pursuit for their flesh or their feathers, they have formed the habit, and then have had the instinct of taking flight.
Let us return to insects. Two instincts, the most remarkable among all, are presented by them; that of the bee, with its mathematical architecture, and that of the ant, with its mixed societies. Before inquiring whether it might not be possible to explain even such amazing instincts by habit and inherited tendency, it is important at the outset to remove an objection that might be supposed unanswerable. Those individuals that have these instincts in the hive or in the ant-hill are neuters—that is to say, they are neither male nor female, and must consequently die without posterity. How explain the way in which a habit acquired by a neuter can be transmitted, can grow into an instinct, in the neuters of following generations, which will not descend from that first one? Yet the difficulty is not so great as it seems, and Darwin points it out very well. Indeed, it is not the instincts of the neuters which concern him, but it is the special organic modifications that these present, in connection with their social duties—with some, labor, and with others, fighting; but the reasoning he employs can be applied as well to instincts, behind which there always appears, as we see by a little reflection, that latent modification of the cerebral organ through which the transmission has taken place.
Darwin begins with a reminder that the principle of natural selection is true as well for communities as for individuals. The strength of a single male in a wild herd, the extraordinary fecundity of a single female, will be the elements of prosperity. The herd will succeed better than the rest. The qualities of the individual from which it draws its advantage will have a chance of being transmitted at first to all the herd, and this, more and more favored in the struggle against the outer world, will absorb the rest. The modification, at first individual, will become general. It would be the same if the member of the herd benefited in the beginning had been a neuter. We are still speaking of external forms. Let us suppose that a certain number of neuters may have brought from birth a favorable organic modification into a community of insects, and that by this the community has prospered; the males and females who have produced these neuters will then have, by them, the greatest possible chances of posterity. It may happen thenceforward that they transmit to their descendants what they had themselves—that is to say, the property of procreating neuters having the same favorable organic modification—and we thus fall back into the common process of natural selection. Such is Darwin's explanation; he is well aware, when he gives it, complex as it is, that it is the touchstone of his theory, the side whence attack will come; therefore, how he strengthens his arguments! He is no longer satisfied with explaining, he demonstrates; he is supposed to have exhausted his reasons, and this is the very moment he chooses for an appeal to experiment, and to the proof of that kind of paradox that might be called "hereditary tendency in sterility." There are oxen with horns a little longer than those of the bulls and heifers that produced them. "Well," says Darwin, "pair together, by attentive selection, the fertile descendants of the bulls and heifers that produced the oxen with longer horns, and before long you will have a race of oxen in which length of horns will he hereditary, although the animal is sterile." The experiment has yet to be made, and is worthy of being a temptation to some one of the great English lords who know so well how to spend their fortunes for the advance of science. There is every reason to believe that it would succeed; and, if this striking instance ever comes, to justify Darwin's theories in their points most difficult of explanation, how can we avoid accepting them in their completeness, as well for external forms as for instinct?
Neuters in a community bring at their birth an intellectual disposition, a special tendency. The community benefits by it, and prospers; but the parents of these neuters have produced, besides, males and females, who will be able to inherit in their turn the property of giving life to neuters having the same disposition or the same tendency with the first. This becomes hereditary; it fixes itself in the race; it is thenceforward an instinct; and it will be able to continue developing itself thus by a sort of collateral inheritance. The source of it will continue in the parents without its being necessary that they should have it themselves, exactly as the reason for the long horns of the oxen is in the parent bull and heifer which have only short ones themselves.
Even after confuting this great objection of the neuters, the problem of explaining the architecture of bees by natural conditions seemed still to defy every attempt. Yet Darwin undertook to solve it. Aided by the experiments of his countryman Waterhouse, he shows that all this labor, worthy of the most practised geometrician, can be reduced, in the last analysis, to a certain number of very simple habits, taken in succession, so that by a linking together of facts, hypothetical, it is true, yet all perfectly plausible and possible, we arrive at the discovery, in the biological laws already known, of a natural explanation of that instinct which seems to share in the miraculous. We know the subject in question. The cells of the bee are six-sided prisms of perfect regularity. The most interesting point is the bottom of the cell; it is formed of a hollow pyramid of three equal sides, and arranged in such a manner that each contributes its share, on the other side of the comb, to make the bottom of a distinct cell; the bottom of each cell thus rests on three cells on the other side of the comb. Buffon did not remark this combination; he only spoke of the regular hexagonal design of the whole, and on this subject he had a singular idea. "The bees," he said, "all want to make a cylindrical chamber for themselves in the wax, but room is wanting; on the comb, which is too small, each one attempts to settle itself in the way most convenient for itself, at the same time that all are equally in each other's way. The cells are hexagonal only on account of reciprocal obstacles. For the same reason," he adds, "as, if we fill a vessel with peas or cylindrical grains, shut it tightly after pouring in as much water as the intervals between these grains can receive, and set the water boiling, all these cylinders will become six-sided columns." Buffon's comparison has been a good deal laughed at, yet it is not altogether bad. He understood that each cell with its sides cut at regular angles was not an individual work, nor the direct execution of the original plan; that it was a kind of resultant brought about by the forced neighborhood, the mutual crowding and hindering of constructions conceived on a simpler plan, and one more usual among insects, the cylindrical chamber.
The humble-bees, which are hymenopterous insects, like honey-bees, put their store of honey away in their old cocoons. When the vessel is too small, they add to it at the opening a prolongation of wax. It may even occur that they build single cells, of an irregular globular form; this is a first step, the primitive wax-working. There is nothing very remarkable yet in this; but the next step becomes more important. Between this rude simplicity and the work, so finished, of the bee, we find something intermediate, the honey-cells of the domestic mélipone, of Mexico. The insect itself forms a transition, by its external marks, between the honey-bee and the humble-bee, and is nearer to the latter. To preserve its honey, it builds a pile of large spherical cells, all placed at equal distances apart, only that this distance is everywhere less than twice the radius of the spheres, so that they all encroach on each other, and are kept apart by a perfectly flat partition, having exactly the same thickness as the curved wall that bounds the free and spherical portion of each cell. If three are found to adjoin, the lines of separation cross at equal angles, and their common meeting-point rests on the top of a pyramid with three walls formed by the three cells, exactly as in a honeycomb. Reflecting on all this, Darwin says the thought occurred to him that, if the mélipone, which already builds its spheres at equal distances apart, were to come to disposing them symmetrically and back to back upon two opposite sides, there would result from this fact a construction as admirable as the bottom of a double rank of cells in the hive.
Has the constructive genius of the wasp and the bee passed through these transitions? It is impossible to assert it; but the evidence shows, and calculation confirms it, that some modifications, slight enough definitely, occurring in the instincts of the mélipone, might lead it, after an indefinite number of ages—we must always calculate on such periods of time—to build those three-angled pyramids which are already found in its constructions, in two or three ranks; then to build upon those pyramids, on each side, prolongations cylindrical in principle, like those which the humble-bee puts on its cocoons, and prism-shaped from their nearness to each other. Besides, such a construction upon a flat surface of its honey-cells by the mélipone would be nothing very extraordinary; in this way it builds the little chambers where it deposits its grubs.
In the general effort that produces the honey-comb, it is important to make allowance for that supreme law of necessity which Buffon refers to, and which compels each insect, if it makes a mistake in its measurements, to begin its work again, under penalty of seeing it destroyed by its neighbors. The bee's cell is no more an individual work than it is a work finished all at once. At the beginning, the six-sided plan is scarcely indicated; the original wall is clumsy, oftentimes too thick; it is attempted a second time, made thinner at the bottom, thickened at the top, crowded by force into its right place, and worked over and over constantly to the last perfection. The geometric regularity of the whole is the result of long tentative work. A multitude of bees are laboring on it at once, each for a time at one cell, then at another, and so on; twenty insects at least busy themselves with the first chamber, which at the outset is very irregular; new chambers are added, and the first remade. On all these points Darwin and other English naturalists have made very curious experiments, which deserve to be cited along with the observations of Francis Huber. He observed, to learn; they experimented, to explain. By dealing with swarms or individuals properly isolated, by modifying their conditions of labor, by deceiving their instinct, we should doubtless succeed in decomposing it by a kind of physiological analysis, at the same time that we should ascertain more clearly the tolerably large share that intelligence probably has in this industry of the bee. This is an aspect of the problem that is perhaps too much neglected by Darwin, but indicated by Mdlle. Clémence Royer in the notes added by her to the French translation of the "Origin of Species." We may ask, Why should not the bee itself be sensitive to that harmony of lines which strikes our eye in its work? Why deny so simple an impression as that which springs from regularity, to that brain which is of tiny dimensions, it is true, but which is quick to seize relations of far greater complexity between cause and effect, quick to choose the best place, to avoid an obstacle, to pursue with eye and sting the enemy of the hive? We have seen how the ant understands when an object is too large to pass through the entrance to its cave. The bee, to which we would attribute sensitiveness to regularity of lines, certainly has the notion of relations of length. There is a large moth, the death's-head sphinx, very fond of honey, and which asks nothing better than to make its way into the hive; its body, hairy and covered with horny plates, defies the sting. The bees, dreading this unwelcome visit, know very well how to protect themselves from it in regions where the sphinx abounds. As soon as the earliest ones begin to show themselves in the evenings of the longest days, as M. Blanchard relates, the bees narrow the opening of the hive in such a way that the robber can no longer get in. When the season for this moth has gone by, they destroy the new construction, and rebuild the passage of its original size. Certainly these are creatures that have a measuring eye! Is there, then, so wide a distance between this power of eye and the sense of symmetry, which the lowest savage has who is sensitive to the harmony in the lines of a carving or a tattooing? Is it not simpler to suppose that the bee has something of the same sensibility, rather than a sort of mathematical instinct, such as is sometimes attributed to it? The whole cerebral physiology of insects remains to be created. While we are no further advanced, it is perhaps rash to allow much to their intellectual faculties, but it is certainly unreasonable to degrade them too much. And, besides, there is still in us that old sin of pride, on which Montaigne rallies us so delicately, just with respect to the reason of animals. He understood animals much better than Descartes; he loves them, he plays with his cat, and this intercourse enlightens him; he speaks with sound judgment of the too narrow share of intelligence allowed to animals by man, while he himself "goes soaring in imagination beyond the orbit of the moon."
As to the legionary ants, the connection of the successive phenomena serving to explain the appearance and development of their instinct was far more difficult to conceive. We might well have despaired of any reasonable deduction, had not certain facts, here and there in Nature, come to our aid and put us on the right track, by showing us elsewhere the same instinct, less developed, or modified in different ways. These observations, coördinated by Darwin, have been like flashes of light, and have allowed us to conceive the evolution of these singular habits in a manner at least plausible. Thus, it is not uncommon that certain ants, which do not usually take auxiliaries, carry away to their hills nymphæ that are found by chance in their neighborhood. It is not unlikely that some of these nymphæ may have happened to come out, and may have performed the functions of their special instinct in their adopted city. If, now, it is admitted that these services may be of some use to the hill, then it will thrive better, and afterward it may happen that the same chance captives and chance comings-out of nymphæ may be repeated. At last, the habit will be formed—then the instinct will supervene, of carrying off stolen nymphæ. At the same time the presence of these strangers will almost necessarily react upon the robber-ants. Their instincts and their organs will be simultaneously modified, always upon the same principle, in the direction most favorable to the special duty that they perform in the community. From step to step, by a succession of scarcely-perceptible modifications, accumulating through centuries and ages, we shall arrive at races of legionaries as dependent on their comrades' labors as the species studied by Peter Huber.
Each instinct that we study displays itself to us, in a manner, under an absolute form; we never see it change; therefore it is said to be unchangeable. This is the illusion common to all phenomena that are too slow for their progress to be measured by the life or the memory of man. Yet the European beaver and the oriole give us examples of instincts that go back to a date relatively not very ancient. We know now, too, that the nests of the same species of birds sometimes present remarkable enough variations in different countries. That Darwin should point out with great care these instincts, varying with latitudes, is very natural; but we should less naturally expect to find a similar fact, in the book of a partisan, of the unchangeableness of instincts. The leaf-cutter, another hymenopterous insect, lays its eggs in little chambers made of bits of leaves which it has rapidly cut. In our country it is always a rose-leaf. Yet, "we are assured," says M. Blanchard, "that our cutter of rose-leaves, finding itself in some place in Russia where there are no rose-bushes, makes its nest with willow or osier leaves." Therefore, instinct must vary in space as it has varied in time! It is not at all the case that the same legionaries are everywhere as dependent on their comrades as those that Peter Huber saw in the environs of Geneva. In England, as in Switzerland, the auxiliaries reared by the dark-red ants take complete care of the larvae, while the legionaries alone go on expeditions; but in Switzerland the two castes together busy themselves about all works of construction or supply, while in England the legionaries alone go out to gather provisions and materials; the auxiliaries remain shut up within; they thus render less service to the community than they do in Switzerland.
It will be said, perhaps, that these differences are a very trifling matter. They are, at least, enough to show how the ancient doctrine of Cuvier has been shaken, and how, in the infinite lapse of time, those instincts may have become developed, which mere geographical accidents suffice to modify slightly. The grand solution of instinct is—Time; that immeasurable duration of those geological epochs which our mind holds in contemplation, but of which it can no more form an idea than of the measure of the heavenly spaces. Modern science begins to be amazed at those figures of ages which it must count since the rude attempts at primitive human industry. What shall we think of those times, measured by the planet's growth, through which the instinct of the legionary ants may have been originated, defined, and perfected? The ant not only saw the epoch of the reindeer and the mammoth, and the glaciers of the Jura creeping down the valley of the Rhone—it was a contemporary of that period which geologists mark by the lifting of the Alps. The ant is older on the earth than Mont Blanc. They existed already in the Jurassic period, very little different from what they are in our own times. While an inland sea still flowed over the site where later Paris was to stand, they were multitudinous in the central regions of Europe that were out of water. We may judge of this by the mass of their remains; they fill thick layers of territory at Oeningen, on the shores of Lake Constance, and at Radoboj, in Croatia; the rock is black with ants, all wonderfully preserved, with their claws and delicate antennae. Entomologists now count fifty species in Europe. More than a hundred have been found by Heer, of Zurich, and Mayr, of Vienna, in the cantons of Oeningen and Radoboj alone; several seem identical with existing species. Most of them have wings; these are males and females. Workers are rare; and that is explained by the nature of the rock, deposited at the bottom of still waters. The winged insects fell into it by thousands; the workers, more lowly in existence, attached to earth, have left fewer victims in the streams that preserve the record of that age. For the same reason, those sepulchres, so rich in species, teach us nothing of the habits or abodes of the ants of that time. What we do know is, that there were also plant-lice in the country, and that the larvæ of phryganes made for themselves even then, as they do now, those cases in which they live, and which they carry about everywhere with them. Some of these have been found at Oeningen. We have butterflies' wings of that era with their marks, if not with their coloring. Who knows whether we shall not some day discover a wasps'-nest dropped from a bough, and a trifle less regular than those of to-day? Even were it just as perfect, that would in nowise weaken the hypothesis of progressive development in the instinct by which it was built. Should we not have, beyond the Jurassic epoch, an enormous past, beside which the actual age now of the deposits of Oeningen and Radoboj is, perhaps, like a day or an hour in the history of man?
The grand result which the introduction of Darwin's ideas into biological science has had is, beyond question, to have transformed a subject hitherto deemed unapproachable and insolvable into a question of development that may be attacked by our investigations. Instinct, like the outward forms of animals, has always been made dependent on those first causes too high for man to raise his look to them. The observations of the English naturalist have brought the problem upon new ground; his logic, his science, have forced the world to accept at last the ideas formerly defended by Cuvier's opponents, by Lamarck and Geoffroy Saint-Hilaire. The doctrine of the immutability of animal forms has had its time, and that of the invariability of instinct is falling into ruin. Darwin proves, in fact, that it suffices to admit the principle of intelligence, which no one now denies to animals, and then the twofold influence of habit and hereditary tendency, and last that law, stated by himself, of absorption of the poorly-endowed races by these better endowed, to reach the conclusion that the finely-perfected instinct of the bee or the ant is nothing more than a purely natural phenomenon, a necessary consequence of life. The most complex instinct is merely an hereditary accumulation of very simple habits, of which the first source was always in the spontaneous intelligence of the individual. Instinct, then, including that of neuter animals, may be defined, "a group of habits, slowly acquired, and fixed by inheritance." Then it appears to us as independent, in some degree, of the forms of the animal; the variations it presents find their explanation; it is contingent, it originates, it is modified, through circumstances, aided by time, and through ages helped by scarcely-perceptible accidents. In its turn, it insensibly leads the organs to become perfect in the direction conformed to the use made of them by the animal. Regarded in this way, connected in the last analysis with other first properties from which it results, instinct, instead of baffling investigation by the human mind, as they do, becomes a possible and proper object of research by experimental science. It is a new horizon opening before the physiologist for the discovery of the laws of life.—Revue des Deux Mondes.