The instability thus variously ill.u.s.trated becomes still more manifest if we consider its rationale. It is consequent on the fact that the several parts of any h.o.m.ogeneous ma.s.s are necessarily exposed to different forces--forces which differ either in their kinds or amounts; and being exposed to different forces they are of necessity differently modified. The relations of outside and inside, and of comparative nearness to neighbouring sources of influence, imply the reception of influences which are unlike in quant.i.ty or quality or both; and it follows that unlike changes will be wrought in the parts dissimilarly acted upon. The unstable equilibrium of any h.o.m.ogeneous aggregate can thus be shown both inductively and deductively.

And now let us consider the bearing of this general truth on the evolution of organisms. The germ of a plant or animal is one of these h.o.m.ogeneous aggregates--relatively h.o.m.ogeneous if not absolutely so--whose equilibrium is unstable. But it has not simply the ordinary instability of h.o.m.ogeneous aggregates: it has something more. For it consists of units which are themselves specially characterized by instability. The const.i.tuent molecules of organic matter are distinguished by the feebleness of the affinities which hold their component elements together. They are extremely sensitive to heat, light, electricity, and the chemical actions of foreign elements; that is, they are peculiarly liable to be modified by disturbing forces.

Hence then it follows, _a priori_, that a h.o.m.ogeneous aggregate of these unstable molecules will have an excessive tendency to lose its equilibrium. It will have a quite special liability to lapse into a non-h.o.m.ogeneous state. It will rapidly gravitate towards heretogeneity.

Moreover, the process must repeat itself in each of the subordinate groups of organic units which are differentiated by the modifying forces. Each of these subordinate groups, like the original group, must gradually, in obedience to the influences acting on it, lose its balance of parts--must pa.s.s from a uniform into a multiform state. And so on continuously.

Thus, starting from the general laws of things, and the known chemical attributes of organic matter, we may conclude deductively that the h.o.m.ogeneous germs of organisms have a peculiar proclivity towards a non-h.o.m.ogeneous state; which may be either the state we call decomposition, or the state we call organization.

At present we have reached a conclusion only of the most general nature.

We merely learn that _some_ kind of heterogeneity is inevitable; but as yet there is nothing to tell us _what_ kind. Besides that _orderly_ heterogeneity which distinguishes organisms, there is the _disorderly_ or _chaotic_ heterogeneity, into which a loose ma.s.s of inorganic matter lapses; and at present no reason has been given why the h.o.m.ogeneous germ of a plant or animal should not lapse into the disorderly instead of the orderly heterogeneity. But by pursuing still further the line of argument hitherto followed we shall find a reason.

We have seen that the instability of h.o.m.ogeneous aggregates in general, and of organic ones in particular, is consequent on the various ways and degrees in which their const.i.tuent parts are exposed to the disturbing forces brought to bear on them: their parts are differently acted upon, and therefore become different. Manifestly, then, a rationale of the special changes which a germ undergoes, must be sought in the particular relations which its several parts bear to each other and to their environment. However it may be masked, we may suspect the fundamental principle of organization to be, that the many like units forming a germ acquire those kinds and degrees of unlikeness which their respective positions entail.

Take a ma.s.s of unorganized but organizable matter--either the body of one of the lowest living forms, or the germ of one of the higher.

Consider its circ.u.mstances. It is immersed in water or air; or it is contained within a parent organism. Wherever placed, however, its outer and inner parts stand differently related to surrounding existences--nutriment, oxygen, and the various stimuli. But this is not all. Whether it lies quiescent at the bottom of the water, whether it moves through the water preserving some definite att.i.tude, or whether it is in the inside of an adult; it equally results that certain parts of its surface are more directly exposed to surrounding agencies than other parts--in some cases more exposed to light, heat, or oxygen, and in others to the maternal tissues and their contents. The destruction of its original equilibrium is therefore certain. It may take place in one of two ways. Either the disturbing forces may be such as to overbalance the affinities of the organic elements, in which case there results that chaotic heterogeneity known as decomposition; or, as is ordinarily the case, such changes are induced as do not destroy the organic compounds, but only modify them: the parts most exposed to the modifying forces being most modified. Hence result those first differentiations which const.i.tute incipient organization. From the point of view thus reached, suppose we look at a few cases: neglecting for the present all consideration of the tendency to a.s.sume the inherited type.

Note first what appear to be exceptions, as the _Amoeba_. In this creature and its allies, the substance of the jelly-like body remains throughout life unorganized--undergoes no permanent differentiations.

But this fact, which seems directly opposed to our inference, is really one of the most significant evidences of its truth. For what is the peculiarity of the Rhizopods, exemplified by the _Amoeba_? They undergo perpetual and irregular changes of shape--they show no persistent relations of parts. What lately formed a portion of the interior is now protruded, and, as a temporary limb, is attached to some object it happens to touch. What is now a part of the surface will presently be drawn, along with the atom of nutriment sticking to it, into the centre of the ma.s.s. Thus there is an unceasing interchange of places; and the relations of inner and outer have no settled existence.

But by the hypothesis, it is only in virtue of their unlike positions with respect to modifying forces, that the originally-like units of a living ma.s.s become unlike. We must not therefore expect any established differentiation of parts in creatures which exhibit no established differences of position in their parts.

This negative evidence is borne out by abundant positive evidence. When we turn from these ever-changing specks of living jelly to organisms having unchanging distributions of substance, we find differences of tissue corresponding to differences of relative position. In all the higher _Protozoa_, as also in the _Protophyta_, we meet with a fundamental differentiation into cell-membrane and cell-contents, answering to that fundamental contrast of conditions implied by the words outside and inside. And on pa.s.sing from what are roughly cla.s.sed as unicellular organisms to the lowest of those which consist of aggregated cells, we equally observe the connexion between structural differences and differences of circ.u.mstance. In the sponge, permeated throughout by currents of sea-water, the absence of definite organization corresponds with the absence of definite unlikeness of conditions. In the _Thala.s.sicolla_ of Professor Huxley--a transparent, colourless body, found floating pa.s.sively at the surface of the sea, and consisting essentially of "a ma.s.s of cells united by jelly"--there is displayed a rude structure obviously subordinated to the primary relations of centre and surface: in all of its many and important varieties, the parts exhibit a more or less concentric arrangement.

After this primary modification, by which the outer tissues are differentiated from the inner, the next in order of constancy and importance is that by which some part of the outer tissues is differentiated from the rest; and this corresponds with the almost universal fact that some part of the outer tissues is more directly exposed to certain environing influences than the rest. Here, as before, the apparent exceptions are extremely significant. Some of the lowest vegetable organisms, as the _Hematococci_ and _Protococci_, evenly imbedded in a ma.s.s of mucus, or dispersed through the Arctic snow, display no differentiations of surface: the several parts of the surface being subjected to no definite contrasts of conditions. The _Thala.s.sicolla_ above mentioned, unfixed, and rolled about by the waves, presents all its sides successively to the same agencies; and all its sides are alike. A ciliated sphere like the _Volvox_ has no parts of its periphery unlike other parts; and it is not to be expected that it should have; seeing that as it revolves in all directions, it does not, in traversing the water, permanently expose any part to special conditions. But when we come to creatures that are either fixed, or while moving, severally preserve a definite att.i.tude, we no longer find uniformity of surface. The gemmule of a Zoophyte, which during its locomotive stage is distinguishable only into outer and inner tissues, no sooner takes root than its upper end begins to a.s.sume a different structure from its lower. The free-swimming embryo of an aquatic annelid, being ovate and not ciliated all over, moves with one end foremost; and its differentiations proceed in conformity with this contrast of circ.u.mstances.

The principle thus displayed in the humbler forms of life, is traceable during the development of the higher; though being here soon masked by the a.s.sumption of the hereditary type, it cannot be traced far. Thus the "mulberry-ma.s.s" into which a fertilized ovum of a vertebrate animal first resolves itself, soon begins to exhibit a difference between the outer and inner parts answering to the difference of circ.u.mstances. The peripheral cells, after reaching a more complete development than the central ones, coalesce into a membrane enclosing the rest; and then the cells lying next to these outer ones become aggregated with them, and increase the thickness of the germinal membrane, while the central cells liquefy. Again, one part of the germinal membrane presently becomes distinguishable as the germinal spot; and without a.s.serting that the cause of this is to be found in the unlike relations which the respective parts of the germinal membrane bear to environing influences, it is clear that we have in these unlike relations an element of disturbance tending to destroy the original h.o.m.ogeneity of the germinal membrane. Further, the germinal membrane by and by divides into two layers, internal and external; the one in contact with the liquefied interior part or yelk, the other exposed to the surrounding fluids: this contrast of circ.u.mstances being in obvious correspondence with the contrast of structures which follows it. Once more, the subsequent appearance of the vascular layer between these mucous and serous layers, as they have been named, admits of a like interpretation. And in this and the various complications which now begin to show themselves, we may see coming into play that general law of the multiplication of effects flowing from one cause, to which the increase of heterogeneity was elsewhere ascribed.[9]

Confining our remarks, as we do, to the most general facts of development, we think that some light is thus thrown on them. That the unstable equilibrium of a h.o.m.ogeneous germ must be destroyed by the unlike exposure of its several units to surrounding influences, is an _a priori_ conclusion. And it seems also to be an _a priori_ conclusion, that the several units thus differently acted upon, must either be decomposed, or must undergo such modifications of nature as may enable them to live in the respective circ.u.mstances they are thrown into: in other words--_they must either die or become adapted to their conditions_. Indeed, we might infer as much without going through the foregoing train of reasoning. The superficial organic units (be they the outer cells of a "mulberry-ma.s.s," or be they the outer molecules of an individual cell) must a.s.sume the function which their position necessitates; and a.s.suming this function, must acquire such character as performance of it involves. The layer of organic units lying in contact with the yelk must be those through which the yelk is absorbed; and so must be adapted to the absorbent office. On this condition only does the process of organization appear possible. We might almost say that just as some race of animals, which multiplies and spreads into divers regions of the earth, becomes differentiated into several races through the adaptation of each to its conditions of life; so, the originally h.o.m.ogeneous population of cells arising in a fertilized germ-cell, becomes divided into several populations of cells that grow unlike in virtue of the unlikeness of their circ.u.mstances.

Moreover, it is to be remarked in further proof of our position, that it finds its clearest and most abundant ill.u.s.trations where the conditions of the case are the simplest and most general--where the phenomena are the least involved: we mean in the production of individual cells. The structures which presently arise round nuclei in a blastema, and which have in some way been determined by those nuclei as centres of influence, evidently conform to the law; for the parts of the blastema in contact with the nuclei are differently conditioned from the parts not in contact with them. Again, the formation of a membrane round each of the ma.s.ses of granules into which the endochrome of an alga-cell breaks up, is an instance of a.n.a.logous kind. And should the recently-a.s.serted fact that cells may arise round vacuoles in a ma.s.s of organizable substance, be confirmed, another good example will be furnished; for such portions of substance as bound these vacant s.p.a.ces are subject to influences unlike those to which other portions of the substance are subject. If then we can most clearly trace this law of modification in these primordial processes, as well as in those more complex but a.n.a.logous ones exhibited in the early changes of an ovum, we have strong reason for thinking that the law is fundamental.

But, as already more than once hinted, this principle, understood in the simple form here presented, supplies no key to the detailed phenomena of organic development. It fails entirely to explain generic and specific peculiarities; and leaves us equally in the dark respecting those more important distinctions by which families and orders are marked out. Why two ova, similarly exposed in the same pool, should become the one a fish, and the other a reptile, it cannot tell us. That from two different eggs placed under the same hen, should respectively come forth a duckling and a chicken, is a fact not to be accounted for on the hypothesis above developed. Here we are obliged to fall back upon the unexplained principle of hereditary transmission. The capacity possessed by an unorganized germ of unfolding into a complex adult which repeats ancestral traits in minute details, and that even when it has been placed in conditions unlike those of its ancestors, is a capacity impossible for us to understand. That a microscopic portion of seemingly structureless matter should embody an influence of such kind, that the resulting man will in fifty years after become gouty or insane, is a truth which would be incredible were it not daily ill.u.s.trated. But though the _manner_ in which hereditary likeness, in all its complications, is conveyed, is a mystery pa.s.sing comprehension, it is quite conceivable that it is conveyed in subordination to the law of adaptation above explained; and we are not without reasons for thinking that it is so. Various facts show that acquired peculiarities resulting from the adaptation of const.i.tution to conditions, are transmissible to offspring. Such acquired peculiarities consist of differences of structure or composition in one or more of the tissues. That is to say, of the aggregate of similar organic units composing a germ, the group going to the formation of a particular tissue, will take on the special character which the adaptation of that tissue to new circ.u.mstances had produced in the parents. We know this to be a general law of organic modifications. Further, it is the _only_ law of organic modifications of which we have any evidence.[10] It is not impossible then that it is the universal law; comprehending not simply those minor modifications which offspring inherit from recent ancestry, but comprehending also those larger modifications distinctive of species, genus, order, cla.s.s, which they inherit from antecedent races of organisms. And thus it _may be_ that the law of adaptation is the sole law; presiding not only over the differentiation of any race of organisms into several races, but also over the differentiation of the race of organic units composing a germ, into the many races of organic units composing an adult. So understood, the process gone through by every unfolding organism will consist, partly in the direct adaptation of its elements to their several circ.u.mstances, and partly in the a.s.sumption of characters resulting from a.n.a.logous adaptations of the elements of all ancestral organisms.

But our argument does not commit us to any such far-reaching speculation as this; which we introduce simply as suggested by it, not involved. All we are here concerned to show, is, that the deductive method aids us in interpreting some of the more general phenomena of development. That all h.o.m.ogeneous aggregates are in unstable equilibrium is a universal truth, from which is deducible the instability of every organic germ. From the known sensitiveness of organic compounds to chemical, thermal, and other disturbing forces, we further infer the _unusual_ instability of every organic germ--a p.r.o.neness far beyond that of other h.o.m.ogeneous aggregates to lapse into a heterogeneous state. By the same line of reasoning we are led to the additional inference, that the first divisions into which a germ resolves itself, being severally in a state of unstable equilibrium, are similarly p.r.o.ne to undergo further changes; and so on continuously. Moreover, we have found it to be equally an _a priori_ conclusion, that as, in all other cases, the loss of h.o.m.ogeneity is due to the different degrees and kinds of force brought to bear on the different parts; so, in this case too, difference of circ.u.mstances is the primary cause of differentiation. Add to which, that as the several changes undergone by the respective parts thus diversely acted upon, are changes which do not destroy their vital activity, they must be changes which bring that vital activity into subordination to the incident forces--they must be adaptations; and the like must be in some sense true of all the subsequent changes. Thus by deductive reasoning we get some insight into the method of organization. However unable we are, and probably ever shall be, to comprehend the way in which a germ is made to take on the special form of its race, we may yet comprehend the general principles which regulate its first modifications; and, remembering the unity of plan so conspicuous throughout nature, we may _suspect_ that these principles are in some way concerned in succeeding modifications.

A controversy now going on among zoologists, opens yet another field for the application of the deductive method. We believe that the question whether there does or does not exist a _necessary correlation_ among the several parts of an organism is determinable _a priori_.

Cuvier, who first a.s.serted this necessary correlation, professed to base his restorations of extinct animals upon it. Geoffroy St. Hilaire and De Blainville, from different points of view, contested Cuvier's hypothesis; and the discussion, which has much interest as bearing on paleontology, has been recently revived under a somewhat modified form: Professors Huxley and Owen being respectively the a.s.sailant and defender of the hypothesis.

Cuvier says--"Comparative anatomy possesses a principle whose just development is sufficient to dissipate all difficulties; it is that of the correlation of forms in organized beings, by means of which every kind of organized being might, strictly speaking, be recognized by a fragment of any of its parts. Every organized being const.i.tutes a whole, a single and complete system, whose parts mutually correspond and concur by their reciprocal reaction to the same definite end. None of these parts can be changed without affecting the others; and consequently each taken separately, indicates and gives all the rest." He then gives ill.u.s.trations: arguing that the carnivorous form of tooth necessitating a certain action of the jaw, implies a particular form in its condyles; implies also limbs fit for seizing and holding prey; therefore implies claws, a certain structure of the leg-bones, a certain form of shoulder-blade. Summing up he says, that "the claw, the scapula, the condyle, the femur, and all the other bones, taken separately, will give the tooth or one another; and by commencing with any one, he who had a rational conception of the laws of the organic economy, could reconstruct the whole animal."

It will be seen that the method of restoration here contended for, is based on the alleged physiological necessity of the connexion between these several peculiarities. The argument used is, not that a scapula of a certain shape may be recognized as having belonged to a carnivorous mammal because we always find that carnivorous mammals _do_ possess such scapulas; but the argument is that they _must_ possess them, because carnivorous habits would be impossible without them. And in the above quotation Cuvier a.s.serts that the necessary correlation which he considers so obvious in these cases, exists throughout the system: admitting, however, that in consequence of our limited knowledge of physiology we are unable in many cases to trace this necessary correlation, and are obliged to base our conclusions upon observed coexistences, of which we do not understand the reason, but which we find invariable.

Now Professor Huxley has recently shown that, in the first place, this empirical method, which Cuvier introduces as quite subordinate, and to be used only in aid of the rational method, is really the method which Cuvier habitually employed--the so-called rational method remaining practically a dead letter; and, in the second place, he has shown that Cuvier himself has in several places so far admitted the inapplicability of the rational method, as virtually to surrender it as a method. But more than this, Professor Huxley contends that the alleged necessary correlation is not true. Quite admitting the physiological dependence of parts on each other, he denies that it is a dependence of a kind which could not be otherwise. "Thus the teeth of a lion and the stomach of the animal are in such relation that the one is fitted to digest the food which the other can tear, they are physiologically correlated; but we have no reason for affirming this to be a necessary physiological correlation, in the sense that no other could equally fit its possessor for living on recent flesh. The number and form of the teeth might have been quite different from that which we know them to be, and the construction of the stomach might have been greatly altered; and yet the functions of these organs might have been equally well performed."

Thus much is needful to give an idea of the controversy. It is not here our purpose to go more at length into the evidence cited on either side.

We simply wish to show that the question may be settled deductively.

Before going on to do this, however, let us briefly notice two collateral points.

In his defence of the Cuvierian doctrine, Professor Owen avails himself of the _odium theologic.u.m_. He attributes to his opponents "the insinuation and masked advocacy of the doctrine subversive of a recognition of the Higher Mind." Now, saying nothing about the questionable propriety of thus prejudging an issue in science, we think this is an unfortunate accusation. What is there in the hypothesis of _necessary_, as distinguished from _actual_, correlation of parts, which is particularly in harmony with Theism? Maintenance of the _necessity_, whether of sequences or of coexistences, is commonly thought rather a derogation from divine power than otherwise. Cuvier says--"None of these parts can be changed without affecting the others; and consequently, each taken separately, indicates and gives all the rest." That is to say, in the nature of things the correlation _could not_ have been otherwise. On the other hand, Professor Huxley says we have no warrant for a.s.serting that the correlation _could not_ have been otherwise; but have not a little reason for thinking that the same physiological ends might have been differently achieved. The one doctrine limits the possibilities of creation; the other denies the implied limit. Which, then, is most open to the charge of covert Atheism?

On the other point we lean to the opinion of Professor Owen. We agree with him in thinking that where a rational correlation (in the highest sense of the term) can be made out, it affords a better basis for deduction than an empirical correlation ascertained only by acc.u.mulated observations. Premising that by rational correlation is not meant one in which we can trace, or think we can trace, a design, but one of which the negation is inconceivable (and this is the species of correlation which Cuvier's principle implies); then we hold that our knowledge of the correlation is of a more certain kind than where it is simply inductive. We think that Professor Huxley, in his anxiety to avoid the error of making Thought the measure of Things, does not sufficiently bear in mind the fact, that as our notion of necessity is determined by some absolute uniformity pervading all orders of our experiences, it follows that an organic correlation which cannot be conceived otherwise, is guaranteed by a much wider induction than one ascertained only by the observation of organisms. But the truth is, that there are relatively few organic correlations of which the negation is inconceivable. If we find the skull, vertebrae, ribs, and phalanges of some quadruped as large as an elephant; we may indeed be certain that the legs of this quadruped were of considerable size--much larger than those of a rat; and our reason for conceiving this correlation as necessary, is, that it is based, not only upon our experiences of moving organisms, but upon all our mechanical experiences relative to ma.s.ses and their supports. But even were there many physiological correlations really of this order, which there are not, there would be danger in pursuing this line of reasoning, in consequence of the liability to include within the cla.s.s of truly necessary correlations, those which are not such. For instance, there would seem to be a necessary correlation between the eye and the surface of the body: light being needful for vision, it might be supposed that every eye must be external. Nevertheless it is a fact that there are creatures, as the _Cirrhipedia_, having eyes (not very efficient ones, it may be) deeply imbedded within the body. Again, a necessary correlation might be a.s.sumed between the dimensions of the mammalian uterus and those of the pelvis. It would appear impossible that in any species there should exist a well-developed uterus containing a full-sized foetus, and yet that the arch of the pelvis should be too small to allow the foetus to pa.s.s. And were the only mammal having a very small pelvic arch, a fossil one, it would have been inferred, on the Cuvierian method, that the foetus must have been born in a rudimentary state; and that the uterus must have been proportionally small. But there happens to be an extant mammal having an undeveloped pelvis--the mole--which presents us with a fact that saves us from this erroneous inference. The young of the mole are not born through the pelvic arch at all; but in front of it! Thus, granting that some quite _direct_ physiological correlations may be necessary, we see that there is great risk of including among them some which are not.

With regard to the great ma.s.s of the correlations, however, including all the _indirect_ ones, Professor Huxley seems to us warranted in denying that they are necessary; and we now propose to show deductively the truth of his thesis. Let us begin with an a.n.a.logy.

Whoever has been through an extensive iron-works, has seen a gigantic pair of shears worked by machinery, and used for cutting in two, bars of iron that are from time to time thrust between its blades. Supposing these blades to be the only visible parts of the apparatus, anyone observing their movements (or rather the movement of one, for the other is commonly fixed), will see from the manner in which the angle increases and decreases, and from the curve described by the moving extremity, that there must be some centre of motion--either a pivot or an external box equivalent to it. This may be regarded as a necessary correlation. Moreover, he might infer that beyond the centre of motion the moving blade was produced into a lever, to which the power was applied; but as another arrangement is just possible, this could not be called anything more than a highly probable correlation. If now he went a step further, and asked how the reciprocal movement was given to the lever, he would perhaps conclude that it was given by a crank. But if he knew anything of mechanics, he would know that it might possibly be given by an eccentric. Or again, he would know that the effect could be achieved by a cam. That is to say, he would see that there was no necessary correlation between the shears and the remoter parts of the apparatus. Take another case. The plate of a printing-press is required to move up and down to the extent of an inch or so; and it must exert its greatest pressure when it reaches the extreme of its downward movement. If now anyone will look over the stock of a printing-press maker, he will see half a dozen different mechanical arrangements by which these ends are achieved; and a machinist would tell him that as many more might readily be invented. If, then, there is no necessary correlation between the special parts of a machine, still less is there between those of an organism.

From a converse point of view the same truth is manifest. Bearing in mind the above a.n.a.logy, it will be foreseen that an alteration in one part of an organism will not necessarily entail _some one specific set of alterations in the other parts_. Cuvier says, "None of these parts can be changed without affecting the others; and consequently, each taken separately, indicates and gives all the rest." The first of these propositions may pa.s.s, but the second, which it is alleged follows from it, is not true; for it implies that "all the rest" can be severally affected in only one way and degree, whereas they can be affected in many ways and degrees. To show this, we must again have recourse to a mechanical a.n.a.logy.

If you set a brick on end and thrust it over, you can predict with certainty in what direction it will fall, and what att.i.tude it will a.s.sume. If, again setting it up, you put another on the top of it, you can no longer foresee with accuracy the results of an overthrow; and on repeating the experiment, no matter how much care is taken to place the bricks in the same positions, and to apply the same degree of force in the same direction, the effects will on no two occasions be exactly alike. And in proportion as the aggregation is complicated by the addition of new and unlike parts, will the results of any disturbance become more varied and incalculable. The like truth is curiously ill.u.s.trated by locomotive engines. It is a fact familiar to mechanical engineers and engine-drivers, that out of a number of engines built as accurately as possible to the same pattern, no two will act in just the same manner. Each will have its peculiarities. The play of actions and reactions will so far differ, that under like conditions each will behave in a somewhat different way; and every driver has to learn the idiosyncrasies of his own engine before he can work it to the greatest advantage. In organisms themselves this indefiniteness of mechanical reaction is clearly traceable. Two boys throwing stones will always differ more or less in their att.i.tudes, as will two billiard-players.

The familiar fact that each individual has a characteristic gait, ill.u.s.trates the point still better. The rhythmical motion of the leg is simple, and on the Cuvierian hypothesis, should react on the body in some uniform way. But in consequence of those slight differences of structure which consist with ident.i.ty of species, no two individuals make exactly similar movements either of the trunk or the arms. There is always a peculiarity recognizable by their friends.

When we pa.s.s to disturbing forces of a non-mechanical kind, the same truth becomes still more conspicuous. Expose several persons to a drenching storm; and while one will subsequently feel no appreciable inconvenience, another will have a cough, another a catarrh, another an attack of diarrhoea, another a fit of rheumatism. Vaccinate several children of the same age with the same quant.i.ty of virus, applied to the same part, and the symptoms will not be quite alike in any of them, either in kind or intensity; and in some cases the differences will be extreme. The quant.i.ty of alcohol which will send one man to sleep, will render another unusually brilliant--will make this maudlin, and that irritable. Opium will produce either drowsiness or wakefulness: so will tobacco.

Now in all these cases--mechanical and other--some force is brought to bear primarily on one part of an organism, and secondarily on the rest; and, according to the doctrine of Cuvier, the rest ought to be affected in a specific way. We find this to be by no means the case. The original change produced in one part does not stand in any necessary correlation with every one of the changes produced in the other parts; nor do these stand in any necessary correlation with one another. The functional alteration which the disturbing force causes in the organ directly acted upon, does not involve some _particular set_ of functional alterations in the other organs; but will be followed by some one out of various sets. And it is a manifest corollary, that any _structural alteration_ which may eventually be produced in the one organ, will not be accompanied by _some particular set of structural alterations_ in the other organs. There will be no necessary correlation of forms.

Thus Paleontology must depend upon the empirical method. A fossil species that was obliged to change its food or habits of life, did not of necessity undergo the particular set of modifications exhibited; but, under some slight change of predisposing causes--as of season or lat.i.tude--might have undergone some other set of modifications: the determining circ.u.mstance being one which, in the human sense, we call fortuitous.

May we not say then, that the deductive method elucidates this vexed question in physiology; while at the same time our argument collaterally exhibits the limits within which the deductive method is applicable. For while we see that this extremely _general_ question may be satisfactorily dealt with deductively; the conclusion arrived at itself implies that the more _special_ phenomena of organization cannot be so dealt with.

There is yet another method of investigating the general truths of physiology--a method to which physiology already owes one luminous idea, but which is not at present formally recognized as a method. We refer to the comparison of physiological phenomena with social phenomena.

The a.n.a.logy between individual organisms and the social organism, is one that has from early days occasionally forced itself on the attention of the observant. And though modern science does not countenance those crude ideas of this a.n.a.logy which have been from time to time expressed since the Greeks flourished; yet it tends to show that there _is_ an a.n.a.logy, and a remarkable one. While it is becoming clear that there are not those special parallelisms between the const.i.tuent parts of a man and those of a nation, which have been thought to exist; it is also becoming clear that the general principles of development and structure displayed in organized bodies are displayed in societies also. The fundamental characteristic both of societies and of living creatures, is, that they consist of mutually-dependent parts; and it would seem that this involves a community of various other characteristics. Those who are acquainted with the broad facts of both physiology and sociology, are beginning to recognize this correspondence not as a plausible fancy, but as a scientific truth. And we are strongly of opinion that it will by and by be seen to hold to an extent which few at present suspect.

Meanwhile, if any such correspondence exists, it is clear that physiology and sociology will more or less interpret each other. Each affords its special facilities for inquiry. Relations of cause and effect clearly traceable in the social organism, may lead to the search for a.n.a.logous ones in the individual organism; and may so elucidate what might else be inexplicable. Laws of growth and function disclosed by the pure physiologist, may occasionally give us the clue to certain social modifications otherwise difficult to understand. If they can do no more, the two sciences can at least exchange suggestions and confirmations; and this will be no small aid. The conception of "the physiological division of labour," which political economy has already supplied to physiology, is one of no small value. And probably it has others to give.

In support of this opinion, we will now cite cases in which such aid is furnished. And in the first place, let us see whether the facts of social organization do not afford additional support to some of the doctrines set forth in the foregoing parts of this article.

One of the propositions supported by evidence was that in animals the process of development is carried on, not by differentiations only, but by subordinate integrations. Now in the social organism we may see the same duality of process; and further, it is to be observed that the integrations are of the same three kinds. Thus we have integrations which arise from the simple growth of adjacent parts that perform like functions: as, for instance, the coalescence of Manchester with its calico-weaving suburbs. We have other integrations which arise when, out of several places producing a particular commodity, one monopolizes more and more of the business, and leaves the rest to dwindle: witness the growth of the Yorkshire cloth-districts at the expense of those in the west of England; or the absorption by Staffordshire of the pottery-manufacture, and the consequent decay of the establishments that once flourished at Worcester, Derby, and elsewhere. And we have those yet other integrations which result from the actual approximation of the similarly-occupied parts: whence result such facts as the concentration of publishers in Paternoster Row, of lawyers in the Temple and neighbourhood, of corn-merchants about Mark Lane, of civil engineers in Great George Street, of bankers in the centre of the city. Finding thus that in the evolution of the social organism, as in the evolution of individual organisms, there are integrations as well as differentiations, and moreover that these integrations are of the same three orders; we have additional reason for considering these integrations as essential parts of the developmental process, needed to be included in its formula. And further, the circ.u.mstance that in the social organism these integrations are determined by community of function, confirms the hypothesis that they are thus determined in the individual organism.

Again, we endeavoured to show deductively, that the contrasts of parts first seen in all unfolding embryos, are consequent upon the contrasted circ.u.mstances to which such parts are exposed; that thus, adaptation of const.i.tution to conditions is the principle which determines their primary changes; and that, possibly, if we include under the formula hereditarily-transmitted adaptations, all subsequent differentiations may be similarly determined. Well, we need not long contemplate the facts to see that some of the predominant social differentiations are brought about in an a.n.a.logous way. As the members of an originally-h.o.m.ogeneous community multiply and spread, the gradual separation into sections which simultaneously takes place, manifestly depends on differences of local circ.u.mstances. Those who happen to live near some place chosen, perhaps for its centrality, as one of periodical a.s.semblage, become traders, and a town springs up; those who live dispersed, continue to hunt or cultivate the earth; those who spread to the sea-sh.o.r.e fall into maritime occupations. And each of these cla.s.ses undergoes modifications of character fitting to its function. Later in the process of social evolution these local adaptations are greatly multiplied. In virtue of differences of soil and climate, the rural inhabitants in different parts of the kingdom, have their occupations partially specialized; and are respectively distinguished as chiefly producing cattle, or sheep, or wheat, or oats, or hops, or cider. People living where coal-fields are discovered become colliers; Cornishmen take to mining because Cornwall is metalliferous; and the iron-manufacture is the dominant industry where ironstone is plentiful. Liverpool has a.s.sumed the office of importing cotton, in consequence of its proximity to the district where cotton goods are made; and for a.n.a.logous reasons Hull has become the chief port at which foreign wools are brought in. Even in the establishment of breweries, of dye-works, of slate-quarries, of brick-yards, we may see the same truth.

So that, both in general and in detail, these industrial specializations of the social organism which characterize separate districts, primarily depend on local circ.u.mstances. Of the originally-similar units making up the social ma.s.s, different groups a.s.sume the different functions which their respective positions entail; and become adapted to their conditions. Thus, that which we concluded, _a priori_, to be the leading cause of organic differentiations, we find, _a posteriori_, to be the leading cause of social differentiations. Nay further, as we inferred that possibly the embryonic changes which are not thus directly caused, are caused by hereditarily-transmitted adaptations; so, we may actually see that in embryonic societies, such changes as are not due to direct adaptations, are in the main traceable to adaptations originally undergone by the parent society. The colonies founded by distinct nations, while they are alike in exhibiting specializations caused in the way above described, grow unlike in so far as they take on, more or less, the organizations of the nations they sprung from. A French settlement does not develop exactly after the same manner as an English one; and both a.s.sume forms different from those which Roman settlements a.s.sumed. Now the fact that the differentiation of societies is determined partly by the direct adaptation of their units to local conditions, and partly by the transmitted influence of like adaptations undergone by ancestral societies, tends strongly to enforce the conclusion, otherwise reached, that the differentiation of individual organisms, similarly results from immediate adaptations compounded with ancestral adaptations.

From confirmations thus furnished by sociology to physiology, let us now pa.s.s to a suggestion similarly furnished. A factory, or other producing establishment, or a town made up of such establishments, is an agency for elaborating some commodity consumed by society at large; and may be regarded as a.n.a.logous to a gland or viscus in an individual organism. If we inquire what is the primitive mode in which one of these producing establishments grows up, we find it to be this. A single worker, who himself sells the produce of his labour, is the germ. His business increasing, he employs helpers--his sons or others; and having done this, he becomes a vendor not only of his own handiwork, but of that of others. A further increase of his business compels him to multiply his a.s.sistants, and his sale grows so rapid that he is obliged to confine himself to the process of selling: he ceases to be a producer, and becomes simply a channel through which the produce of others is conveyed to the public. Should his prosperity rise yet higher, he finds that he is unable to manage even the sale of his commodities, and has to employ others, probably of his own family, to aid him in selling; so that, to him as a main channel are now added subordinate channels. Moreover, when there grow up in one place, as a Manchester or a Birmingham, many establishments of like kind, this process is carried still further.

There arise factors and buyers, who are the channels through which is transmitted the produce of many factories; and we believe that primarily these factors were manufacturers who undertook to dispose of the produce of smaller houses as well as their own, and ultimately became salesmen only. Under a converse aspect, all the stages of this development have been within these few years exemplified in our railway contractors.

There are sundry men now living who ill.u.s.trate the whole process in their own persons--men who were originally navvies, digging and wheeling; who then undertook some small sub-contract, and worked along with those they paid; who presently took larger contracts, and employed foremen; and who now contract for whole railways, and let portions to sub-contractors. That is to say, we have men who were originally workers, but have finally become the main channels out of which diverge secondary channels, which again bifurcate into the subordinate channels, through which flows the money (representing the nutriment) supplied by society to the actual makers of the railway. Now it seems worth inquiring whether this is not the original course followed in the evolution of secreting and excreting organs in an animal. We know that such is the process by which the liver is developed. Out of the group of bile-cells forming the germ of it, some centrally-placed ones, lying next to the intestine, are transformed into ducts through which the secretion of the peripheral bile-cells is poured into the intestine; and as the peripheral bile-cells multiply, there similarly arise secondary ducts emptying themselves into the main ones; tertiary ones into these; and so on. Recent inquiries show that the like is the case with the lungs,--that the bronchial tubes are thus formed. But while a.n.a.logy suggests that this is the _original_ mode in which such organs are developed, it at the same time suggests that this does not necessarily continue to be the mode. For as we find that in the social organism, manufacturing establishments are no longer commonly developed through the series of modifications above described, but now mostly arise by the direct transformation of a number of persons into master, clerks, foremen, workers, &c.; so the approximate method of forming organs, may in some cases be replaced by a direct metamorphosis of the organic units into the destined structure, without any transitional structures being pa.s.sed through. That there are organs thus formed is an ascertained fact; and the additional question which a.n.a.logy suggests is, whether the direct method is subst.i.tuted for the indirect method.

Such parallelisms might be multiplied. And were it possible here to show in detail the close correspondence between the two kinds of organization, our case would be seen to have abundant support. But, as it is, these few ill.u.s.trations will sufficiently justify the opinion that study of organized bodies may be indirectly furthered by study of the body politic. Hints may be expected, if nothing more. And thus we venture to think that the Inductive Method, usually alone employed by most physiologists, may not only derive important a.s.sistance from the Deductive Method, but may further be supplemented by the Sociological Method.

FOOTNOTES:

[Footnote 6: Carpenter's _Principles of Comparative Physiology_, pp.

616-17.]