Popular Science Monthly/Volume 48/February 1896/Imitation Among Atoms and Organisms



DURING the past dozen years scientific writers, American as well as European, have given a certain amount of attention to the part played in human life by imitation, with especial reference to the conditions under which children acquire from parents and associates the salient characters of individual and social habit. But the examples drawn upon for illustration have been narrow in their range, while the analogous process of assimilation among the lower animals and in the realm of the inorganic has received but scant recognition. It is proposed in the present article to connect the three classes of phenomena by formulating a general law which may serve, provisionally at any rate, to cover them all, and then to group the phenomena in their various natural divisions.

Without taking note of its unimportant and obvious qualifications, the statement may be made that all things free to move, capable of becoming closely associated, and impelled to movement by the system to which they belong, tend to come together when they are likes, and to be separated when they are unlikes. If we regard this tendency from the point of view of the movement, we shall say that likeness of things involves association of them in the degree of their likeness, and that unlikeness of them involves dissociation of them in the degree of their unlikeness; while, if we regard the tendency from the point of view of the things themselves, we shall say that association of things involves likeness of them in the degree of the association, while dissociation of things involves or implies unlikeness of them in the degree of the dissociation. These truths may be expressed in a more general way by saying that in systems the association of likes involves the least degree of resistance among them, while the association of unlikes involves the utmost degree of resistance. It follows that where unlikes are associated, the resistance offered to their union will tend (1), where they are forcibly held in association, to make them likes; and (2) where they are free to move, to dissociate them. This law of assimilation, as we may briefly call it, finds illustration over so vast a field that it may fairly be described as a universal character.

If we begin, then, with the most complex of all the phenomena known to us, our first illustrations will be drawn from the realm of mind. The fact that cognition is a process of the association of like and the dissociation of unlike impressions, and the further fact that all the activities of thought, from reasoning of the lowest to reasoning of the highest kind, involve the association of like and dissociation of unlike elements—these are psychological truths of the utmost certainty. At the outset of all knowing is the indispensable condition that unless we can connect the thing perceived with some other things already known, and thus recognize our object as a like of those things, we can not know it at all, and it can not become a part of our store of mental experiences; while the very act by which we know it involves dissociation of it as an impression from all the impressions which it does not resemble. From this, the simplest form of knowing, to the most elaborate process of the reasoning faculty; from the recognition of single objects as like others to the recognition of classes of objects as like other classes; from the discovery of a causal relation between one set of objects or changes that is like the causal relation between another set of objects or changes to the discovery of the causal likeness connecting great groups of objects and activities, and finally all objects and changes whatsoever, there is throughout the same process at work—the process of the association of likes and the dissociation of unlikes—that conditions the mode of all our mental operations. So-called thought or reflection, for example, is simply the recovery into consciousness, for the purposes of knowledge, of cognitions more or less simple, more or less complex; cognitions recoverable as images, symbols, or terms from the classes in which they have been first associated by the mind; and the sense of pleasure felt by a thinker in discovering analogies can spring only from satisfaction of the demand, even in mental processes, that likes shall be brought together and unlikes separated. Classification in all its forms, whether in the ordinary business of life, as a means to scientific investigation, or for the ends of philosophic thought, illustrates the same necessity: at first unlikes and likes are mingled indiscriminately, and at first the mind regards them, if roughly, as being alike; but a sense of unrest leads to further examination of the aggregated elements until, by a more conscious and complete knowledge of them, they are found to be likes and unlikes, and are associated and dissociated accordingly.

The elements of coherent systems, whether of thought or language, always have their relations facilitated by likeness and impeded by unlikeness; hence we find in such systems that it is resemblance which is the bond of their union. Just as views in science or philosophy, schools in literature or art, are related to each other by varying degrees of likeness, the whole forming a group through which each of the parts thereof is rendered intelligible, so concepts and the words representing them are interrelated by degrees of resemblance, and so every term we can use is really intelligible to us only through its connection—a connection of likeness more or less x>roximate, more or less remote—with all other terms whatsoever. This is true, moreover, not only of all words as they exist at a particular moment, and are in use for particular concepts, but also for all the forms through which particular words have passed in their structural development. We bring new words into existence by connecting them, through likeness either of sound or of form, with words already familiar to us; and such new words, when we meet with them, become intelligible to us largely because of the likeness which connects them with known and intelligible elements of speech. When, moreover, a word is unfamiliar, the mental system rejects it as long as it remains a stranger and an unlike; but as soon as the mind, insisting on assimilation, obtains the satisfaction it seeks by change of form, the modified term, no longer meeting with resistance, becomes one of the system of likes. It is the same kind of assimilation as that seen in the use of Shotover for Château Vert which leads races to spell any foreign word they adopt in accordance with the analogy of their own tongue, and children to construct such grotesque plural forms as "foots," "mouses," "goodest," and "bringed." That in nearly all languages the vowels within a word tend to be assimilated to one another is a phenomenon encountered very early in the study of linguistics. The use of metaphor, again, which is largely the mental assimilation of a thing more or less unknown to something much better known, is universal.

Passing from the allied realms of cognition and speech to the sphere of human relationships, we shall find that here also likes tend to be associated and unlikes dissociated; that the resistance is greatest where the association is of unlikes, and least where the association is of likes. At the outset we must understand that likeness or unlikeness between human beings is a likeness or unlikeness not merely in structure but also in manner of acting; not only in structure and acting, as these are popularly understood, but also in thinking, in belief not only, that is to say, in body, but also in mind—not only in permanent and fundamental characters, but also in temporary and superficial characters. If men were exactly alike in every respect, we should find the greatest ease of association between men and men generally, as distinguished from association of beings human and beings not human. Yet upon the fundamental characters in which all men are alike there are superposed by advancing social and industrial complexity those superficial characters in which, for such characters, the members of particular groups come to be, on the one hand, more alike one another than, for the same characters, they can be alike men in general; and on the other, more unlike each other than, for the same characters, they can be alike men in general. Hence, even among human beings fundamentally alike we shall find abundant scope for movements both of association and dissociation.

First note the superior ease, even pleasure, which characterizes the relations of likes temporarily or permanently associated, and observe how, in the very facility of such relations, we are entitled to see a satisfaction of the demand that likes shall be brought together. The illustrations naturally take a wide range. Men of the same vocation, for example, find intercourse with each other, for the scope and ends of such vocations, much easier than with those engaged in other vocations. It is from likeness within industrial groups that the camaraderie of the trades and professions draws its spirit. The pleasure, again, with which artists, musicians, scientific and literary men come together manifestly has its origin in likeness of common pursuits, tastes, and aspirations. That facility of association is at its highest between men of like politics, men of like faith, men of like aims in any field of activity, has long been proverbial. Societies and clubs of every kind constitute so many classes of likes within which the elements of resistance are at a minimum, and this not because each individual is always or even often wholly like his fellow-member, but because there is a resemblance between them on some one or more sides which, being permanent, at least for the time, and valid for the ends of the association, facilitates the intercourse of all who during that time and for those ends come into contact with each other. Even when men gather for only temporary and general purposes—such as for those of social enjoyment—we often hear of religion or politics being tabooed, and this is done obviously as a means of avoiding the resistances of unlikeness—of bringing forward only those sides of human nature on which the associated individuals are for the time being alike. Intercourse is also much easier within than without the limits of class; the poor find among the poor the greatest facility of relation with their fellow-beings; so the possessors of wealth are usually most at their ease in intercourse with the well-to-do. Who has not witnessed how quickly two abnormally stout men enter into confiding relations with one another; or how easily, between persons afflicted with a like disease or deformity, friendship is set up? Caste, group, and race prejudices of every kind, together with such distinctions as those implied in the names of aristocrat, commoner, bourgeois, official, proletariat, all imply and involve the coming together of likes and the separation of unlikes. In most countries there is still a strong though diminishing prejudice against alliances with foreigners; even among natives, marrying "below one's station" or "out of one's religion" meets with a certain amount of social as well as with much family resistance. Most people usually fall in love with those about their own age. Deaf-mutes almost invariably marry deaf-mutes.

Highly temporary likenesses are equally potent in promoting association. Their influence is seen in the pleasure we feel at the discovery in others of some character like to our own, even if that character be no more than a gleam of intelligence in the lower animals. We note it constantly in the unifying effect which the wearing of a common badge or uniform has upon large bodies of men brought together for special ends. So in moments of exaltation or depression, as in the excitement of conflict or intoxication; under the influence of music or during religious worship; the ordinary differences belonging to more permanent states lose their separative power, and men come into unwonted facility, even pleasure of relation with each other. It is supposed that even between deadly enemies chancing to meet at the dinner table there passes a flash of momentary reconciliation. It is certain that shipwrecked men do not quarrel over politics; that the starving are usually free from religious bigotry; and that in the presence of a general bereavement, the clamors of faction and the prejudices of class are silent. The power of a war and of the comradeship of battle to annihilate distinctions usually recognized in time of peace is notorious. In all these cases, and in all the cases which they represent, we see great importance suddenly given to one side of men's natures to some all-a,bsorbing feeling or experience in the presence of which the ease of association with likes becomes extended for the moment to each of the human beings by whom the same experience is shared, whether they be few or many; in other words, the unlikenesses which normally separate men into more or less narrow classes lose their importance in proportion as some suddenly developed feeling gives prominence to resemblances either wider in character or more strongly felt.

Observe next how unlikes tend to be separated from the human systems in which they appear, and to be associated, in the act of separation, according as they are likes to each other. From whatever causes the unlikenesses which differentiate men from each other take their rise, it is manifest that even in the most minute of them the law of likes and unlikes finds illustration. Perhaps the most common and familiar example of this simultaneous association and dissociation is afforded by the breaking up of a social gathering into minor groups of likes that do not easily coalesce. In all such cases the individuals who so naturally fall into pairs or groups move from associations that involve the greatest resistance into associations that involve the least resistance, while the movement itself (given the need of association) is manifestly the product of the stress of the greater resistance.

In what multifarious ways this stress is exerted may be easily shown. As a small group of men surrounded in battle by a larger force naturally come together and are compacted for defense by the stress of those who attack, so the individual members of classes or races despised, hated, or feared, when persecuted by the community in which they live, are in like manner thrust and welded together by the stress which they encounter. Individuals, again, who differ from their fellows to the extent of being unable or unwilling to work—to take part in those co-operative activities that are necessary for individual as well as social maintenance—meet with the resistances which failure in self-maintenance involves, and are by them, though nominally by society, thrust into hospitals or poor-houses; while men whose actions are unlike those of others beyond the amount of difference permitted by law are dissociated, either by exile or imprisonment, from the communities which they injure. When expulsion of the unlikes is not practicable, there is association of them as prisoners within the system. Savages, again, often abandon or destroy individuals deformed at birth or incapacitated from social duties by age; even a comparatively advanced people like the Spartans exposed their weaklings to death by starvation. That there is still a tendency to offer undue resistance to the unlikenesses of physical and mental deformity is shown by the treatment of lunatics and criminals even in the most civilized countries; not many years ago positively cruel, it is still in many cases culpably careless and inadequate. The treatment ordinarily given by human beings to the lower animals; offenses against the helplessness of children and women; the oppression of individuals by autocrats, by governments, by official tyrants abusing power intrusted to them by the people, and by capitalists taking unfair advantage of the economic condition of those whom they employ—these are all cases of stress directed against unlikes. So shortcomings of men of every kind veritable unlikenesses are in numberless ways made the goal of individual or social stress; even ridicule and slander are forms of aggression, based on the real or fancied discovery of fault, and therefore unlikeness in the person assailed. But the action here illustrated, so far as human beings are concerned, is not necessarily moral and not necessarily immoral. Men may be in advance of as well as behind their age—may be larger as well as smaller than their fellows; yet the social system offers the same resistance to the individual too greatly contemplating the good of his kind as to him who wantonly plots its harm.

We thus come to note how the social group or system tends constantly to the production of conditions of least resistance within itself—how, that is to say, its resistance to unlikeness acts as a stress compelling likeness among its units. The fact that profound differences between individuals are not to be eradicated by the social stress after their appearance is quite consistent with the power of that stress to assimilate human beings to each other in their more superficial and temporary characters. The tendency to do as others do is universally felt, no matter to what extent, in individual cases, it may or can be resisted. It appears both as imitation of the particular acts of particular persons and of the acts in which a number of persons are generically alike; as imitation not only by the one of the many, but also by the many of the one. It usually begins—for wholly voluntary actions, at any rate—in that interesting process by which people are assimilated to each other in their views and beliefs.

Submission of opinion, whether accompanied by imitative action or not, is clearly a path of least resistance—a way in which men avoid the difficulties of differing from the community in which they live. And when action is involved, as it usually is, the process shows us the enormous assimilative influence which it brings to bear upon human development. It may be said, indeed, that civilized human beings acquire their normal activities as such largely through the molding influence which the social community exerts upon them from their earliest years.

This stress impelling individuals to imitate society is well seen in industrial organization, and is thus obvious in the lowest as well as the highest stages of human development. The civilized human being who enters a savage community will be compelled to go half naked through very lack of any means of producing clothes to which he has been accustomed; he will be forced to hunt or fish for a living for the reason that there is yet no system of co-operative supply in existence; in the absence of those functionaries, he must become his own farmer, his own soldier, his own tailor and shoemaker, his own doctor, even his own priest. So a savage entering civilized society will be forced to wear clothes, partly by social intolerance for human nudity in public, and partly because of the facilities for obtaining clothes, with all which that implies; there will now be no game to pursue and no need to pursue it, since the man's food supplies will come from the agents of the industrial system; he will soon lose his primitive, many-sided industrial capacity, for he will have an army of stock-raisers, farmers, tailors, shoemakers, house-builders, and doctors at his service; while in time society will impose, in place of the powers which it abstracts, new functions and activities, assimilating him industrially to the units of which it is already composed. In both these cases, moreover, likeness will be enforced by the resistance offered to unlikeness, and not by any inherent superiority of one system or any inherent inferiority of another; for savages who have not yet learned to cooperate are as much held to their rude industrial methods by the stresses that impel to likeness as civilized men are held by such stresses to the vastly more complex and highly organized activities of the modern society.

The same is true of nonindustrial forms of social organization. As a community imposes its industrial methods upon its own and upon intruding members, whether it represent a high or a low stage of human society, so does a community, quite irrespective of the degree of its development, insist upon a certain likeness of habits and customs in the units of which it is composed. A civilized man forced to live among savages will find it impossible to avoid barbarous methods of living, just as a savage compelled to sojourn in a civilized community will inevitably adopt the manners of the race among whom his lot is cast. In all communities whatsoever, and under ordinary circumstances valid for the majority of men, it is vastly easier to imitate others in general characters than to differ from them in those characters, while the difficulty of differing becomes almost insuperable when the stresses tending to assimilate the individual to the sum of individuals are exerted directly by the group or community as a whole. All acts of race assimilation in history—many of them already accomplished, some of them still going on—all so-called civilizing processes, whether carried out by individuals or by peoples, and all proselytizing movements, by whomsoever conducted, are but so many illustrations of the general process. Such modifications, moreover, as have been wrought in the native races of India, and in the negro of the United States, by dominant populations in those countries, are going on within each nation, each city, and every social group, however large or small its dimensions may be. For if the traveler, when living in strange countries, finds it expedient, if only temporarily, to conform to the customs of the people he meets—such customs being imposed upon him by the very resistances which divergence from them entails—he none the less, on his return home, finds his actions in social intercourse determined by the same need of conforming to some larger or smaller group of which he may happen for the time to be a member. It is because of this "doing at Rome as Rome does" that social gatherings are said to succeed best and to be most enjoyable when the guests are all alike each other on certain social sides of human nature, or are willing to appear to be thus alike during the period of their association. The fact that the social code, as it is sometimes called, frowns upon the guest who would take more than his share of the attention or time of the company, and encourages the host to an equal distribution of his favors to all of them alike—this shows how thoroughly, even in the social circle, imitation of the group is the direction of greatest ease, and how it is the stress of the resistances offered to unlikeness by the group as a whole which impels the members of it to those acts of imitation by which they are more or less temporarily assimilated.

The very description, again, of costumes as de rigueur for certain special occasions contains a suggestion of the resistance which the social group opposes to unlikenesses in dress. The attacks sometimes made upon strangely attired persons in so highly conservative a country as China have had their parallels even in the highly progressive countries of Europe and America. A similar antagonism is manifested to nonconformity in social manners; and all formulæ of such manners—the etiquette at baptisms, weddings, and funerals; established methods of paying and receiving visits; prescriptions of what to do and what not to do at the dinner table—are simply means, among those who attach importance to these minutiæ, of avoiding the resistances that would inevitably be encountered were there many ways, instead of a generically common manner, of behaving on social occasions.

The resistance offered to unlikenesses among associated individuals is also announced in that universal human character, the passion for equality among men—the tendency, however vaguely or vividly it may be felt, to insist upon it that those with whom we come into contact shall, in as many respects as possible, be likes of ourselves. Jealousy of special privilege, with its spirit embodied in such phrases as "fair play," "start equal," "share and share alike," "a fair field and no favor," begins to manifest itself very early in the life of the individual: mere children, when associated, insist in multifarious ways on likeness of treatment; anything like favoritism in the distribution of gifts or the bestowal of attention, as well as all unfair advantage in games, they resent with surprising promptness and vigor, often also with indignation. In adults this jealousy of unequalness finds expression over a much wider field, and finally gives character to the whole social system. All rules governing the activities of large or small bodies of men, such as regulations in factories, hospitals and prisons, by-laws in cities, and general laws framed for communities and nations, are manifestly means of providing against unequal acting of individuals—of securing, that is to say, the like acting, under particular and specified circumstances, of all the individuals associated. The sense, early developed, in the individuals of a community that they ought to be equal before the law, with the restiveness which they manifest under all inequalities of its operation; the claim that men shall be politically as well as legally equal; and the demand always made, if not always satisfied, for universal suffrage, first for men, and then for women—at first with limitations of race, property, or sex, and finally without such limitations—all these explain the progress which the world has been making during the last fifty years toward an ideal democracy—toward a condition of ideal political equality between the units that make up a human society as the very condition and means of least resistance between those units.

But men do not remain satisfied with likenesses set up by statute law, with resemblances imposed by a governing body. They aim, more or less consciously, more or less outspokenly, at a common degree of social well-being for all the individuals of a community, and, rightly or wrongly, regard inequalities of wealth, even inequalities of industrial power, as imperfect stages of human development that are to be outgrown. Hence it is that men denounce monopolies, and declare abnormally large accumulations of wealth in private hands to be iniquitous, because inequitous; hence, too, it is that, in the common jealousy of privilege there is to be found every form of hostility to exceptional social power and rank, from the dislike in which it is merely envied to the anger by which it is openly attacked. Socialistic schemes of reform are really schemes for diminishing those inequalities which still offer resistance to the association of men—schemes that is to say, for making men in social, political, and legal relations, in powers, privileges, and responsibilities—more than ever before likes to each other. Even religions illustrate the progress from a belief and acquiescence in special privileges held out to a favored few to an expectation and demand for the salvation of all; for if the pagan had to content himself with the satisfaction of knowing death as the universal leveler, asking with Cicero (Tusculan Disputations) "Can what is necessary for all be a source of misery to one?" the Christian, who claims the equality of all men before God, looks forward to a life hereafter in which all earthly unlikenesses are to be removed.

We shall next note that when unlikes are forced to remain associated, such unlikes being kept together for longer or shorter periods, the resistances which arise operate as a power tending to assimilate them. This process of assimilation is manifested in all degrees of our intercourse with others. Men rarely come into contact with each other, even for special and temporary ends, without feeling the molding influence of more or less unlike habits, manners, opinions, and speech. Unless our ways of thinking, acting, and speaking are so firmly established as to be unchangeable, we can not long remain in the society of others without feeling that the differences which separate us from them are gradually being worn down, if not disappearing altogether: strenuous as our determination may be. Nature herself seeks to lessen resistance, and thus guides us insensibly into the path which offers the least. Thus it is (in indifferent things, at any rate) that we gain our opinions and beliefs from those about us, that we unconsciously acquire the gestures and mannerisms of relatives and friends—that, in a word, we come at last to be profoundly modified by the more permanent characters of our human surrounding. In speech alone the change wrought is often considerable. Few succeed in avoiding the use of colloquialisms constantly heard, and fewer still escape the insidious influence of phrases and idioms peculiar to districts and countries: newcomers may at first regard them as strange, even barbarous, yet in the end they employ the novel forms as frequently and as unconsciously as the native. The very features of human beings living in close association with each other are known to undergo assimilation. The fact, again, that jockeys, hostlers, and cowherds sometimes betray more or less distant resemblances to the animals with which they habitually associate is well known; and although animals do not in feature grow to be like human beings by contact with them, it is certain that in the character of intelligence, as in the case of the dog and the cat, as well as in tameness generally, as in the case of cattle and poultry, a real and profound assimilation of them to men has undoubtedly taken place.

Fashion in all its forms illustrates the same assimilative process. The first fashions are seen under the domestic roof: there, children imitate their parents not only in speech, gesture, and action, but also sometimes in opinion and bent of mind, and (for the sons, at any rate) occasionally also in vocation. Men imitate individuals as well as communities; they borrow from each other mannerisms of dress, of conduct, of opinion, and even of literary style, whenever attention is strongly called to any of these; even maladies have been known to become fashionable. A remarkable exploit in athletics usually sends a fever of emulation through the sympathetic members of a social group, just as a fashion set in literature, science, art, or philosophy inspires a thousand imitators. All composition, whether of prose or poetry, is in the ground of it imitative—a fact sufficently suggested by the grotesquely moral turn ordinarily given to college and school essays, by the accession of literary power which always follows much reading, and by the imitation of any particular author or style to which great liking for one or the other inevitably leads. What has been called the contagiousness of example is really the power of a strong impression, and therefore practically of stress, to produce likeness. This may be noticed at public meetings, where people applaud or cheer gregariously, and do other acts, such as those of sitting or standing, with obvious reference to what others are doing. The ease with which a stammerer communicates his defect to another is a matter of common observation. Movements or actions of persons sitting together, such as yawning, coughing, and the like, tend to be propagated more or less through the whole of them by unconscious imitation. To the same process is due the spread of more or less hysterical ailments among a company of persons, such as the often-recorded mania for mewing among nuns, or the propagation of convulsions among girls in factories. Simple movements, like the shifting of a chair, the rustling of a paper, audible change in the position of the body often follow involuntarily in others after they have occurred in one of the persons associated. If in a thoroughfare a man be encountered staring intently and conspicuously at some portion of the sky, most of those who see him will at once direct their gaze in the same direction. A man watching an athletic feat, or a stroke at billiards, in which he is deeply interested, will often at a critical moment imitate the attitudes or action of the performer.

The law is further abundantly illustrated by facts relating to the lower animals. These find the greatest ease of association as likes, and come together everywhere in Nature on the ground of likeness. The general evidence of this is familiar, and mention need be made here only of a representative example. In the Falkland Islands, for instance, where the cattle have run wild, and where they are of several different colors, each color keeps in a separate herd, often restricted to one part of the island; among the wild horses of Paraguay, those of the same color and size associate with each other; in Circassia three races of horses exist which, when living in freedom, always refuse to mingle and cross; on the Färöe Islands the half-wild, native black sheep resist attempts to breed them with imported white sheep; in the Forest of Dean and the New Forest the dark and pale colored herds of fallow deer have never been known to mingle; the merino and heath sheep of Scotland, if the two flocks are mixed together, will each breed with its own variety; Ancon sheep have been observed to keep together, separating themselves from the rest of the flock when put into inclosures with other sheep; "the female of the dog" (according to Prof. Low, an authority on domesticated animals), "when not under restraint, makes selection of her mate, the mastiff selecting the mastiff, the terrier the terrier, and so on"; pigeons pair, when choice is free, with their own kind; flocks of white and Chinese geese, even when associated by the breeder, keep themselves distinct. The fact that organisms prefer to associate and breed with their likes is also widely shown by the habits of birds. Describing a molluscan fauna in the Sandwich Islands, Mr. Gulick says: "We frequently find a genus represented in several successive valleys by allied species, sometimes feeding on the same, sometimes on different plants. In every such case the valleys that are nearest to each other furnish the most nearly allied forms; and a full set of the varieties of each species presents a minute gradation of forms between the more divergent types found in the more widely separated localities."

The recognition marks of insects, birds, and other animals imply and involve the association of likes and the separation of unlikes. The mimicry of one set of organisms by another, whether the imitation be of color, shape, or both, is manifestly a means of acquiring the superior ease of living which assimilation offers to the imitating kind: the mimickers always occupy the same region as the species mimicked, and the resemblance wrought is sometimes (Darwinism, page 256) advantageous to both. All association of organisms involves likeness between them, and when, by unequal conditions, unlikeness is set up in sufficient degree, that unlikeness involves dissociation of the unlike members of the group, accompanied by the resistance which sterility offers to the intercrossing of those members. The like individuals, moreover, are only held together by the constant operation of assimilative processes: one of these is the free intercrossing of all the associated members, whereby the individual is maintained true to the average; the other is the resultant tendency of the organism to transmit and perpetuate only those characters due to influences and conditions that are the common experiences of all the members of a kind, as opposed to characters arising out of special individual experiences.

The group of animals, like the group of men, expels unlikes that appear in its midst. Thus ants eject intruding members of alien kinds; geese and hens drive away strange birds of their own species; crows and rooks expel wrongdoers from their midst; domestic pigeons attack and badly wound sick, young, or fallen birds; beavers dissociate idle members from their colony; a wounded herbivorous animal returning to its companions is usually attacked and gored to death; elephants not only expel a "rogue" from the herd, but have been known to refuse refuge to one of their number who had escaped into the jungles with the bandages of the capturer still on its legs.

It should be added that the organism itself, and the organic structure, everywhere illustrates the principle of the association of likes and the dissociation of unlikes. All tissue systems in the organism are systems whose units are likes to each other, and the likeness which those units exhibit is a likeness maintained by the assimilative force of the organism acting a whole. So all food material taken into the organic system which is really a community of cells—must undergo assimilation as the very condition of its admission to association, while all material that can not be associated is expelled, either by dissociation without or by dissociation within the system.

In the inorganic world the same law holds good. Its most general evidence is afforded by the fact that matter is always associated with matter, and that where free to move it always constitutes a system of which the parts of like density are associated and the parts of unlike density dissociated. In a general way the bodies which constitute the solar system occupy positions in that system according to the degree of their density, the denser planets lying nearer to, those less dense farther away from, the sun. The ring system of Saturn shows a like collocation, due to differences and likenesses of density. In the earth itself (omitting complications of gravitation produced by internal heat) the denser solids always tend to move nearer to the earth's center than the liquids; the liquids thus displaced take precedence in position over the atmosphere; while even the air shows a gradually diminishing density in a direction away from the earth. That which, moreover, is observable in our own solar system may reasonably be predicated of stellar aggregations generally: most of the planetary nebulæ and star clusters display a gradually increasing density from circumference to center; comets, densest at their nucleus, visibly diminish in mass as their fanlike streamers recede from the sun; in meteor orbits the densest bodies travel first, the remainder, roughly speaking, following in the order of their density. In the star system itself, there are signs of the same kind of segregation, one region of the Milky Way being as remarkable for the multiplicity of its suns and its almost complete absence of nebulae as are the other regions of the heavens for the fewness of their stars and the large number of incipient systems which they contain.

What is true of matter in the varying degrees of its density is also true of the different types of matter; for whether the atom constitutes the molecule, as in the case of a few of the elements, or the molecule be made up of two or more atoms, as in the case of most of the elements, it is undisputed, not only that the uniting parts are likes to each other, but that the molecules which they form are also likes to each other. That which, moreover, is especially obvious in the case of the elements, where the molecule is always made up of the same kind of atoms, is none the less true of compounds; for, however superficially unlike the atoms or molecules may be that enter into combination to form such compounds, and whether the matter formed be what we know as inorganic or what we know as organic—whether it be extremely simple, as in the case of water, or highly complex, as in the case of albumin—the resultant system is in every instance a system whose unit parts all possess the same general character.

The power of material systems to dissociate unlikes is best seen when the dissociating stress is exerted by matter in the liquid or gaseous state—in a form, that is to say, in which the power to dissociate is at its maximum and the resistance to dissociation at a minimum. If we thrust a ball of wood into a volume of water, the entering mass is immediately expelled to the surface, and it is thus expelled for the reason that it possesses less density than water, and is therefore an unlike: the act of dissociation, moreover, is manifestly an act of the water system; for, if we remove the water, our wooden ball will descend freely through the space previously occupied by the fluid. So a balloon ascends through air because, being less dense than air, it appears in the air system as an unlike, and is by that system expelled to those regions of the atmosphere which resemble it in density: in this case, as well, the act of dissociation is an act of the air system, for, if the air could be removed, the balloon would not rise at all.

Unlikes, sufficiently free to move, often repel one another when forced into association. Zinc and lead, or zinc and bismuth, may be melted together, but they separate more or less completely during the process. Chloroform and water may be mechanically mixed, but when they are no longer shaken the two fluids part into distinct layers. If, upon oil placed in a receptacle, water be poured, the water will displace the oil; while, if mercury be added, it will displace both oil and water; the final result of the experiment—three layers occupying positions in the order of their density—being reached by acts involving both association and dissociation association of the like parts of each layer, and dissociation of the unlike layers.

How likes are associated and unlikes dissociated may also be shown in most of the phenomena of cohesion, and especially in those wherein particles of matter assume the spherical or globular form. Where the medium is not apparent, as in the case of the sun, moon, and planets, the association of likes is alone displayed; where the medium is more accessible to us, there is more or less suggestion of an act of dissociation by the system in which the parts become associated. Thus an atmosphere overcharged with moisture literally expels water from itself, first into fine vesicles, and later, by cumulative aggregation of these, into the drops which constitute rain; so when the vapor of water issues from the spout of a kettle, the air which it traverses condenses it into the droplets visible as steam. Water thrown upon a dusty surface; molten lead let fall from a tower in the process of shot-making; melted glass dropped into water—all these assume a more or less spherical shape under circumstances which suggest, not only cohesion of the parts, but also repulsion by the medium. The spherical shape assumed by oil dropped on water is not wholly due to cohesion of the parts of oil, but is also due in very large measure to the repulsive action of the water system itself. A still more striking illustration of these acts of simultaneous association and dissociation is yielded by mixing small quantities of water with large quantities of oil: here the water, in descending, breaks up into spherelike globules, each of which exemplifies at once cohesion of the intruding parts and repulsion by the receiving system. There is also to be added the evidence of smoke and vapor rings, the forms of which are largely determined by the action of the atmospheric system in which they are produced.

That assimilative action takes place within material systems is also to be noted. Such action is of several kinds, and includes (1) assimilation of movement, (2) assimilation of substance by diffusion, and (3) assimilation of mass or structure, the latter being divisible into (a) assimilation by change of form in the case of gross aggregates rudely associated, and (b) assimilation by changes of arrangement in the case of minute parts closely associated. The simplest form of assimilation is seen when volumes of two different gases are brought together within a closed receptacle; for, though the molecules of the two gases may originally possess different "kinetic" energy, they undergo in association a change by which the molecules of both gases come to have a like degree of the energy of movement. What is true of gases is true also of matter in each of its states: heat communicated to an aggregate is more or less rapidly distributed through it until all the parts possess, roughly speaking, like degrees or amounts of movement; a mass of metal heated in a furnace becomes gradually assimilated to the character of its surroundings; by a precisely similar process, the overheated earth radiates energy into the atmosphere. There is also the assimilative distribution of heat through liquids by means of convection currents: as the surface of the sea becomes cooled in winter, the cooler layers, grown heavier, sink, and are constantly being replaced by warmer water from below. Vertical exchanges of temperature also take place between higher and lower layers of the atmosphere, while the difference of temperature between the polar regions and the equatorial zone results in the assimilative movements of the general atmospheric circulation. As ocean currents, like the Gulf Stream of the Atlantic and the Kuro Siwo of the Pacific, carry heat from the warmer to the colder regions of the earth, so air currents arise out of those differences of heat and of barometrical pressure which it is obviously their function, as far as possible, to remove. All atmospheric movements, in fact, however local or general their character may be, are either movements of direct assimilation by which the atmosphere is seeking, so to speak, to bring all its areas into like temperature and pressure with each other, or are disturbances involved and arising indirectly out of such acts of assimilation. It is only because the work done by these movements is being constantly undone through the agency of influences, permanent and temporary, that differentiate areas of the atmosphere in every part of the globe—setting up, for example, unlikenesses of temperature and pressure between the equatorial and polar regions, between continents or islands and the surrounding oceans, or between any area of the earth's surface abnormally heated or cooled and the surrounding parts of that surface, as well as between seasonal variations in such inequalities—that we have cyclones and anticyclones, tornadoes, blizzards, land and sea breezes, mountain and valley winds, sand spouts and dust whirlwinds, as well as various periodical and more or less local disturbances all over the world. It should be added that meteorological phenomena do but illustrate the wider interchanges that take place in the ether system, since the constant distribution, as electro-magnetic disturbances, of movement differentially accumulated in material aggregates—whether such disturbances take place within purely local limits, as in circuits artificially set up, or on a universal scale, as by diffusion from solar bodies—are all cases of the distribution of movement, and therefore cases of assimilation.

The diffusion of molecules through each other is also a common form of assimilation. Gases, if brought together, permeate each other until a tolerably like constitution for every larger or smaller area of the total volume has been reached; gas distributes itself equably through liquids, as in the case of effervescing drinks; solutions of salts brought into contact gradually intermingle. A soluble solid, when introduced into a liquid, usually assumes the liquid state to the extent of the capacity of the fluid for taking it up, as in the familiar case of sugar in tea or alum in water, while the liquid itself undergoes modification by the equable distribution of the particles absorbed. The uniform hardness of "hard" water, due to the presence of bicarbonate of lime, and the uniform tenacity of solutions of soap in water, as shown in experiments with soap bubbles, both illustrate how equably substances held in solution are diffused. The evaporation of fluids into air, like the dissolution of solids in water, is in its results, at any rate, a case of assimilation to the character of the surrounding or adjacent medium; so that, the more we heat a bar of iron, the more progress does it make toward that vaporous condition in which it can easily be diffused through its environment. The mixing of metals also illustrates diffusion; for, whether it results in amalgams or alloys, whether the mixture be a merely mechanical association of the parts brought together or a chemical combination of those parts, the fact remains that for given areas, which may be large or small, the average degree of diffusion is the same. This is shown with especial clearness in those compounds of carbon and manganese with iron needed for a variety of industrial pur]30ses, since such compounds would have none of their present commercial value were it not for the uniform diffusion through the iron of the substance employed to modify it.

Here, then, our treatment of the subject must draw to a close. While necessarily brief, it has been complete enough to reveal a process far reaching in its scope and of cosmical significance. We have seen how like units everywhere tend to be associated and unlikes dissociated; how unlikes, held in forcible association, tend to be more or less profoundly assimilated to one another; and how disturbances of prevailing uniformity tend to be equably distributed through the several media in which they occur. But we have also noted that the power impelling to these multifarious acts of assimilation, to these movements of association and dissociation, is not the power of the units themselves, but the power of the system to which they immediately belong; and we are thus warned of the important bearing which our law has upon two problems of the utmost generality in physics—namely, the problems of chemical affinity and gravitation. It is true that we have as yet no formula for explaining these manifestations of power by a single principle; that we do not yet know the real structure of ether; and that there is still needed a definite account of gravitation as an intelligible mechanical process. Nevertheless, the causal connection of both gravitative and chemical actions with the ether system is already obvious to physicists. That the power which accomplishes these actions does not reside in matter alone, but resides also in the ether system—is, in fact, a function of that system regarded as including both ether and matter—seems to be increasingly pointed to by the trend of recent physical research. Basing our final conjecture, therefore, on generalization from a wide array of facts, it may not be premature to look to the ether system, not only for an illustration on the widest scale of the law of assimilation, but also for the ultimate source of all the phenomena in which the operation of that law can be traced. For whether the ether be continuous or granular, it clearly satisfies, by its very nature and uniformity, the demand that likes shall be associated, the while that, by the actions known as chemical and gravitative, it also fulfills the requirement that there shall be dissociation of unlikes. Lacking, as we do, all explanation of the actual mechanism of gravitation, we may none the less find its form suggested to us when we describe it as an act of dissociation by the ether system. And if this view be tenable, we should be justified in regarding the ether as primarily embodying the power manifested in the multifarious changes which we call evolution.