Popular Science Monthly/Volume 51/July 1897/The Principle of Economy in Evolution


ONE of the many interesting things about evolution, oftener taken for granted than formally recognized, is the fact that the changes which everywhere accompany and constitute it have their rise in a simple excess of pressure in one direction over the pressure in another. For all movement, whether it be of simple or of complex matter, whether it be of an inorganic or an organic system, whether it involve will and conscious perception or not, is in every case and under every conceivable set of circumstances movement in a single mode—that is to say, movement in the direction of the least resistance, or from the direction of the greatest traction or stress.[1] If we look to the origin of the movement, we shall speak of acting as in the line of the greatest stress; if we consider the resistances in the presence of which movement is produced, we shall regard acting as in the direction of the least resistance. But, however we may describe it, the truth of the law is obvious, since it follows from the very nature of movement. For if a body be equally stressed from all directions it will not move, while if it be stressed differentially—in one direction more than in other directions—it will move in the line of, or away from, the greatest stress. Now, as all movement must take place in the presence of or against resistance, a body which moves in the line of the greatest stress necessarily moves in the direction of the least stress, and it is this movement in the direction of the least stress which we mean when we speak of movement in the direction of the least resistance.

We have next to note what is meant by the greatest stress. This is not necessarily a stress applied at a single moment in time or at a single point in space. The movement of a billiard ball, for example, may be determined for part of its course wholly by the blow given with the cue, but the cushions soon come into action, and thus the total course of the ball is decided, not solely by the cue, but by the cue and all subsequent stresses of the cushions and balls that happen to be struck. In like manner, the initial impulse is given to the cannon ball by the exploding gunpowder, yet this initial stress is immediately complicated with gravitative action; and when we say that such a ball moves in the line of the greatest stress, we mean not simply the direction originally given by the cannon, but the whole direction as determined by cannon, gravity, and atmosphere. The greatest stress determining the direction of movement, then, is a stress made up, not only of the initial stress, but also of all subsequent determinations encountered as resistances by the moving body in its course; and when we say that a body moves in the direction of the least resistance, we mean that its total movement is determined by the total of greatest stresses. It is true that a distinction may be made between the original impulse given to a body and the subsequent stress or stresses entailed upon it by its own movement, and due to contact with other bodies at rest or in motion. It is an active stress, for example, which gives the initial impulse to the billiard ball; it is a reactive stress by which the cushion deflects the ball from its original course. But this distinction is little more than formal; the whole of the stresses determining movement, however easy it may be to analyze them into parts, must be regarded in their totality; for if we have to account for movements that take place in time in their totality, we must consider the determinations to those movements in their totality.

The law of least resistance, as we may briefly call it, finds exemplification alike in the realm of life and in the world of inanimate things. Not only are all movements of masses and their parts—from the descent of a bowlder down the hillside to the revolutions of planets in their orbits; from the activities of gas molecules in a chemist's laboratory to the movements of cosmical aggregation out of which suns arise—due to a differential stress producing motion in the presence of resistance to that motion: the law is valid also for the activities of animals, since, if the molecular forces embodied in an organic system impel that system to move to particular ends in the interest of maintenance, it can not move to such ends save in the direction of the least resistance. Thus, if a stone thrown at a mark takes the shortest route, having regard to the whole of the influences which act upon it, so a pedestrian goes to his destination by the shortest way which the circumstances permit. A volume of steam finds exit from an overstrained boiler at the weakest point; so by the weakest point an animal escapes from its cage. As a river flows through its channel, determined to that path by the resistance which prevents deviation from it, so the traveler is held to the beaten track by the broken and difficult ground on each side of it. A bullet is diverted by some obstacle suddenly encountered; the root of a plant coils round the stone it meets; the railway engineer usually carries his line round an obstruction instead of through it; the secondary current of an induction coil avoids a journey of many miles by leaping through a flaw in the insulation; a dishonest pupil avoids work at examination by copying the replies of a fellow-student. The light wave makes its way, roughly speaking, spirally through ether; objects of large surface and slow descent, such as certain suitably shaped pieces of paper, descend through the atmosphere in a spiral path; a bubble of air ascends spirally through water; the plant climbs a tree by spiral windings; a horse mounting a steep ascent with a heavy load takes a zigzag or spiriform course; men ascend and descend by spiral stair-ways; water sinks through an orifice spirally, and the descent of a whirlpool is a spiral; boring instruments, such as gimlets, augers, corkscrews, have spiral blades. The hunter seeks particular animals at pools and watercourses which they frequent, as certain medusæ throng to water traversed by a beam of light because the illumination attracts thither small crustacea upon which they feed. Earthworms, in drawing leaves into their holes, seize the leaf at such a point as will permit its passage into the hole with the least amount of resistance; a man carrying a ladder on his shoulder through a crowded thoroughfare carefully regulates his movements so as to avoid collisions. Men escape from an invested city by utilizing the wind; the invested dandelion balloons its seed to a place where it can grow in safety. Certain organisms wear the garb of others in order to increase the ease of their existence; certain men mimic their fellows to the like end of diminishing resistance. As a mother disguises her child's medicine in sugar or sirup, so plants offer their seeds to animals in sweetly flavored fruits. Bees construct their combs in the form that secures the utmost capacity for storage with the smallest expenditure of building material and therefore of energy; so human builders attain in their constructions a maximum of needed effect with the lowest minimum expenditure of material and labor. A general carrying on war, a statesman conducting affairs of government, a merchant engaged in business negotiations, alike take the path which, having regard to the whole of the circumstances, offers the least amount of resistance to the attainment of the ends in view.

All inorganic and organic movements are therefore alike in the fact that each is due to a greatest stress, and takes place in the direction of a least resistance. It is true, of course, that a pedestrian does not rebound, like the billiard ball, from the resistances which he encounters in trying to find the easiest path through a forest or over the mountains; yet he consciously seeks the path of least resistance, and does so because he is diverted into it by the greater resistances of all other paths; these greater resistances become part of the greatest stress which determines the form of his movement, just as the reactive stress of the cushions forms part of the greatest stress that determines the path of the billiard ball. Inorganic and organic movements differ from each other simply in the fact that by living animals the path of least resistance is more or less consciously chosen, while in the inorganic world the path of least resistance is not chosen. And this unlikeness arises out of a more fundamental unlikeness still, from the fact that movement in the realm of the organic has end for its concomitant, though not necessarily conscious end, while in the motion of things inanimate end is wholly absent. Organic movements, that is to say, are all directed to some end, while in the realm of the inorganic, movements are simply unintelligent effects, results, or products of differential stress. In the form of organic movement, end plays a most important part, while in inorganic movement it has no part at all. Thus a pedestrian may find a circuitous route through a forest the easiest if his only end be to pass through it as quickly as possible; yet, should botanizing be his object, the form of his movement will be quite different, and may very well be the direction of greatest resistance, so far as physical obstacles to movement are concerned. In the case, moreover, of particular ends, numerous opportunities for the exercise of choice present themselves. The more direct path up a mountain is chosen in preference to the one less direct, yet, when the "easier" path is the more dangerous, the traveler takes the safer and more difficult passage. So the more efficient tool is preferred to the less perfect instrument; and so, out of numberless ways in which the ends of life are to be reached, men instinctively and consciously choose those which, by encountering the least possible resistances, involve the minimum expenditure of effort. In the case of organic movements, economy of energy is possible because of the presence of end, the existence of various ways of reaching it, and the possibility of choosing the one which involves the least expenditure of energy. In the case of inorganic movements there is no such economy, since those movements are mere effects, and comply unvaryingly with the laws of mechanics. Finally, the exertion of choice by an organism does not determine whether movement shall take place in the direction of the least resistance or not—for that is the inevitable mode of all movements, organic as well as inorganic—but whether the energy expended in the differential or greatest stress producing movement shall be a larger or a smaller quantity.

We have next to note that the economy of energy which is possible in organic movements has two forms. There is economy in the realm of the conscious will, exemplified in movements by which animals reach various ends; and there is an economy in the realm of the unconscious life of the organism by which the parts thereof rearrange themselves in such a way as to lessen the expenditure of effort in the work of maintenance. For, whenever function is imposed by the organism upon certain of its parts, such parts, moving into configurations of least resistance, set up the intelligent adaptations which we know as organs. The only difference between a tool and an organ is that the former has been consciously shaped by man, whereas the latter has arisen through the unconsciously effected rearrangements of living molecules upon which function has been imposed by the organism. All organs, like all tools, are paths of least resistance, ways of reaching ends of organic maintenance with a minimum expenditure of effort. Simultaneously, moreover, with the saving of energy spared through the gradual perfecting of organs, there goes on a gradual improvement of the ends which such organs are unconsciously produced to reach. For this is simply to say that all effort saved by an organism through increase of the efficiency of its organs and processes goes—the circumstances being favorable—to increase the complexity and delicacy of its relation to the environment, as well as to enlarge the scope of the activities of maintenance.

The way in which organic molecules move into configurations that offer the least resistance to their special activities may be seen in similar structural formations which are more or less unconsciously assumed by human beings. One of these is the habit of taking turn by people waiting, say, at the box office of a theatre—a configuration which is assumed more or less unconsciously, because it is the one which, under the whole of the circumstances, involves conditions of least resistance. There is a similar selection of conformations involving a maximum of ease in the manner in which pedestrians avoid collision with each other. The throng in movement on the crowded sideways of a great city divides itself naturally and without conscious deliberation into two streams going in contrary directions, each pursuing its particular course without the slightest resistance from the other, and to the manifest advantage, both in amount of energy expended and speed of movement, of every individual concerned.

The history of social and industrial ascent is, throughout, a record of the lessening of the resistance encountered in the attainment of human ends, as well as of the constant improvement of those ends. Social ascent not only diminishes resistance within the tribe, community, or nation; it everywhere lessens external aggression, substitutes mutual aid for the antagonisms of conflict, and enables men to devote energy spent in war to the pursuits of peace. Step by step with this lessening of resistance by the reduction of conflict, there goes on within the social body, and between the group of social bodies, those cooperative movements which, by tending to unify men, gradually bring to the aid of the individual the whole power of the social organism. In the beginning there is little industrial co-operation: each man is his own agriculturist, hunter, tailor, shoemaker, and soldier—each, that is to say, discharges for himself the work which individual maintenance involves. But little by little men discover the superior ease of mutual aid, and as they learn the value of the division of labor, the function of maintenance, originally exercised almost wholly by each individual for himself, comes more and more to be distributed among sets of individuals specially differentiated for the tasks allotted to them, and finally there arise those wider interdependencies of industrial and commercial co-operation that bind the inhabitants of almost every clime under the sun in bonds of mutual indebtedness. That the whole of this movement is a movement of constantly increasing economy of energy in the reaching of special ends, and of constant ascent in the scope and perfection of those ends, will be evident when we remember that the lower we go down in the scale of human existence—to the stages where coacting is least developed—the more rudely and imperfectly are the ends of maintenance reached, and the more completely is the time of the organism exhausted in attaining them, while the higher we look in co-operation the more efficiently are those ends accomplished, and the less time is taken up in their performance.

The way in which labor is reduced and end perfected, both as to the quality of the work and the time in which it is performed, has been shown in many familiar examples of co-operative acting. The advantages of giving particular tasks to specialized sets of workmen in such processes as those of coining and pin-making is well known. The gradual improvement of tools—which are really means to the attainment of the ends of the individual and of the community of individuals, and must therefore share in the movement of ascent in which those individuals are engaged—yields in its every detail an illustration of the mode of all movement. At first, tools were of the rudest kind, and men reached their ends with labor enormous compared with that needed for the attainment of the same ends to-day; but in proportion as they acquired knowledge of the external world, of the properties of things, of how things act and may be acted upon, and of the means and methods by which desired results may be brought about—in proportion, moreover, as human need, widening and becoming more varied with human ascent, made demand for a larger number and a greater variety of implements—in such proportion did men perfect, not only their tools, but also the ends possible of attainment therewith. To the implements, moreover, once used only by individuals, there have been added the tools called into service as social appliances by groups of men, and finally by the whole community. Thus the progress of tools has been an ascent, not only from the sandals of rawhide to the shoe of civilized races, from the knife of stone to the modern blade of steel, from the sticks rubbed together to the lucifer match, from the sling to the rifle, from the bone needle to the sewing machine, and from the gnomon and the clepsydra to the timepiece—it has also meant the gradual development of such social mechanisms as steamboats, railways, street cars, post offices, telegraphs, and the like. Finally, all such improvement, whether of the individual or the social appliance, has been, from first to last, progress in the economy of the labor needed for particular ends and perfection of the ends themselves.

Illustrations of the law of least resistance may also be drawn from the realm of mind. The need of economizing energy in thought is one which, however conscious or unconscious we may be of it, dominates and directs, so to speak, all our mental activities. This is suggested by the familiar antithesis between breadth and profundity of acquirements—by the fact that artistic genius is usually divorced from depth of intellect, that speculative ability is rarely associated with knowledge of the world, that the thinker who is deeply versed in general principles is almost never a specialist, that the poet is only phenomenally a man of affairs, and that the power to think originally and philosophically and the power to excel in the graces of literary style are rarely allied in one and the same individual, or present in any individual at one and the same moment. In a general way, we can concentrate the mind, so to speak, upon any particular object only by abstracting it from all other objects; our attention to a speaker, or a book, ebbs and flows according to the interest we take in particular passages; more than half the familiar activities of our daily life are performed without any attention to them which can properly be called conscious. We are constantly, on the one hand, reserving voluntary effort for the less habitual processes and activities, and on the other committing such processes and activities, to the extent that they become habitual, to the realm of the subconscious.

What is true of our bodily activities is equally true of the mental processes through which we form judgments and reach conclusions. To men in the mass, partial aspects of reality are easier to seize than complete verities; they find "concrete facts" more comprehensible than general principles; the gently undulating slopes of belief offer them a less arduous path than that which leads over the steep cliffs of knowledge; for most of them, the rosy streamers that herald morning are more beautiful than the full lights of day:

L'homme est de glace aux vérités—
II est de feu pour les mensonges!

Hence it is that in their earlier thoughts men explain the invisible parts of the external world in terms of the parts visible to them; that they confound the object with the garb woven for it by the subject; that they conceive anthropopathically of things and activities in the external world and that most of their ideas of the universe and of its parts presuppose the human organism as the source of the analogies which alone make them intelligible. There can be thus no theory of the universe, however crude, and no religious belief, however barbarous, which may not find its justification in the fact that, for a particular stage of mental ascent, it is an expression of the law of least resistance. If, moreover, the beliefs and theories of individuals and races, at first of the simplest kind, become more complex as men ascend in mental power and knowledge; and, if, as the spheres of feeling and knowing draw near to one another, each grows richer in content until in both the mind is enabled to range in a world of ideas inaccessible to man on a lower plane of development—these results are reached in every stage of the progress they constitute not only by the saving of energy through the improvement of mental operations, but also by the enlarging and perfection of the ends compassed by those operations.

The history of the concept is itself full of evidence to the same effect. In the early stage of mental development, men attach high validity to appearances, and thus form concepts which connect things only on the basis of their superficial likenesses and differences; the stage is one in which, while there are terms for the members of a class, those descriptive of the class are either very imperfect or do not exist at all—one in which, for example, there are names for particular trees, particular plants, particular animals, but no general name for tree, for plant, or for animal. Not only are objects imperfectly known in the absence of the power to form these general concepts; it is impossible to think of them in their proper relations to one another, and thus there is at once imperfect knowledge of the external world on the one hand, and on the other, through lack of the bond of likenesses between classes, that comparative slowness of mental processes which must have been a character of all early thought. The more imperfect, in fact, are the links of likeness which binds concepts together, the more the mind tends to resemble the confusion of an unclassified library, where the needed volume can be obtained only by great expenditure of time and effort; the more complete is mental segregation, the more the mind may be said to resemble the same library properly classified. Ascent, therefore, from the knowing of things by their superficial characters to knowledge of them also in their fundamental characters enormously increases, not only the ends reached by thought, but also the ease and rapidity of mental operations.

There is another economy to be noted in mental operations—the economy wrought by the increasing content and the growing symbolism of the concept. The name first given to any object simply expresses the most prominent out of a very small number of qualities by which we know that object. In onomatopoetic words, for example, the quality perceived and named is one of sound, and the process gives rise to such terms as kolokol, the Russian word for "bell"; gunguma, the Gallas name for "drum"; kwalalkwalal, used for "bell" by the natives of Yakama (North America); tumtum, also a Gallas word, meaning "workman," or, more literally, "hammerer"; krakra, the name of a Dahoman watchman's rattle; chacha, the Aino word for "to saw"; the Peruvian ccaccaccahay, signifying "thunderstorm"; the Australian bungbungween, used for "thunder"; hou-hou-hou-gitcha, the Botocudo word for "to suck"; kakakkaka, which in Dyak means "to go on laughing loudly"; shiriushiriukanni, used by the Ainos in the sense of "a rasp"; and the Quichua chiuiuiuinichi, indicating the noise made by the wind among trees. At first, that is to say, the name means no more than the most prominent character, and perhaps the only known character, of the object to which it is applied, whether that character be one of sound, of acting, or of appearance; but, as men come to learn more of the qualities and relations of such object, the name gradually loses its descriptive value, and becomes a mere symbol or word counter for the total content of the concept. Thus, "the Russian called the duck utka because he saw it plunge its beak into the water; the Pole called it kaczka, because he noticed that it waddled in walking; the Bosnian gave it the name of plovka, because he saw it swimming"; yet in their survival none of these terms for the duck retain or even suggest the character which originally gave rise to them—they imply the duck in all its characters and activities. It is for like reason that the various symbol values of a vast number of terms in our own language have gradually emerged from their original meaning as words descriptive of a single quality of the thing named—sheep from "bleater," its original meaning; dawn from "shine," pig from "grunter," or "the maker of the su sound," mortality from "a wasting away," mother from "fashioner," sky from "cover," mouse from "stealer," ant from "swarmer," bird from "upstriver," father from "nourisher" or "protector," ground from "the trodden," foot from "treader," woman from "bearer" (gune), "soft one" (mulier), or "the suckler" (femina), night from "the blind" or "dangerous" earth from "the dry" (terra), house from the "built," horse from "the neigher," picture from "scratching," stars from "strewn," fetters from "footers," fingers from "seizers" (Fänger), language from "tongue," imply from "folding in," apprehend from "taking hold of," develop from "unwrap." The gain of the process is obviously this—that the mind, instead of describing a single quality by its name—instead of having to deal with all the qualities separately—is enabled to include in a single concept all the characters which the thing named is known to possess, and to bring such concept into true relation with other concepts equally rich in the number of qualities which they connote. That the economy thus attained is no small one—that it means enlargement and perfection of end as well as saving of energy—may be realized by remembering the enormous increase which has taken place even in recent years in the meaning of such simple terms, for example, as stone and star. "Stone," to the uncultured man, is merely a hard substance of a particular color, size, shape, and weight; to the geologist the concept "stone" has a rich content of both chemical and physical characters, and demands for its thorough comprehension a familiarity with the whole history of the planet. So to the ignorant man "stars" are little more than

specks of tinsel fixed in heaven,
To light the midnights of his native town;

while to the educated, and above all to the scientific mind, the concept is rich with thoughts of cosmic processes and solar evolution, and has a content of materials drawn from well-nigh every department of knowledge.

Economy in language (which throughout implies economy in mental processes) is probably shown as much by that which escapes as by that which attains to expression in speech. Words are brought into use only to describe things, actions, and relations that are of habitual or frequent occurrence. A vast number of phenomena are left unnamed for the reason that they do not recur with sufficient frequency to demand formal attention for the social purposes of language. Thus, if only one railway collision had ever occurred, the word "telescoped" would never have been invented; so a single case of "marauding" or of "boycotting" would have been totally insufficient to bring into existence these now familiar terms. It is because most emotional states are too complex ever to recur a second time in the same form and sequence that they can never become fixed by language, and that the feelings excited by the sight of a beautiful landscape, or an Alpine range, may be but imperfectly suggested only by the multitudinous epithets of a poem, and need a new poem to suggest them every time they are felt. The naming faculty is in fact called into action only as impressions emerge into familiarity: for the changing complex of the activities and relations that never recur twice in the same way, and often never recur at all, the mind has no process of classification, and therefore no concepts that can be named.

Uttered speech is full of the signs of this ever-present striving after economy. Observe the constant omission of particles and words whenever intelligibility is to be attained without them. Where gestures will suffice to convey our meaning—a beck of the hand, it may be, or a shrug of the shoulders—we do not need speech, or, when we do, a "Pooh-pooh!" a "Mind!" or a "Beware!" will often answer all our purposes. We say "in French" for "in the French language"; "Thanks!" for "I thank you"; "Herein!" for "Kommen Sie herein!" In phrases like "I go to-morrow, not you," "Ni l'or ni la grandeur ne nous rendent heureux," "Dove ci è despotismo, non ci è virtù" (Gaetano Filangieri), "Er war ärmlich, aber doch sauber gekleidet," "Me ipsum ames oportet, non mea" (Cicero), we habitually omit words formally necessary to the sentence, but not needed to convey its meaning. As, moreover, words are dropped from phrases, so letters are dropped from words. When there is no literature to stereotype a form, as in the case of the native American languages, degeneration by process of syncope sets in rapidly; it is not delayed long even for classic tongues, like Greek and Latin, or for their successors of the Romance family, on all of which phonetic decay has set its mark; while all literary tongues, ancient and modern, display the process in their colloquial forms. Thus the process which turned anima into áme, femina into femme, and punctum into point, which converted the earlier Latin ad diem into the later Latin of ad die, and in Italian shortened de ab illo monte into dal monte, has its analogue in the Bas-Valais peasant's contraction of génisse into j'ni and éteindre into tède; in the Berlin workman's conversion of "Ich" into "I'"; in the English reduction of "I love-did" to "I loved," "boatswain" to "bos'n," "God be with, you!" to "Good-by!" and in the slang which in portions of the United States has begun to dwarf "How do you do?" into "Howdy?"

There is abundant scope for economy in all the forms of literary expression. Not only do we avoid as far as possible redundant elements, we also choose words calculated to convey our meaning with the minimum of effort on the part of the reader or listener. Where our end is simply that of intelligibility, as in the case of scientific statement, we choose words as simple and as expressive as possible; where to the end of intelligibility are added the ends of style, we employ words more ornate and picturesque in their character. In most prose compositions we are satisfied if we succeed in conveying our meaning; in most poetical compositions we seek, in addition to the end of intelligibility, to produce emotional excitement, to call the imagination into powerful activity, and to give rise to various pleasing effects, such as those of rhyme and alliteration. But whatever are our ends in composition, and however multifarious they may be, we always strive to reach them in the completest way and with the least possible demand upon the attention of the persons whom we are addressing. The sparing use of metaphor and parenthesis, the placing of the stronger epithets after instead of before the weaker, the avoidance of long and involved sentences, the care taken not to repeat words already used instead of their synonyms, the provision for variety which excludes monotony both of thought and of style, the observance of a best arrangement for the words in a sentence, the choice of particular material for the various paragraphs of a composition, and the construction of the links by which unity is secured for the whole treatment—all this is ordered, as far as is possible in each individual case, so as to produce a maximum of effect with a minimum of material.

How intolerant men are of speech elements unnecessary to intelligibility is shown by the drift of the educated and uneducated alike toward a phonetic spelling by the gradual doing away with inversion in both word and sentence, and by the growing tendency to use adjectives as adverbs, to discard subtleties like the subjunctive, and break down the well-established distinction between "shall" and "will." The economy which has taken place in the domain of grammatical forms is shown both in their gradual acquirement as means to the increased intelligibility of speech, and in the haste with which the mind, no longer needing them, hastened to discard the scaffolding of the structure which with their aid it had built up. The enormous gain which has been secured by the dropping of inflection may be appreciated somewhat by reference to the clumsy paraphernalia of such undeveloped languages as Zulu, in which, as translated by Dr. Bleek, the simple sentence, "Our great kingdom appears; we love it," must be expressed as, "The kingdom our dom, which dom is the great dom, the dom appears, we love the dom" (U-bu-kosi b-etu o-bu-kulu bu-ya-bonakala si-bu-tanda). So the saving attained in such a language as English may be easily inferred from the wild luxuriance in analytic distinctions of all tongues in an early stage of development.

It should be added that the gain which comes of the gradual rejection of inflection is a gain not merely in the domain of language alone, it is throughout made possible by mental ascent, and the whole of the progress which it implies is a progress not only in the saving of labor in the intercourse between men, but also in the enlargement and perfection of the ends of that intercourse.

We now return from this brief and highly incomplete account of the various forms of acting to consider the application of our principle to the case of the organic system. That principle admitted, it will be at once obvious that the law of least resistance must be true of all those rearrangements and activities which are imposed upon a living protoplasmic system in the interest of maintenance. If, in other words, such aggregate be impelled by the forces inherent in organic molecules to maintain itself, the various means by which it will maintain itself will be means such as, from the minutest detail of structural rearrangement to the highest organ and process, are best adapted under the whole of the circumstances to accomplish the end of maintenance with the minimum expenditure of energy, and this for the reason that only such means can arise by movement in the direction of the least resistance. It also follows, from the inevitableness of the law and from the character of the organic aggregate as a system of parts, that the means by which maintenance is carried on by such aggregate will undergo progressive improvement, and will therefore illustrate the same gradual advance in the economizing of energy and the perfecting of end as those which are exemplified in the ascent of the human community.

The obvious analogy between the parts of an organic system and the individuals constituting a human society is completely borne out on examination. Whether, in fact, the primitive organic aggregate be viewed as a union of previously separated units, or as an organic mass divided into unit parts that are first likes to each other and only finally differentiated, or as an aggregate that undergoes differentiation of its parts the moment it is sufficiently advanced in complexity to possess organic character, the fact remains that the parts can not constitute an organic system without aiding each other in the work of maintenance. Even if we could regard them as independent of each other, though associated, we should be compelled to say that, acting in accordance with the law of least resistance, they would find it easier to divide that work among their own number than for each to maintain itself apart from the rest. Yet the reality is even stronger than this: since the parts are interdependent, must each act in the interest of the whole of them, and are each by that whole dominated, so to speak, into co-operation with one another for the ends of maintenance. Just in proportion, moreover, as special activities are imposed upon special parts, in that degree are such parts differentiated for the tasks they must perform; special centers and organs arise connecting the various processes with one another, until finally the whole unified system is an aggregate of co-operating but subordinated individualities, of which each is in the service of all, and all act in the interest of each—an aggregate, that is to say, in which each of the parts, instead of having to carry on itself all the activities of maintenance, obtains in exchange for its own small contribution to the general labor the services and power of the whole society. In other words, the parts of such a system, impelled to the activities of maintenance, move into those configurations in which self-maintenance is the easiest and completest for all of them, and do so by a process of gradual adaptation and interadaptation, every stage of which is a stage of increasing efficiency of end and of greater economy of energy in the reaching of that end.

The progressive unification of men in the human society also has its analogy in the progressive unification of the organic system. In the lower planes of life lack of complete solidarity between all the parts and processes of an organism often manifests itself in the well-known phenomenon of iterated organs. The system in this stage consists, so to speak, of groups or segments, and every segment has its special set of organs—such, for example, as the legs of the centiped and the lobster, the multiple breathing holes of insects, and in a variety of organisms the iterated eyes or ocelli, as well as the repeated nerve centers of many of the lower forms. As the organism becomes unified this phenomenon of iteration tends to pass away, and the change is wrought through what may be called the discovery by the organism that it is easier to produce and maintain a single set of organs of each kind for the body as a whole than to produce and maintain and use a separate set of such organs for each segment or group. Hence the ascent of the organism from the stage of iterated organs to the stage of single sets of organs, from the condition of imperfect to the condition of perfect unification, is ascent by diminution of resistance, by perfection of end, by greater economy of energy.

As, moreover, the improvement of tools is a saving of energy to the individual wielding them, so is the improvement of an organ to the system which needs and has produced it for ends of maintenance. In the degree that the organic parts have special activities imposed upon them, in that degree do they become modified by those activities, and therefore adapted to the doing of those activities. An incipient leg, tail, fin, or eye, or any other organ, impelled to a particular thing, to act in a particular way, will do that thing more perfectly, will act in that way more completely and efficiently, with every repetition of the acting, for the reason that the parts of the organ and of the organism become with every such repetition, up to a certain natural limit, more and more adapted to the doing of that particular thing, to acting in that particular way; and this is why use is said to improve organs. The parts of such a system rearrange themselves in such a way as in every case continually to lessen the resistance offered within the system to the acting needed for each particular end. Just as from the simple foot of the snail to the leg of the vertebrates, so from the membrane of the worm sensitive to light, from the ocelli of insects and marine organisms to the highly developed eye of mammals, or from the incipient forms of internal organs to the more perfect and efficient forms of such organs, there have been progressive stages of ascent in the economy of energy with which given ends have been reached, as well as improvement of the ends themselves. In the case of organs, as in that of tools, the improvement has been made possible by a finer sense on the parts of the organism acting of the direction of least resistance, a finer self-adaptation by that organism to the environment, and a more perfect reaching of more perfect ends as the result of that adaptation.

We now see that the advantage gained by the perfection of any given organ or appliance necessary to maintenance is the advantage which, given the end to be reached, is gained by the saving of energy in the reaching of that end—that, in other words, the inducement to the improvement of any given organ is the saving of the energy spent in reaching, with the aid of that organ, the general end of maintenance. The more perfect are the appliances of the organic system, the more easily and completely does that system reach its end of maintenance; hence the gradual improvement of the organs with which maintenance is accomplished is so much movement in the direction of the least resistance. Thus the eye is gradually perfected in successive organisms, not because there is anywhere any foreknowledge that a given configuration of parts will lead to so highly useful an appliance as the organ of vision, but because, given the impulsion to maintenance and the general conditions of organic life, all structural changes leading away from the development of an organ like the eye would involve loss of energy to the organism in the reaching of the general end, and because all rearrangements of the organic parts that lead directly to the development of the eye are favored, as against rearrangements tending in any other direction, by the fact that every successive stage of such rearrangements results in a saving of energy in the reaching of maintenance to the organism bringing them about. In a word, the path of structural movement toward the eye is the easiest path, the path of least resistance, while the path away from the eye is the most difficult path, the path of greatest resistance; and what is true of the eye is true of all other organs and organic appliances whatsoever. Given, therefore, the molecular forces which in some way not yet understood impel the organism to display those activities of maintenance which we call life, and there follow, by virtue of those forces, of the character of organic matter, and of the general conditions of existence, not only the intelligent adaptations which make possible and facilitate maintenance, but also the gradual improvement of those adaptations which constitutes organic ascent.

What, finally, is the outcome? In the biological world at the present moment the great question which interests inquirers is that of the meaning of intelligent adaptations. Thinkers in this field no longer question the existence of intelligence in the unconscious form; they seek to discover what that intelligence means. "What we should like to discover," says one of them in a letter to the writer, "is the seat of the so-called unconscious intelligence which brings about those structures which the older teleologists called designed." That natural selection supplies little if any material for an answer to the question is already recognized. It being impossible to trace these structures to an artificer operating outside, our only recourse is to look to the organism itself for the power to which the fashioning of tissues and organs is due. And though we can do nothing toward solving the fundamental problem in biology, the origin of life itself, we need not despair—given the fact of life—taking the powers of living protoplasm for granted, of comprehending something of the process by which intelligent adaptations arise. For, the rest being assumed, we see how from the operation of the law of least resistance all the mechanisms of life result by necessity. Writ minutely in the tissues of the organism the law is inscribed broadly and grandly on all the features of our modern civilization. Not an activity of the busy industrial life around us, whether it be due directly to travail of brain or hand, or find its realization in that wonderful, external side of human life—the life of machinery—but illustrates the universal mode in which all conscious intelligence reaches its end. And so also in the realm of the unconscious we have only to take for granted the powers of living protoplasm, and the simplicity as well as the exceeding beauty of the process by which intelligent adaptations come into existence flash upon us like a revelation. We look as with vision renewed upon the pine cone in the forest, upon the flower shining amid the expanse of green, upon the sudden lightning of the firefly, and the manifold hues of insect and bird. For, little as we have attended to them before save as the commonplaces of our knowledge, we now see that they are paths of least resistance objectively embodied—protoplasmic tools with which, in the silence of the unconscious world the organic system is slowly but surely reaching its ends. And as we ponder it becomes clear to us that the same system is at work in the making of tools and the fashioning of organs—that, though the one process is conscious, the other unconscious, they are deep down in the heart of them the expressions of but a single method. Everywhere we find the evidences of this likeness—in the awl of the shoemaker and the tool of the boring insect; the earth-trap of the native African and the pitfall of the ant lion; the web of the spider and the net of the fisherman; the digging stick of the Australian, the foot of the mole, and the spade of the navvy; in the single oar of the boatman and the sculling tail of the fish; the sticky tongue of the anteater and the slime pot of the human catcher of birds; in the kayak of the savage and the floating pupa skin of the gnat; the scale armor of the armadillo and the soldier's cuirass; in the climbing hooks of the tiger beetle, the claws of the bat, and the grappling irons used in naval warfare; on the one hand, in the pulley, screw, and wedge; in chisels used in stonecutting, gravers with which wood is carved, axes for felling trees; in screwdrivers, lifting jacks, Nasmyth hammers, battering rams; the cord and weight of the window sash, the wheels of carriages, and the rollers whereon heavy masses are moved from place to place; on the other hand, in the muscles, sinews, and joints of animals; in the wing of the bird, the paddle of the porpoise, the hand of man, the mandible of the ant, the horns of the cow, the lance of the swordfish, the stinging cells of certain cœlenterata, the channeled poison tooth of the snake, or the defensive antennæ of the spider; even in the vertebrate eye itself. For all these, being objective paths of least resistance, are signs of a law that, pervading the realm of living things, has its roots in the inorganic world, since it springs from the very nature of motion as a result of differential stress. And when adequate account is taken of the presence of end in organic activities, of its absence from movements which are inorganic—account, that is to say, of the fundamental difference between living protoplasm and inorganic matter—then the whole of evolution, viewed apart from its secondary processes, may be summed up in the simple formula—movement in the direction of least resistance.

  1. In order to save repetition, the word "stress" will be used throughout in the sense of traction or stress."