ON SOME INTERACTIONS 0F ORGANISMS.“‘ BY S. A. FORBES. While the structural relations of living organisms, as expressed in a classification, can best he figured by a tree ——the various groups, pest and present, being related to each other either as twigs t0 twigs, as twigs t0 branches,' or as branches to the main stem—yet this illustration does not at all express their femctéeeel relations. W'hile the anatomical characters of the various groups may show that they are all branches of a common stock, from Which they have arisen by repeated divisions and contin— ued divergeneies, the history of their lives Will show that they are 110W much more intimately and variously bound together by mutual interactions then are twigs 0f the same hranehb—that with respect to their Vital activities they occupy rather the relation of organs of the same ani— mal body. If for a type of their classification we look to the vegetable world, for an illustration of "their mutual actions and reactions we must 10014; to the animal world. The serious modification of any group, either in 1111m— here, habits, 01' distribution, must modify, considerably, various other groups; and each of these must transmit the change in turn, 01' initiate some other form of change, the disturbance thus propagating itself in a far—extend- ing Circle. While the Whole organic world, viewed as a living unit, thus differs from the single plant by the much greater in— *As details accumulated relating to the feed of animals and similar subjects, it was found that s. preper discussion of them would I‘ieeesseri1},F lead, step by step, to a. full review of certain parts of the general subject 0f the reactions between groups of organisms and their surroundings. erganie and inorganic. Without such a review, the facts can net be safely generalized, ner the conclusions clearly apprehendefl to which they point; It has therefore seemed best te prepare the Way for the diseussien at special subjects by this general discussion of the subject at large. The practical importance of this larger view is illustrated by the feet that. if the current ideas of the value of parasitic and predaeeous insects areeeeepted, we must condemn the bluebird te extermination as a. pest; while if the conclusions of this paper are essentially sound, this, bird is" 3- Very useful species and should be carefully preserved. I 4 On Some Interactions of Organisms. terdependence of its parts, on the other hand, it differs from the single animal in the fact that, notwithstanding this intimate and instant sympathy of part with part, it has an immense Vitality. To out off the leg of an animal is often sufficient to destroy its life, but one might cut off the head of the animal world, so to speak, without seriously impairing its energy. Suddenly to annihilate every living vertebrate would doubtless set on foot some tremendous revolutions in the life of the earth, but it is certain that in time the wound would‘heal—that Nature would finish by readjusting her machinery and would then go on IflflCll as before. In fact, any subkingdom of animals or any class of plants might thus be struck out. without the slightest danger that terrestrial life would perish as a consequence. The functioi'ls of the missing member would be taken on in part by other members, and in part be rendered needless by new adjustments. We see many present illustrations of this fact, as in Australia, where there is but one native carnivorous ani- mal, and that probably not. indigenous; in several Pacific islands where mammals are unknown; and in New Zea— land and the Galapagos, where insects are extremely few and the flowers, therefore, chiefly ooloiless and odorless. _We see, likewise, illustrations of the same truth in the conditions of vegetable and animal life in earlier geologi- eal periods. Plants and insects, for example, existed to- gether through vast periods of time when there were neither mammals nor birds on earth to supervise or regulate their relations. If this is true of such immense and ttwolutionary dis- turbances, it is all the more certain that this same spon~ taneous action of natural forces must in time reduce the smaller disturbances of the primitive order caused every- where by civilized man, and must end by adjusting the whole scheme of organic relations to his interests as com- pletely as to the interests of any other species. It is also plain that if man understands clearly the disorders which arise in the system of Nature as a result of the rapid pro; gressive changes in his own condition and activities, and understands also the processes of Nature which tend to On Some Interactions of Organisms. 5 lessen and remoye these disorders, he may, by his own in— telligent interference, often avoid or greatly mitigate the evils of his situation, as well as hasten their remedy and removal. Some general notion of the original order of Nature, which obtains where civilization has not penetrated, will be needful for an understanding of the most important eonsequenoes of the modifications of that order which man brings to pass—for an understanding" of the rela- tions of our own industrial operations and interests to the general laws and activities of the organic world under whose constant influence we must live and work There is a general eonsent that primeval nature, as in the uninhabited forest or the untilled plain, presents a settled harmony of interaction among organic groups which is in strong contrast with the many serious 1nalad~ justments of plants and animals found in eountries oeeu~ pied by man. This is so familiar a fact that I need not dwell upon it, but will cite the reader to the generally ae— oessible “Ifltl‘OdllCt-litf3fl to Entomology,” :hy Kirby & Spence, for a sufficient statement of it. It will be more to my purpose to discuss the subject from a different stand— point. To determine the primitive order of Nature by in- duction alone requires such a vast number of observa— tions in all parts of-the world, for so long a period of time, that more positive and satisfactory conclusions may perhaps be reached if we call in the aid of first prin- ciples, traveling to our end by the a. prior-i mad. For the purposes of this inquiry I shall assume as established laws of life, the reality of the struggle for ex- istence, the appearance of variations, and the frequent inheritance of such as oonduee to the good of the indi— vidual and the species in short, the evolution of species and higher groups under the influence of natural seleoi tion. I shall also postulate, as an accepted law of Nature, the generalization that the species is maintained at the cost of the individual—that, as a general rule, the rate of reproduction is in inverse ratio to the grade of individual development and activity; or, as Spencer tersely states this law, that “Individuation and Genesis are antagonis- 6 On Some Interactions of Organisms. tie.” Evidently a species cannot long maintain itself in numbers greater than can find sufficient food, year after year. If it is a phytophagous insect, for example, it will soon dwindle if it seriously lessens the numbers of the plants upon which it feeds, either directly, by eating them up, or indirectly, by so weakening them that they labor under a marked disadvantage in the struggle with other plants for foothold, light, air and food. The interest of the insect is therefore identical with the interest of the plant it feeds upon. Wihateyer injuriously affects the lat— ter, equally injures the former; and whatever favors the latter, equally favors the former. This must, therefore, be regarded as the extreme normal limit of the numbers of a phytophagous speeies—a limit such that its depreda- tions shall do no especial harm to the plants upon which it depends for food, but shall remove only the excess of foliage or fruit, or else superfluous individuals which must either perish otherwise, if not eaten, or, surviving, must injure their speeies by overcrowding. if the plant— feeder multiply beyond the above limit, evidently the diminution of its food supply will soon react to diminish its own numbers; a counter reaction will then take place in favor of the plant, and so on through an oscillation of indefinite continuance. 011 the other hand, the reduction of the phytOphagous insect below the normal number will evidently injure the food plant by preventing a reduction of its excess of growth or numbers, and will also set up an oscillation like the preceding“, except that the steps will be taken in reverse orderi‘ ' I next point out the fact that precisely the same reason— ing applies to predaeeous and parasitic insects. Their in- terests, also, are identical with the interests of the species they parasitise or prey upon. A diminution of their food reacts to decrease their own numbers. They are thus vi- tally interested in confining their depredaticns to the ex— cess of individuals produced, or to redundant or other- wise unessential structures. It is only by a sort of un- * See “Principles of Biology.” by Herbert Spencer. Vol IIIr pp. 397-41'8. On Some Iatemetioas of Orgaamas. 7 lucky accident that a destructive species really injures the Species preyed upon. The discussion has thus far afiee—ted only such organ— isms as are confined-to a single species. It remains to see how it applies to sueh as have several sourer—ms of support open to them—such, for instance, as feed indifferently upon several plants or upon a variety of animals, or both. Let us take, first, the ease of a predaeeous beetle feeding upon" a variety of other inseetsheither indifferently, upon whatever species is most numerous or most aeees- sible, or preferably upon certain species, resorting; to others only in case of an insufficiency of "its favorite food. It is at once evident that, taking the group of its food— inseets as a unit, the same reasoning applies as if it were restricted to a single species for food; that is, it is inter- ested in the maintenance of these food—species at the highest number consistent with the general conditions of the environment—interested to confine its own depredai tions to that surplus of its food-whieh would otherwise perish if not eaten-minterested, therefore, in establishing a rate of reproduction for itself which will not unduly I lessen its food supply. Its interest in the Inunbers of each species of the group it eats will evidently he the same as its interest in the group as a whole, sinec the group as a whole can be kept at the highest number possible only by keeping each species at the highest number possible. If the predatory insect prefer some species of the group to others, we need only say that whatever interest it has in any Species of the group, will be an interest in keeping; up its numbers to the highest limit; and any failure in this respect will injure it in precisely the ratio of the value of that species as an element of its food. It would be most injured by anything injuriously affecting" the species it most preferred—the preferences of animals be— ing, according to the doctrine of evolution, like their in— stincts, inherited tendencies toward the things which have proved beneficial to their progenitors. This argument holds for birds as well as for insects, for animals of all kinds, in fact, whether their food be simple or mixed, animal or vegetable, or both. It also applies to 8 On Some Interactions of Organisms. parasitic plants. The ideal adjustment is one in which the reproductive rate of each species should be so exactly adapted to its food supply and to the various drains upon it that the species preyed upon should normally produce an excess su icient for the species it supports And this statement evidently applies throughout the entire scale of being. Among all orders of plants and animals, the ideal balance of Nature is one promotiye of the highest good of all the species. In this ideal state, towards which Nature seems continually striving, every food-producing species of plant or animal would grow and multiply at a rate suf- ficient to furnish the required amount of food, and every (lepredating species would reproduce at a rate no higher than just sufficient to apprOpriate the food thus furnished. We must now point out how this common interest is naturally subseryed—how the mutually beneficialbalance between anin1 ale and their food is ordinarily maintained. Exact adjustment is doubtless never reached anywhere even for a single year. It is usually closely approached in primitive nature, but the chances are practically infinite against its becoming really complete, and maladjustment in some degree is therefore the general rule. All species must oscillate more or less. Even the more stable features of the organic environment are too unstable to allow the establishment of any perfectly uniform habit of growth and increase in any species. The most unvarying species will at one time crowd its boundaries vigorously, and at another, sensibly recede from them. That such an oscilla- tion is injurious to a species may be briefly shown. The most favorable condition of a species is that in which its numbers are maintained at the highest possible average limit; and this, as already demonstrated, requires that its food supplies should likewise be maintained at the highest possible lin1it—that the species should in fact, confine its appropriations to the unessential surplus of its food But when the numbels of an oscillating species are above this average limit, it will devour moreEthan this surplus of its food—its food supplies will be directly lessened. 0n the other hand, when the oscillating species falls below'this limit, its food supplies, reacting, of course cannot in— On Some Interactions of Orgeaisms. 9 crease beyond the highest possible limit, but will reach it and there stop. The average amount of food will there— fore be less than it might be if the species dependent upon it did not oscillateh—and, the food being less, the average number of the species itself must be smaller. Our problem is, therefore, to determine how these innumerable small oscillations, due to imperfect adjustment, are usually kept within bounds—to discover the forces and laws which tend to prevent either inordinate increase or de- crease of any species, and also those by which widely os- cillating species are brought into subjection and reduced to a condition of prosperous uniformity. “Te may know in general that such laws and forces are constantly at onrk, and that the tendency of things is towards this healthful equilibrium, because we see substantially such an equilibrium widely established and steadily main- tained through long periods of time, notwithstanding the great number and kaleidoscopic variability of the forces by which each species is impressed. But this idea will repay more detailed elucidation. We will notice, first, some of the checks upon injurious oscillations arising out of the laws of the individual organism, and afterwards those which are brought to bear upon it from without. It will at once be seen that, in any case, the maladjust- ments possible are of only two kinds—the rate of repro- duction in the species must be either relatively too small or relatively too great. If it be relatively too small, if the species bring forth fewer young than could mature, on the average, under existing circumstances, whatever may be the oscillations arising", they will tend to disappear with the disappearance of the species. The excrane numbels of such a species being in the most far01ableb event, less than they might be it will be at a eeltain disadvantage1n the general Dstruggle fo1 existence—it will eventually yield to some more prolific species with which it comes in competition. If for any reason its rate of multiplication be or become too high, the law of the antagonism between individuation and genesis will constantly tend to bring it within the proper limit. Reproduction being Ino1e active than1s necessary, the individual force and activity will be less than it might be—-the species will be at a disad- 10 On Some Interactions of Organisms. vantage in the search for food, and in all its other activi- ties, as compared with other species more exactly adjust- ed, or, as compared with members of its own species which tend to a better adjustment. As soon as a better— adjusted competitor appears, the other must begin to safe fer, and in the long course of evolution will almost cer— tainly disappear. The fact of survival is therefore usual— ly sufiicient evidence of a fairly complete adjustment of the rate of production to the drains upon the species. For the sake of illustration, let us take an instance—-- and the most difficult we can find for the application of these ideas—the case of a caterpillar and its hymenopu terous parasite. If the rate of increase of the parasite be relatively too great, that is, if more parasites are produced than can find places of deposit for their eggs in the bodies of the mere escess of caterpillars, some of them will deposit their eggs in caterpillars which would otherwise come to maturity—that is the number of caterpillars will be grad- ually diminished. With this diminution of their hosts the parasites will find it more and more difficult favorably to bestow all their eggs, and many of them will fail of devel— opment. The multiplication of the parasites will thus be checked, and their numbers will finally become so far re- duced that less than the then excess of caterpillars will be infested by them, in which case the caterpillars will corn— mence to increase in numbers, and so on indefinitely. Briefly, the excessive rate of increase of the parasite will. keep up an oscillation of numbers in both parasite and host which will cross and recross a certain average line. Let us now look at the method by which Nature may cheek this injurious fluctuation. Let us suppose two groups of a 1.1arasitie species at work on the same species of caterpillar, of which one (A) is distinguished by a tendency to an excessive reproductive rate, while the other (B) multiplies no faster than is con- sistent with the best interest of its best. A, producing- more eggs than B, must either parasitize more caterpil— lars than B, or must deposit a greater number of eggs in. each.- It cannot parasite more caterpillars than B, be- On Some Interactions of Organisms. 11 cause this would require greater activity—a higher in dividuation—and this is contrary to the law that individ— uation and genesis are antagonistic. Instead of being more active than B, it will then be less active, and will, therefore, deposit inore eggs in each caterpillar. B, how- ever, cannot have acquired the habit of depositing too few eggs in each caterpillar, as that would compel it to search habitually for a greater number of larwe than necessary -—to have acquired, that is, a habit of wasting energy— which is, as already said, contrary to evolution. A will“ therefore, sometimes deposit too many eggs in a caterpil- lar, and will then either lose the whole deposit, or bring forth a weakened offspring, which will, in the long run, give way to the more Vigorous progeny of B. This regu— lar production of a wasted excess will constitute an un— compensated drain upon variety A, which will end, like any other radical defect, in its yielding to its better—arl— justed rival. Or if, notwithstanding the foregoing, we suppose this excessive reproductive rate to have become fully estab— lished, then the parasite—ridden species will evidently labor under such a disadvantage in the struggle for exist- ence that it will probably be crowded out, in time, by some more fortunate rival. If the pair are permanently ill~ad— justed, so that permanent loss of numbers follows, they will be treated by the laws of natural selection as a single iinperfect animal—they will be pushed to the wall by some better—adjusted caterpillar and parasite, or by some insect free from troublesome companions. Vile may be sure, therefore, that, as a general rule, in the course of evolution, only those species have been able to survive whose parasites, if any, were not prolific enough sensibly to limit the numbers of their hosts for anylensth of time. We notice incidentally that it is thus made unlikely that an injurious species can be exterminated, can even be permanently lessened in numbers, by a parasite strict— ly dependent upon it—a conclusion which remarkably ”diminishes the economic role of parasitism. The same line of argument will, of course, apply, with slight modi- fications, to- any animal or even to any plant dependent 12 On Some Interactions of Organisms. upon any other animal or any other plant for existence. From the foregoing argument we conclude that, since the interest of a species of plant or animal and the inter- est of its “enemies” are identical, and since the opera- tions of natural selection tend constantly to bring about an adjustment of the species and its enemies which shall best promote this common interest, therefore the oasis hilatioa of all the establishcc “chemise” of a species 11:011Zd, as a role, how 110 efl'ect to tacrcase its final aver— age mtmbers. This being a general law, applying to all organisms, it is plain that the real and final limits of a species are the iaorgo-iiic features of its environment—H soil, climate, seasonal peculiarities, and the like. In treating of the external forces brought to bear upon an oscillating species to restrain its disastrous fluctua- tions, I shall mention only a part of the organic checks to which it is subject. It is a general truth, that those animals and plants are least likely to oscillate widely which are preyed upon by the greatest number of species, of the most varied habit. Then the occasional diminution of a single enemy will not greatly affect them, as any consequent excess of their own numbers will be largely cut down by their other ene— mics, and cs1.1ccially as, in most cases, the backward oscil— lations of one set of enemies will be neutralized by the forward oscillations of another set. But by the ooera— tions of natural selection, most animals are compelled to maintain a 1varied food habit—so that if one element fails others may be available. Thus each species preyed upon is likely to have a number of enemies, which will assist each other in keeping it properly in checli. Against the uprising of inordinate munbers of insects, commonl y harmless but capable of becoming temporarily injurious, the most valuable and reliable protection is un- doubtedly afforded by those predaceous birds and insects which eat a 1-1-tieterl food, so that in the absence or diminu- tion of any one element of their food, their own numbers are not seriously affected. Resorting, then, to other food supplies, they are found ready, on occasion, for imme— diate and oye1whelmina' attack against any threatening Oa Some Interactions of Organisms. 1:; ' foe. Especially does the wonderful locomotive power of birds, enabling them to escape scarcity in one region, ' which might otherwise decimate them, by simply passing to another more favorable one, without the loss of a life, fit them, above all other animals and agencies, to arrest disorder at the start—to head off aspiring and destruc- tive rebellion before it has had time fairly to make head. But we should not therefrom derive the general, but false and mischievous, notion that the indefinite multi- plication of either birds or predaceous insects is good. Too many of either is nearly or quite as harmful as too few. And this brings us to the application of these principles to the interests of civilized man. We must note how the new forces which he brings into the field expend them- selves among; those we have been studying, and to what reactions they are in turn subjected. We must r"rst see how far the primitive natural order of life lends itself to the supply of man’s needs, to the accomplishment of his purposes; and must determine, in a general way, where he may be content to leave it undisturbed, where he should address himself to its improvement, and where he is com" pelled to attempt wholly to set it aside, substituting arti- ficial arrangements of his own, devised solely in his own interest. Some of Nature’s arrangements man finds himself un- able'to improve upon for his own benefit. No one thinks of cultivating the forest to hasten the growth of the wood, or of tiimming the wild oak or the maple or of planting artificially the nuts and acorns in the woods to increase the number of the trees. * We are content to leave things there to go on essential- ly in the old way, merely anticipating the processes of natural death and decay by removing the trees before they spontaneously perish, and glad if the revolutions of or: ganic life which we set up in the country around do not penetrate to the forest, visiting the leaves and trunks of the trees with the scourge of excessive insect depredations. Usually, however, we find the ready-Inade system of 'N‘atare less to our' liking, and all our cultures are at- 14 On Some Interactions of Organisms. tempts to set it aside more or less completely. In the pas~ ture and meadow, it answers our purpose to substitute other species for the grasses growing there spontanebus- ly, and these adapt themselves easily to the circum- stances which have proved favorable to their native pre- decessors. But in the grain—field and fruit-garden the case is different. Not only do we bring in species often very unlike any aboriginal Vegetation and still further altered by long cultivation, but we propose an end quite different from that for whose accomplishment all the ar— rangements of Nature have been made. According to the settled order, the whole economy of every fully—established plant and animal is directed to the production of one more plant or animal to take the place of the first one when it parishes. All the excess of - growth and reproduction is a reward to friends or a trib— ute to powerful enemies, intended to make only this one end secure. But man is not content with this. He does not raise apple-trees for the sake of raising more apple—trees. He would cut off all excess not useful to himself, and all that is useful he would stimulate to the utmost, and apJ propriate to his own benefit. In carrying out this purpose he finds himself opposed and harassed at every step by rules and customs of the natural world established long ages before he was seen upon the earthfllaws certainly too powerful for him wholly to defy, customs too deeply rooted for him to overturn without the most complicated consequences. And yet even here, we see that the primi— tive order is not an evil, it is simply insufficient. It is good as far as it goes, and must be carefully respected in its essence, however far it may be modified in detail. We find abundant reason for a belief in its usual beneficence and for a reluctance to disturbit without urgent necessity. At the best the disturbances we must originate will be tremendous. Old combinations will necessarily be broken up and new ones entered into. As in a country undergo— ing a radical change in its form of government, disorders will almost certainly break out—some of them fearfully destructive and temporarily uncontrollable; but the gen- eral tendency towards a just equilibrium will make itself On Some Interactions of Organisms. 15 felt, and intelligent effort will mitigate some evils and avoid others. Without attempting to go into deatils— which would be quite unnecessary for my purp-ose-mI will. endeavor briefly to show the bearing of some of these ideas upon practical conduct. To man, as to nature at large, the question of adjust ment is of vast importance, since the eminently destruc— tive species are the widely oscillating ones. These insects which are well adjusted to their environments, organic and inorganic, are either harmless or inflict but moderate injury (our ordinary crickets and grasshoppers are er.— amples) ; while those that are imperfectly adjusted, whose numbers are, therefore, subject to wide fluctuations, like the Colorado grasshopper, the chinch-bug and the army— worm: are the enemies which we have reason to dread. Man should then especially address his efforts, first, to prevent any unnecessary disturbance of the settled order of the life of his region which will convert relatively sta- tionary species into widely oscillating ones; second, to destroy or render stationary all the oscillatine‘ species injurious to him; or, failing in this, to restrict their oscil— lations within the narrowest limits possible. For example, remembering that every species oscillates to some extent, and is held to relatively constant num— bers by the joint action of several restraining forces, we see that the removal or weakening of any check or barrier is sufficient to widen and intensify this dangerous oscilla— tion; may even convert a. perfectly harmless species into a frightful pest. Witness the maple bark-louse, which is so rare in natural forests as scarcely ever to be seen, lim* ited there as it is by its feeble locomotive power and the scattered situation of the trees it infests. With the niulti- plication and concentration of its food in towns, it has in- creased enormously, and if it has not done the gravest injury it is because the trees attacked by it are of com- paratively slight economic value, and because it has finally reached new limits which hem it in once more. ,We are therefore sure that the destruction of any spe- cies of insectivorous bird or predaceous insect is a thing to be done, if at all, only after the fullest acquaintance 16 0a Some Interactioris of Organisms. with the facts. The natural presumptions are nearly all in their favor. It is also certain that the species best worth preserving are the mixed feeders and not those of narrowly restricted dietary (parasites, for instance)— that while the destruction of the latter would cause in- jurious oscillations in the species affected by them, they afford a yery uncertain safeguard against the rise of such oscillations. In fact, their undue increase would be finally as dangerous as their diminution. Notwithstanding the strong presumption in favor of the natural system, when we remember that the purposes of man and what, for convenience’s sake, we may call the purposes of Nature do not fully harmonize, we find it in- credible that, acting intelligently, we should not be able to modify existing arrangements to our advantage—— especially since much of the progress of the race is due to such modifications made in the past. We should observe, in passing, that the principal gen— eral problem of economic biology is that of the discovery of the laws of oscillation in plants and animals, and of the methods of Nature for its prevention and control. For all this, evidently, the first, indispensable reouisite is a thorough knowledge of the natural order——an intent geatly conducted aetuml history survey. Without the . general knowledge which such a survey would give us, all our measures must be empirical, temporary, uncertain. and often dangerous. Next we must know the nature, extent, and most im— portant consequences of the disturbances of this order necessarily resulting from human interferenee—we must study the methods by which Nature reduces these dis- turbances, and learn how to second her efforts to our own best advantage. But far the most important general conclusion we have reached is a conviction of the general beneficence of Nature, a profound respect for the natural order, a belief that the part of wisdom is essentially that of practical conservatism in dealing with the system of things by which we are surrounded. ‘ 0% Some Interactions of Or‘qaaisms. 17 Summary. The argument and conclusions of this paper may be thus briefly reeapitulated:—— We find a mutual interdependence of organic groups and a modifiability of their habits, numbers, and distribu- tion which brings them under the control of man. We also see that, after the most violent disturbances of their internal relations, a favorable readjustment eventually occurs. Starting with the general laws of multiplication and natural selection, it is first observed that every spe cies of plant or animal dependent upon living organic food is interested to establish such a rate of reproduction as will, first, meet all the drains to which it is itself sub- jected, and still leave a sufficient progeny to maintain its own numbers, and, second, leave a sufficient supply of its own food—species to keep them undiminished, year after year. That is, we find that the interests of any destruc— tive plant or animal are identical with the interests of its food supply. This common interest of the. organismhnd its organic food is continually promoted by natural selection, by which those that unduly weaken the sources of their own support are eventually crowded out by others with a better-adjusted rate of increase; but, because of the im- mense number, variability, and complexity of the forces involved, a complete adjustment is never reached. Whether the rate of multiplication of the food—producing' species be relatively too great or relatively too small, the result is to cause an oscillation of numbers of both depre— dating species and its food. These oscillations of a spe cies are both directly and indirectly injurious to it: and tend, in various ways, to diminish the average of its num— - bers, especially by lessening the general average amount of the'food available for it. By the operations of natural selection, therefore, widely oscillating species, thus placed at a marked disadvantage as compared with more estable ones, are either eliminated, or else reduced to or— der more or less completely. They tend to become so adjusted to their food supplies as to appropriate only their surplus and excess. 18 On Some Interactions of organisms. Hence, as a general thing, the real limits of a species are not set by its organic environment, but by the inor— ganic; and the removal of the organic checks upon a species would not finally diminish its average numbers. Among the external checks upon the oscillations of spe— cies of insects, the most important are those predaceous insects and insectiyorous birds which eat a varied food, using most freely those elements of their dietary Which are, for the time being, most abundant. When we compare the results of the primitive natural order with the interests of man, we see that, with much coincidence, there is also considerable conflict. While the natural order is directed to the mere maintenance of the species, the necessities of a man usually require much more. They require that .the plant or animal should be urged to excessive and superfluous growth and increase, and that all the surplus, variously and widely distributed in nature, should now be appropriated to the supply of human wants. From the consequent human interferences with the established system of things, numerous disturb- 'aiic‘e;arise—J111any of them full of danger, others fruitful of positive eyil. Oscillations of species appear, not less injurious to man than to the plants and animals more directly involved. Indeed, most of the serious insect i11— juries, for example, are due to species whose injurious oscillations have resulted from changes of the organic balance initiated by man. To avoid or mitigate the evils likely to arise, and to adapt the life of his region more exactly to his purposes, 111an must study the natural order as' a whole, and must understand the i'listurbanccs to which it has been subject. Especially he must know the forces which tend to the re- duction of these disturbances and those which tend to perpetuate or aggravate them, in order that he may rein- force the first and weaken or divert the second. The main lesson of conduct taught us by these facts and rcasonines is that'of conservative action and ex- haustive inquiry. Reasoningr unwarranted by facts, and facts not correctly and sufficiently reasoned out, are equally worthless and dangerous for practical use.