§ 297. The change from the outside of the lips to their inside, introduces us to a new series of interesting and instructive facts, joining on to those with which the last chapter closed. They concern the differentiations of those coats of the alimentary canal which, as we have seen, are physiologically outer, though physically inner.

These coats are greatly modified at different parts; and their modifications vary greatly in different animals. In the lower types, where they compose a simple tube running from end to end of the body, they are almost uniform in their histological characters; but on ascending from these types, we find them presenting an increasing variety of minute structures between their two ends. The argument will be adequately enforced if we limit ourselves to the leading modifications they display in some of the higher animals.

The successive parts of the alimentary canal are so placed with respect to its contents, that the physical and chemical changes undergone by its contents while passing from one end to the other, inevitably tend to transform its originally homogeneous surface into a heterogeneous surface. Clearly, the effect produced on the food at any part of the canal by trituration, by adding a secretion, or by absorbing its nutritive matters, implies the delivery of the food into the next part of the canal in a state more or less unlike its previous states—implies that the surface with which it now comes in contact is differently affected by it from the preceding surfaces—implies, that is, a differentiating action. To use concrete language;—food that is broken down in the mouth acts on the œsophagus and stomach in a way unlike that which it would have done had it been swallowed whole; the masticated food, to which certain solvents or ferments are added, becomes to the intestine a different substance from that which it must have otherwise been; and the altered food, resolved by these additions into its proximate principles, cannot have those proximate principles absorbed in the next part of the intestine, without the remoter parts being affected as they would not have been in the absence of absorption. It is true that in developed alimentary canals, such as the reasoning here tacitly assumes, these marked successive differentiations of the food are themselves the results of pre-established differentiations in the successive parts of the canal. But it is also true that actions and reactions like those here so definitely marked, must go on indefinitely in an undeveloped alimentary canal. If the food is changed at all in the course of its transit, which it must be if the creature is to live by it, then it cannot but act dissimilarly on the successive tracts of the alimentary canal, and cannot but be dissimilarly reacted on by them. Inevitably, therefore, the uniformity of the surface must lapse into greater or less multiformity: the differentiation of each part tending ever to initiate differentiations of other parts.

Not, indeed, that the implied process of direct equilibration can be regarded as the sole process. Indirect equilibration aids; and, doubtless, there are some of the modifications which only indirect equilibration can accomplish. But we have here one unquestionable cause—a cause that is known to work in individuals, changes of the kind alleged. Where, for instance, cancerous disease of the œsophagus so narrows the passage into the stomach as to prevent easy descent of the food, the œsophagus above the obstruction becomes enlarged into a kind of pouch; and the inner surface of this pouch begins to secrete juices that produce in the food a kind of rude digestion. Again, stricture of the intestine, when it arises gradually, is followed by hypertrophy of the muscular coat of the intestine above the constricted part: the ordinary peristaltic movements being insufficient to force the food forwards, and the lodged food serving as a constant stimulus to contraction, the muscular fibres, habitually more exercised, become more bulky. The deduction from general principles being thus inductively enforced, we cannot, I think, resist the conclusion that the direct actions and reactions between the food and the alimentary canal have been largely instrumental in establishing the contrasts among its parts. And we shall hold this view with the more confidence on observing how satisfactorily, in pursuance of it, we are enabled to explain one of the most striking of these differentiations, which we will take as a type of the class.

The gizzard of a bird is an expanded portion of the alimentary canal, specially fitted to give the food that trituration which the toothless mouth of a bird cannot give. Besides having a greatly-developed muscular coat, this grinding-chamber is lined with a thick, hard cuticle, capable of bearing the friction of the pebbles swallowed to serve as grindstones. This differentiation of the mucous coat into a ridged and tubercled layer of horny matter—a differentiation which, in the analogous organs of certain Mollusca, is carried to the extent of producing from this membrane cartilaginous plates, and even teeth—varies in birds of different kinds, according to their food. It is moderate in birds that feed on flesh and fish, and extreme in granivorous birds and others that live on hard substances. How does this immense modification of the alimentary canal originate? In the stomach of a mammal, the macerating and solvent actions are united with that triturating action which finishes what the teeth have mainly done; but in the bird, unable to masticate, these internal functions are specialized, and while the crop is the macerating chamber, the gizzard becomes a chamber adapted to triturate more effectually. This adaptation requires simply an exaggeration of certain structures and actions which characterize stomachs in general, and, in a less degree, alimentary canals throughout their whole lengths. The massive muscles of the gizzard are simply extreme developments of the muscular tunic, which is already considerably developed over the stomach, and incloses also the œsophagus and the intestine. The indurated lining of the gizzard, thickened into horny buttons at the places of severest pressure, is nothing more than a greatly strengthened and modified epithelium. And the grinding action of the gizzard is but a specialized form of that rhythmical contraction by which an ordinary stomach kneads the contained food, and which in the œsophagus effects the act of swallowing, while in the intestine it becomes the peristaltic motion. Allied as the gizzard thus clearly is in structure and action to the stomach and alimentary canal in general; and capable of being gradually differentiated from a stomach where a growing habit of swallowing food unmasticated entails more trituration to be performed before the food passes the pylorus; the question is—Does this change of structure arise by direct adaptation? There is warrant for the belief that it does. Besides such collateral evidence as that mucous membrane becomes horny on the toothless gums of old people, when subject to continual rough usage, and that the muscular coat of the intestine thickens where unusual activity is demanded of it, we have the direct evidence of experiment. Hunter habituated a sea-gull to feed on grain, and found that the lining of its gizzard became hardened, while the gizzard-muscles doubled in thickness. A like change in the diet of a kite was followed by like results. Clearly, if differentiations so produced in the individuals of a race under changed habits, are in any degree inheritable, a structure like a gizzard will originate through the direct actions and reactions between the food and the alimentary canal.

Another case—a very interesting one, somewhat allied to this—is presented by the ruminating animals. Here several dilatations of the alimentary canal precede the true stomach; and in them large quantities of unmasticated food are stored, to be afterwards returned to the mouth and masticated at leisure. What conditions have made this specialization advantageous? and by what process has it been established? To both these questions the facts indicate answers which are not unsatisfactory. [Creatures that obtain their food very irregularly—now having more than they can consume, and now being for long periods without any—must, in the first place, be apt, when very hungry, to eat to the extreme limits of their capacities; and must, in the second place, profit by peculiarities which enable them to compensate themselves for long fasts, past and future. A perch which, when its stomach is full of young frogs, goes on filling its œsophagus also; or a trout which, rising to the fisherman’s fly, proves when taken off the hook to be full of worms and insect-larvæ up to the very mouth, gains by its ability to take in such unusual supplies of food when it meets with them—obviously thrives better than it would do could it never eat more than a stomachful. That this ability to feed greatly in excess of immediate requirement, is one that varies in individuals of the same race, we see in the marked contrast between our own powers in this respect, and the powers of uncivilized men; whose fasting and gorging are to us so astonishing. Carrying with us these considerations, we shall not be surprised at finding dilatations of the œsophagus in vultures and eagles, which get their prey at long intervals in large masses; and we may naturally look for them, too, in birds like pigeons, which, coming in flocks upon occasional supplies of grain, individually profit by devouring the greatest quantity in a given time. Now where the trituration of the food is, as in these cases, carried on in a lower part of the alimentary canal, nothing further is required than the storing-chamber; but for a mammal, having its grinding apparatus in its mouth, to gain by the habit of hurriedly swallowing unmasticated food, it must also have the habit of regurgitating the food for subsequent mastication. This correlation of habits with their answering structures, may, as we shall see, arise in a very simple way. The starting point of the explanation is a familiar fact—the fact that indigestion, often resulting from excess of food, is apt to cause that reversed peristaltic action known as vomiting. From this we pass to the fact, also within the experience of most persons, that during slight indigestion the stomach sometimes quietly regurgitates a small part of its contents as far as the back of the mouth—giving an unpleasant acquaintance with the taste of the gastric juices. Exceptional facts of the same class help the argument a step further. “There are certain individuals who are capable of returning, at will, a greater or smaller portion of the contents of the digesting stomach into the cavity of the mouth.... In some of these cases, the expulsion of the food has required a violent effort. In the majority it has been easily evoked or suppressed. While in others, it has been almost uncontrollable; or its non-occurrence at the habitual time has been followed by a painful feeling of fulness, or by the act of vomiting.” Here we have a certain physiological action, occasionally happening in most persons and in some developed into a habit more or less pronounced: indigestion being the habitual antecedent. Suppose, then, that gregarious animals, living on innutritive food such as grass, are subject to a like physiological action, and are capable of like variations in the degree of it. What will naturally happen? They wander in herds, now over places where food is scarce and now coming to places where it is abundant. Some masticate their food completely before swallowing it, while some masticate it incompletely. If an oasis, presently bared by their grazing, has not supplied to the whole herd a full meal, then the individuals which masticate completely will have had less than those which masticate incompletely—will not have had enough. Those which masticate incompletely and distend their stomachs with food difficult to digest, will be liable to these regurgitations; but if they re-masticate what is thus returned to the mouth (and we know that animals often eat again what they have vomited), then the extra quantity of food taken, eventually made digestible, will yield them more nourishment than is obtained by those which masticate completely at first. The habit initiated in this natural way, and aiding survival when food is scarce, will be apt to cause modifications of the alimentary canal. We know that dilatations of canals readily arise under habitual distensions. We know that canals habitually distended become gradually more tolerant of the contained masses that at first irritated them. And we know that there commonly take place adaptive modifications of their surfaces. Hence if a habit of this kind and the structural changes resulting from it, are in any degree inheritable, it is clear that, increasing in successive generations, both immediately by the cumulative effect of repetitions and mediately by survival of the individuals in which they are most decided, they may go on until they end in the peculiarities which Ruminants display.

§ 298. There are structures belonging to the same group which cannot, however, be accounted for in this way. They are the organs that secrete special products facilitating digestion—the liver, pancreas, and various smaller glands. All these appendages of the alimentary canal, large and independent as some of them seem, really arise by differentiations from its coats. The primordial liver consists of nothing more than bile-cells scattered along a tract of the intestinal surface. Accumulation of these bile-cells is accompanied by increased growth of the surface which bears them—a growth which at first takes the form of a cul-de-sac, having an outside that projects from the intestine into the peri-visceral cavity. As the mass of bile-cells becomes greater, there arise secondary lateral cavities opening into the primary one, and through it into the intestine; until, eventually, these cavities with their coatings of bile-cells, become ramifying ducts distributed through the solid mass we know as a liver. How is this differentiation caused?

Before attempting any answer to this question, it is requisite to inquire the nature of bile. Is that which the liver throws into the intestines a waste product of the organic actions? or is it a secretion aiding digestion? or is it a mixture of these? Modern investigations imply that it is most likely the last. The liver is found to have a compound function. Bernard has proved to the satisfaction of physiologists, that there goes on in it a formation of glycogen—a substance which is transformed into sugar before it leaves the liver and is afterwards carried away by the blood to eventually disappear in the active organs, chiefly the muscles. It is also shown, experimentally, that there are generated in the liver certain biliary acids; and by the aid either of these or of some other compounds, it is clear that bile renders certain materials more absorbable. Its effect on fat is demonstrable out of the body; and the greatly diminished absorption of fat from the food when the discharge of bile into the intestine is prevented, is probably one of the causes of that pining away which results. But while recognizing the fact that the bile consists in part of a solvent, or solvents, aiding digestion, there is abundant evidence that one element of it is an effete product; and probably this is the primary element. The yellow-green substance called biliverdine in herbivora and bilirubin in man and carnivora, which gives its colour to bile, is a product the greater part of which is normally cast out from the system continually, as is shown by the contrast between the normal and abnormal colours of fæcal matters, and as is still more strikingly shown by the effects on the system when there is a stoppage of the excretion, and an attack of jaundice. Hence we are warranted in classing biliverdine as a waste product, and we may fairly infer that the excretion of it is the original function of the liver.

One further preliminary is requisite. We must for a moment return to those physico-chemical data set down in the first chapter of this work (§§ 7–8). We there saw that the complex and large-atomed colloids which mainly compose living organic matter, have extremely little molecular mobility; and, consequently, extremely little power of diffusing themselves. Whereas we saw not only that those absorbed matters, gaseous and liquid, which further the decomposition of living organic matter, have very high diffusibilities, but also that the products of the decomposition are much more diffusible than the components of living organic matter. And we saw that, as a consequence of this, the tissues give ready entrance to the substances which decompose them, and ready exit to the substances into which they are decomposed. Hence it follows that, under its initial form, uncomplicated by nervous and other agencies, the escape of effete matters from the organism, is a physical action parallel to that which goes on among mixed colloids and crystalloids that are dead or even inorganic. Excretion is a specialized form of this spontaneous action; and we have to inquire how the specialization arises.

Two causes conspire to establish it. The first is that these products of decomposition are diffusible in widely different degrees. While the carbonic acid and water permeate the tissues with ease in all directions, and escape more or less from the exposed surfaces, urea, and other waste substances incapable of being vaporized, cannot escape thus readily. The second is that the different parts of the body, being subject to different physical conditions, are from the outset sure severally to favour the exit of these various products of decomposition in various degrees. How these causes must have co-operated in localizing the excretions, we shall see on remembering how they now co-operate in localizing the separation of morbid materials. The characteristic substances of gout and rheumatism have their habitual places of deposit. Tuberculous matter, though it may be present in various organs, gravitates towards some much more than towards others. Certain products of disease are habitually got rid of by the skin, instead of collecting internally. Mostly, these have special parts of the skin which they affect rather than the rest; and there are those which, by breaking out symmetrically on the two sides of the body, show how definitely the places of their excretion are determined by certain favouring conditions, which corresponding parts may be presumed to furnish in equal degrees. Further, it is to be observed of these morbid substances circulating in the blood, that having once commenced segregating at particular places, they tend to continue segregating at those places. Assuming, then, as we may fairly do, that this localization of excretion, which we see continually commencing afresh with morbid matters, has always gone on with the matters produced by the waste of the tissues, let us take a further step, and ask how localizations become fixed. Other things equal, that which from its physical conditions is a place of least resistance to the exit of an effete product, will tend to become established as the place of excretion; since the rapid exit of an effete product will profit the organism. Other things equal, a place at which the excreted matter produces least detrimental effect will become the established place. If at any point the excreted matter produces a beneficial effect, then, other things equal, survival of the fittest will determine it to this point. And if facility of escape anywhere goes along with utilization of the escaping substance, then, other things equal, the excretion will be there localized still more decisively by survival of the fittest.

Such being the conditions of the problem, let us ask what will happen with the lining membrane of the alimentary canal. This, physiologically considered, is an external surface; and matters thrown off from it make their way out of the body. It is also a surface along which is moving the food to be digested. Now, among the various waste products continually escaping from the living tissues, some of the more complex ones, not very stable in composition, are likely, if added to the food, to set up changes in it. Such changes may either aid or hinder the preparation of the food for absorption. If an effete matter, making its exit through the wall of the intestine, hinders the digestive process, the enfeeblement and disappearance of individuals in which this happens, will prevent the intestine from becoming the established place for its exit. While if it aids the digestive process, the intestine will, for converse reasons, become more and more the place to which its exit is limited. Equally manifest is it that if there is one part of this alimentary canal at which, more than at any other part, the favourable effect results, this will become the place of excretion.

Thus, then, reverting to the case in question, we may understand how a product to be cast out, such as biliverdine, if it either directly or indirectly serves a useful purpose, when poured into a particular part of the intestine, may lead to the formation of a patch of excreting cells on its wall; and once this place of excretion having been established, the development of a liver is simply a question of time and natural selection.

§ 299. A differentiation of another order occurring in the alimentary canal, is that by which a part of it is developed into a lateral chamber or chambers, through which carbonic acid exhales and oxygen is absorbed. Comparative anatomy and embryology unite in showing that a lung is formed, just as a liver or other appendage of the alimentary canal is formed, by the growth of a hollow bud into the peri-visceral cavity, or space between the alimentary canal and the wall of the body. The interior of this bud is simply a cul-de-sac of the alimentary canal, with the mucous lining of which its own mucous lining is continuous. And the development of this cul-de-sac into an air-chamber, simple or compound, is merely a great extension of area in the internal surface of the cul-de-sac, along with that specialization which fits it for excreting and absorbing substances different from those which other parts of the mucous surface excrete and absorb. These lateral air-chambers, universal among the higher Vertebrata and very general among the lower, and everywhere attached to the alimentary canal between the mouth and the stomach, have not in all cases the respiratory function. In most fishes that have them they are what we know as swim-bladders. In some fishes the cavities of these swim-bladders are completely shut off from the alimentary canal: nevertheless showing, by the communications which they have with it during the embryonic stages, that they are originally diverticula from it. In other fishes there is a permanent ductus pneumaticus, uniting the cavity of the swim-bladder with that of the gullet: the function, however, being still not respiratory in an appreciable degree, if at all. But in certain still extant representatives of the sauroid fishes, as the Lepidosteus, the air-bladder is “divided into two sacs that possess a cellular structure,” and “the trachea which proceeds from it opens high up in the throat, and is surrounded with a glottis.” In the Amphibia the corresponding organs are chambers over the surfaces of which there are saccular depressions, indicating a transition towards the air-cells characterizing lungs; and accompanying this advance we see, as in the common Triton, the habit of coming up to the surface and taking down a fresh supply of air in place of that discharged.

How are the internal air-chambers, respiratory or nonrespiratory, developed? Upwards from the amphibian stage, in which they are partially refilled at long intervals, there is no difficulty in understanding how, by infinitesimal steps, they pass into complex and ever-moving lungs. But how is the differentiation that produces them initiated? How comes a portion of the internal surface to be specialized for converse with a medium to which it is not naturally exposed? The problem appears a difficult one; but there is a not unsatisfactory solution of it.

When many gold-fish are kept in a small aquarium, as with thoughtless cruelty they frequently are, they swim close to the surface, so as to breathe that water which is from instant to instant absorbing fresh oxygen. In doing this they often put their mouths partly above the surface, so that in closing them they take in bubbles of air; and sometimes they may be seen to continue doing this—the relief due to the slight extra aëration of blood so secured, being the stimulus to continue. Air thus taken in may be detained. If a fish that has taken in a bubble turns its head downwards, the bubble will ascend to the back of its mouth, and there lodge; and coming within reach of the contractions of the œsophagus, it may be swallowed. If, then, among fish thus naturally led upon occasion to take in air-bubbles, there are any having slight differences in the alimentary canal that facilitate lodgment of the air, or slight nervous differences such as in human beings cause an accidental action to become “a trick,” it must happen that if an advantage accrues from the habitual detention of air-bubbles, those individuals most apt to detain them will, other things equal, be more likely than the rest to survive; and by the survival of descendants inheriting their peculiarities in the greatest degrees, and increasing them, an established structure and an established habit may arise. And that they do in some way arise we have proof. The common Loach swallows air, which it afterwards discharges loaded with carbonic acid.

From air thus swallowed the advantages that may be derived are of two kinds. In the first place, the fish is made specifically lighter, and the muscular effort needed to keep it from sinking is diminished—or, indeed, if the bubble is of the right size, is altogether saved. The contrast between the movements of a Goby, which, after swimming up towards the surface, falls rapidly to the bottom on ceasing its exertions, and the movements of a Trout, which remains suspended just balancing itself by slight undulations of its fins, shows how great an economy results from an internal float, to fishes which seek their food in mid-water or at the surface. Hence the habit of swallowing air having been initiated in the way described, we see why natural selection will, in certain fishes, aid modifications of the alimentary canal favouring its lodgment—modifications constituting air-sacs. In the second place, while from air thus lodged in air-sacs thus developed, the advantage will be that of flotation only if the air is infrequently changed or never changed, the advantage will be that of supplementary respiration if the air-sacs are from time to time partially emptied and refilled. The requirements of the animal will determine which of the two functions predominates. Let us glance at the different sets of conditions under which these divergent modifications may be expected to arise.

The respiratory development is not likely to take place in fishes that inhabit seas or rivers in which the supply of aërated water never fails: there is no obvious reason why the established branchial respiration should be replaced by a pulmonic respiration. Indeed, if a fish’s branchial respiration is adequate to its needs, a loss would result from the effort of coming to the surface for air; especially during those first stages of pulmonic development when the extra aëration achieved was but small. Hence in fishes so circumstanced, the air-chambers arising in the way described would naturally become specialized mainly or wholly into floats. Their contained air being infrequently changed, no advantage would arise from the development of vascular plexuses over their surfaces; nothing would be gained by keeping open the communication between them and the alimentary canal; and there might thus eventually result closed chambers the gaseous contents of which, instead of being obtained from without, were secreted from their walls, as gases often are from mucous membranes. Contrariwise, aquatic vertebrates in which the swallowing of air-bubbles, becoming habitual, had led to the formation of sacs that lodged the bubbles; and which continued to inhabit waters not always supplying them with sufficient oxygen, might be expected to have the sacs further developed, and the practice of changing the contained air made regular, if either of two advantages resulted—either the advantage of being able to live in old habitats that had become untenable without this modification, or the advantage of being able to occupy new habitats. Now it is just where these advantages are gained that we see the pulmonic respiration coming in aid of the branchial respiration, and in various degrees replacing it. Shallow waters are liable to three changes which conspire to make this supplementary respiration beneficial. The summer’s sun heats them, and raising the temperatures of the animals they contain, accelerates the circulation in these animals, exalts their functional activities, increases the production of carbonic acid, and thus makes aëration of the blood more needful than usual. Meanwhile the heated water, instead of yielding to the highly carbonized blood brought to the branchiæ the usual quantity of oxygen, yields less than usual; for as the heat of the water increases, the quantity of air it contains diminishes. And this greater demand for oxygen joined with smaller supply, pushed to an extreme where the water is nearly all evaporated, is at last still more intensely felt in consequence of the excess of carbonic acid discharged by the numerous creatures congregated in the muddy puddles that remain. Here, then, it is, that the habit of taking in air-bubbles is likely to become established, and the organs for utilizing them developed; and here it is, accordingly, that we find all stages of the transition to aërial respiration. The Loach before-mentioned, which swallows air, frequents small waters liable to be considerably warmed. The Amphipnous Cuchia, an anomalous eel-shaped fish, which has vascular air-sacs opening out at the back of the mouth, “is generally found lurking in holes and crevices, on the muddy banks of marshes or slow-moving rivers”; and though its air-sacs are not morphological equivalents of those above described, yet they equally well illustrate the relation between such organs and the environing condition. Still more significant is the fact that the Lepidosiren, or “mudfish” as it is called from its habits, though it is a true fish nevertheless has lungs. But it is among the Amphibia that we see most conspicuously this relation between the development of air-breathing organs, and the peculiarities of the habitats. Pools, more or less dissipated annually, and so rendered uninhabitable by most fishes, are very generally peopled by these transitional types. Just as we see, too, that in various climates and in various kinds of shallow waters, the supplementary aërial respiration is needful in different degrees; so do we find among the Amphibia many stages in the substitution of the one respiration for the other. The facts, then, are such as give to the hypothesis a vraisemblance greater than could have been expected.

The relative effects of direct and indirect equilibration in establishing this further heterogeneity, must, as in many other cases, remain undecided. The habit of taking in bubbles is scarcely interpretable as a result of spontaneous variation: we must regard it as arising accidentally during the effort to obtain the most aërated water; as being persevered in because of the relief obtained; and as growing by repetition into a tendency bequeathed to offspring, and by them, or some of them, increased and transmitted. The formation of the first slight modifications of the alimentary canal favouring the lodgment of bubbles, is not to be thus explained. Some favourable variation in the shape of the passage must here have been the initial step. But the gradual increase of this structural modification by the survival of individuals in which it is carried furthest, will, I think, be all along aided by immediate adaptation. The part of the alimentary canal previously kept from the air, but now habitually in contact with the air, must be in some degree modified by the action of the air; and the directly-produced modification, increasing in the individual and in successive individuals, cannot cease until there is a complete balance between the actions of the changed agency and the changed tissue.

§ 300. We come now to differentiations among the truly inner tissues—the tissues which have direct converse neither with the environment nor with the foreign substances taken into the organism from the environment. These, speaking broadly, are the tissues which lie between the double layer forming the integument with its appendages, and the double layer forming the alimentary canal with its diverticula. We will take first the differentiation which produces the vascular system.

Certain forces producing and aiding distribution of liquids in animals, come into play before any vascular system exists; and continue to further circulation after the development of a vascular system. The first of these is osmotic exchange, acting locally and having an indirect general action; the second is local variation of pressure, which movement of the body throws on the tissues and their contained liquids. A few words are needed in elucidation of each. If in any creature, however simple, different changes are going on in parts that are differently conditioned—if, as in a Hydra, one surface is exposed to the surrounding medium while the other surface is exposed to dissolved food; then between the unlike liquids which the dissimilarly-placed parts contain, osmotic currents must arise; and a movement of liquid through the intermediate tissue must go on as long as an unlikeness between the liquids is kept up. This primary cause of re-distribution remains one of the causes of re-distribution in every more-developed organism: the passage of matters into and out of the capillaries is everywhere thus set up. And obviously in producing these local currents, osmose must also indirectly produce general currents, or aid them if otherwise produced. In the absence of a pumping organ, this force is probably an important aid to that movement of the nutritive liquids which the functions set up. How the second cause—the changes of internal pressure which an animal’s movements produce—furthers circulation, will be sufficiently manifest. That parts which are bent or strained necessarily have their contained vessels squeezed, has been shown (§ 281); and whether the bend or strain is caused, as in a plant, by an external force, or, as usually in an animal, by an internal force, there must be a thrusting of liquids towards places of least resistance—commonly places of greatest consumption. This which in animals without hearts is a main agent of circulation, continues to further it very considerably even among the highest animals. In these the effect becomes as it were systematized. The valves in the veins necessitate perpetual propulsions towards the heart.

Even in such simple types as the Hydrozoa, cavities in the tissues faintly indicate a structure which facilitates the transfer of nutritive matters. These cavities become reservoirs filled with the plasma that slowly oozes through the substance of the body; and every movement of the animal, accompanied as it must be by changed pressures and tensions on these reservoirs, tends here to fill them and there to squeeze out their contents in that or the other direction—possibly aiding to produce, by union of several cavities, those lacunæ or irregular canals which the body in some cases presents.

Irregular canals of this kind, not lined with any membranes but being simply cavities running through the flesh, mainly constitute the vascular system in Polyzoa and Brachiopoda and some Mollusca. Though the central parts of a vascular system are rudely developed, yet its peripheral parts consist of sinuses permeating the tissues. The higher orders of Mollusca have a more-developed system of vessels or arteries, which run into the substance of the body and end in lacunæ or simple fissures. This ending in lacunæ takes place at various distances from the vascular centre. In some genera the arterial structure is carried to the periphery of the blood-system, while in others it stops short midway. Throughout most orders of the Mollusca the back current of blood continues to be carried by channels of the original kind: there are no true veins, but the blood having been delivered into the tissues, finds its way back to the peri-visceral cavity through inosculating sinuses. Among the Cephalopods, however, the afferent blood-canals, as well as the efferent ones, acquire distinct walls. On putting together these facts, we may conceive pretty clearly the stages of vascular development. From the original reservoir of nutritive liquid between the alimentary canal and the wall of the body, a portion partially shut off becomes a contractile vessel; and by its actions there is produced a more rapid transfer of the nutritive liquid than was originally produced by the motions of the animal. Clearly, the extension of this contractile tube and the development from it of branches running hither and thither into the tissues, must, by defining the channels of blood throughout a part of its course, render its distribution more regular and active. As fast as this centrifugal growth advances, so fast are the efferent currents of blood, prevented from escaping laterally, obliged to move from the centre towards the circumference; and so fast also does the less developed set of channels become, of necessity, occupied by afferent currents. When, by a parallel increase of definiteness, the lacunæ and irregular sinuses through which the afferent currents pass, become transformed into veins, the accompanying disappearance of all stagnant or slow-moving collections of blood, implies a further improvement in the circulation.

By what agency is effected this differentiation of a definite vascular system? No sufficient reply is obvious. The genesis of the primordial heart is not comprehensible as a result of direct equilibration, and we cannot readily see our way to it as a result of indirect equilibration; for it is difficult to imagine what favourable variation natural selection could have seized hold of to produce such a structure. A contractile tube that aided the distribution of nutritive liquid, having been once established, survival of the fittest would suffice for its gradual extension and its successive modifications. But what were the early stages of the contractile tube, while it was yet not sufficiently formed to help circulation, and while it must nevertheless have had some advantage without which no selective process could go on? The question seems insoluble. To another part of the question, however, an answer may be ventured. If we ask the origin of these ramifying channels which, first appearing as simple lacunæ, eventually become vessels having definite walls, a reply admitting of considerable justification, is, that the currents of nutritive liquid forced and drawn hither and thither through the tissues, themselves initiate these channels. We know that streams running over and through solid and quasi-solid inorganic matter, tend to excavate definite courses. We saw reason for concluding that the development of sap-channels in plants conforms to this general principle. May we not then suspect that the nutritive liquid contained in the tissue of a simple animal, made to ooze now in this direction and now in that by the changes of pressure which the animal’s movements cause, comes to have certain lines along which it is thrust backwards and forwards more than along other lines; and must by repeated passings make these more and more permeable until they become lacunæ? Such actions will inevitably go on; and such actions appear competent to produce some, at least, of the observed effects. The leading facts which indicate that this is a part-cause of vascular development are these.

Growths normally recurring in certain places at certain intervals, are accompanied by local formations of blood-vessels. The periodic maturation of ova among the Mammalia supplies an instance. Through the stroma of an ovarium are distributed innumerable minute vesicles, which, in their early stages, are microscopic. Of these, severally contained in their minute ovi-sacs, any one may develop: the determining cause being probably some slight excess of nutrition. When the development is becoming rapid, the capillaries of the neighbouring stroma increase and form a plexus on the walls of the ovi-sac. Now since there is no typical distribution of the developing ova; and since the increase of an ovum to a certain size precedes the increase of vascularity round it; we can scarcely help concluding that the setting up of currents towards the point of growth determines the formation of the blood-vessels. It may be that having once commenced, this local vascular structure completes itself in a typical manner; but it seems clear that this greater development of blood-vessels around the growing ovum is initiated by the draught towards it. Abnormal growths show still better this relation of cause and effect. The false membranes sometimes found in the bronchial tubes in inflammatory diseases, may perhaps fairly be held abnormal in but a partial sense: it may be said that their vascular systems are formed after the type of the membranes to which they are akin. But this can scarcely be said of the morbid growths classed as malignant. The blood-vessels in an encephaloid cancer, are led to enlarge and ramify, often to an immense extent, by the unfolding of the morbid mass to which they carry blood. Alien as is the structure as a whole to the type of the organism; and alien in great measure as is its tissue to the tissue on which it is seated; it nevertheless happens that the growth of the alien tissue and accompanying abstraction of materials from the blood-vessels, determine a corresponding growth of these blood-vessels. Unless, then, we say that there is a providentially-created type of vascular structure for each kind of morbid growth (and even this would not much help us, since the vascular structure has no constancy within the limits of each kind), we are compelled to admit that in some way or other the currents of blood are here directly instrumental in forming their own channels. One more piece of evidence, before cited as exemplifying adaptation (§ 67), may be called to mind. When any main channel for blood, leading to or from a certain part of the body, has been rendered impervious, others among the channels leading to or from this same part, enlarge to the extent requisite for fulfilling the extra function that falls upon them: the enlargement being caused, as we must infer, by the increase of the currents carried.

Here, then, are facts warranting inductively the deduction above drawn. It is true that we are left in the dark respecting the complexities of the process. How the channels for blood come to have limiting membranes, and many of them muscular coats, the hypothesis does not help us to say. But the evidence assigned goes far to warrant the belief that vascular development is initiated by direct equilibration; though indirect equilibration may have had the larger share in establishing the structures which distinguish finished vascular systems.

§ 301. Of the inner tissues which remain let us next take bone. In what manner is differentiated this dense substance serving in most cases for internal support?

When considering the vertebrate skeleton under its morphological aspect (§ 256), it was pointed out that the formation of dense tissues, internal as well as external, is, in some cases at least, brought about by the mechanical forces to be resisted. Through what process it is brought about we could not then stay to inquire: this question being not morphological but physiological. Answers to some kindred questions have since been attempted. Certain actions to which the internal dense tissues of plants may be ascribed, have been indicated; and more recently, analogous actions have been assigned as causes of some external dense tissues of animals. We have now to ask whether actions of the same nature have produced these internal dense tissues of animals.

The problem is an involved one. Bones have more than one stage. They are membranous or cartilaginous before they become osseous; and their successive component substances so far differ that the effects of mechanical actions upon them differ. And having to deal with transitional states in which bone is formed of mixed tissues, having unlike physical properties and unlike minute structures, the effects of strains become too complicated to follow with precision. Anything in the way of interpretation must therefore be regarded as tentative. If analysis and comparison show that the phenomena are not inconsistent with the hypothesis of mechanical genesis, it is as much as can be expected. Let us first observe more nearly the mechanical conditions to which bones are subject.

The endo-skeleton of a mammal with the muscles and ligaments holding it together, may be rudely compared to a structure built up of struts and ties; of which, speaking generally, the struts bear the pressures and the ties bear the tensions. The framework of an ordinary iron roof will give an idea of the functions of these two elements, and of the mechanical characters required by them. Such a framework consists partly of pieces which have each to bear a thrust in the direction of its length, and partly of pieces which have each to bear a pull in the direction of its length; and these struts and ties are differently formed to adapt them to these different strains. Further, it should be remarked that though the rigidity of the framework depends on the ties which are flexible, as much as on the struts which are stiff, yet the ties help to give the rigidity simply by so holding the struts in position that they cannot escape from the thrusts which fall on them. Now the like relation holds with a difference among the bones and muscles: the difference being that here the ties admit of being lengthened or shortened and the struts of being moved about upon their joints. The mechanical relations are not altered by this, however. The actions are of essentially the same kind in an animal that is standing, or keeping itself in a strained attitude, as in one that is changing its attitude—the same in so far that we have in each a set of flexible parts that are pulling and a set of rigid parts that are resisting. It needs but to remember the sudden collapse and fall which take place when the muscles are paralyzed, or to remember the inability of a bare skeleton to support itself, to see that the struts without the ties cannot suffice. And we have but to think of the formless mass into which a man would sink when deprived of his bones, to see that the ties without the struts cannot suffice. To trace the way in which a particular bone has its particular thrust thrown upon it, may not always be practicable. Though it is easy to perceive how a flexor or extensor of the arm causes by its tension a reactive pressure along the line of the humerus, and is enabled to produce its effect only by the rigidity of the humerus; yet it is not so easy to perceive how such bones as those of a horse’s pelvis are similarly acted upon. Still, as the weight of the hind quarters has to be transferred from the back to the feet, and must be so transferred through the bones, it is manifest that though these bones form a very crooked line, the weight must produce a pressure along the axis of each: the muscles and ligaments concerned serving here, as in other cases, so to hold the bones that they bear the pressure instead of being displaced by it. Not forgetting that many processes of the bones have to bear tensions, we may then say that generally, though by no means universally, bones are internal dense masses that have to bear pressures—pressures which in the cylindrical bones become longitudinal thrusts. Leaving out exceptional cases, let us consider bones as masses thus circumstanced.

When giving reasons for the belief that the vertebrate skeleton is mechanically originated, one of the facts put in evidence was, that in the vertebrate series the transition from the cartilaginous to the osseous spine begins peripherally (§ 257): each vertebra being at first a ring of bone surrounding a mass of cartilage. And it was pointed out that this peripheral ossification is ossification at the region of greatest pressures. Now it is not vertebræ only that follow this course of development. In a cylindrical bone, though it is differently circumstanced, the places of commencing ossification are still the places on which the severest stress falls. Let us consider how such a bone that has to bear a longitudinal pressure is mechanically affected. If the end of a walking-cane be thrust with force against the ground, the cane bends; and partially resuming its straightness when relieved, again bends, usually towards the same side, when the thrust is renewed. A bend so caused acts on the fibres of the cane in nearly the same way as does a bend caused by supporting the cane horizontally at its two ends and suspending a weight from its middle. In either case the fibres on the convex side are extended and the fibres on the concave side compressed. Kindred actions occur in a rod that is so thick as not to yield visibly under the force applied. In the absence of complete homogeneity of its substance, complete symmetry in its form, and an application of a force exactly along its axis, there must be some lateral deflection; and therefore some distribution of tensions and pressures of the kind indicated. And then, as the fact which here specially concerns us, we have to note that the strongest tensions and pressures are borne by the outer layers of fibres. Now the shaft of a long bone, subject to mechanical actions of this kind, similarly has its outer layer most strained. In this layer, therefore, on the mechanical hypothesis, ossification should commence, and here it does commence—commences, too, midway between the ends, where the bends produce on the superficial parts their most intense effects. But we have not in this place simply to observe that ossification commences at the places of greatest stress, but to ask what causes it to do this. Can we trace the physical actions which set up this deposit of dense tissue? It is, I think, possible to indicate a “true cause” that is at work; though whether it is a sufficient cause may be questioned. We concluded that in certain other cases, the formation of dense tissue indirectly results from the alternate squeezing and relaxation of the vessels running through the part; and the inquiry now to be made is, whether, in developing bone, the same actions go on in such ways as to produce the observed effects. At the outset we are met by what seems a fatal difficulty—cartilage is a non-vascular tissue: this substance of which unossified bones consist is not permeated by minute canals carrying nutritive liquid, and cannot, therefore, be a seat of actions such as those assigned. This apparent difficulty, however, furnishes a confirmation. For cartilage that is wholly without permeating canals does not ossify: ossification takes place only at those parts of it into which the canals penetrate. Hence, we get additional reason for suspecting that bone-formation is due to the alleged cause; since it occurs where mechanical strains can produce the actions described, but does not occur where mechanical strains cannot produce them. Let us consider more closely what the several factors are. It will suffice for the argument if we commence with the external vascular layer as already existing, and consider what will take place in it. Cartilage is elastic—is somewhat extensible, and spreads out laterally under pressure, but resumes its form when relieved. How, then, will the minute channels traversing it in all directions be affected at the places where it is strained by a bend? Those on the convex side will be laterally squeezed, in the same way that we saw the sap-vessels on the convex side of a bent branch are squeezed; and as exudation of the sap into the adjacent prosenchyma will be caused in the one case, so, in the other, there will be caused exudation of serum into the adjacent cartilage: extra nutrition and increase of strength resulting in both cases. The parallel ceases here, however. In the shoot of a plant, bent in various directions by the wind, the side which was lately compressed is now extended; and hence that squeezing of the sap-vessels which results from extension, suffices to feed and harden the tissue on all sides of the shoot. But it is not so with a bone. Having yielded on one side under longitudinal pressure, and resumed as nearly as may be its previous shape when the pressure is taken off, the bone yields again towards the same side when again longitudinally pressed. Hence the substance of its concave side, never rendered convex by a bend in the opposite direction, would not receive any extra nutrition did no other action come into play. But if we consider how intermittent pressures must act on cartilage, we shall see that there will result extra nutrition of the concave side also. Squeeze between two pieces of glass a thin bit of caoutchouc which has a hole through it. While the caoutchouc spreads out away from the centre, it also spreads inwards, so as partially to close the hole. Everywhere its molecules move away in directions of least resistance; and for those near the hole, the direction of least resistance is towards the hole. Let this hole stand for the transverse section of one of the minute canals or channels passing through cartilage, and it will be manifest that on the side of the unossified bone made concave in the way described, the compressed cartilage will squeeze the canals traversing it; and, in the absence of perfect homogeneity in the cartilage, the squeeze will cause extra exudation from the canals into the cartilage. Thus every additional strain will give to the cartilage it falls upon, an additional supply of the materials for growth. So that presently the side which, by yielding more than any other, proves itself to be the weakest, will cease to be the weakest. What further will happen? Some other side will yield a little—the bends will take place in some other plane; and the portions of cartilage on which repeated tensions and pressures now fall will be strengthened. Thus the rate of nutrition, greatest at the place where the bending is greatest, and changing as the incidence of forces changes, will bring about at every point a balance between the resistances and the strains. Thus, too, there will be determined that peripheral induration which we see in bones so circumstanced. As in a shoot we saw that the woody deposit takes place towards the outside of the cylinder, where, according to the hypothesis, it ought to take place; so, here, we see that the excess of exudation and hardening, occurring where the strains are most intense, will form a cylinder having a dense outside and a porous or hollow inside. These processes will be essentially the same in bones subject to more complex mechanical actions, such as sundry of the flat bones and others that serve as internal fulcra. Be the strains transverse or longitudinal, be they torsion strains or mixed strains, the outer parts of the bone will be more affected by them than its inner parts. They will therefore tend everywhere to produce resisting masses having outer parts more dense than their inner parts. And by causing most growth where they are most intense, they will call out reactive forces adequate to balance them. There are doubtless obstacles in the way of this interpretation. It may be said that the forces acting on the outer layers in the manner described, would compress the canals too little to produce the alleged effects; and if evenly distributed along the whole lengths of the layers, they would probably do so. But it needs only to bend a flexible mass and observe the tendency to form creases on the concave surface, to feel assured that along the surface of an ossifying bone, the yielding of the tissue when bent will not be uniform. In the absence of complete homogeneity, the interstitial yielding will take place at some points more than others, and at one point above all others. When, at the weakest point—the centre of commencing ossification—an extra amount of deposit has been caused, it will cease to be the weakest; and adjacent points, now the weakest, will become the places of yielding and induration. It may be further objected that the hypothesis is incompatible with the persistence of cartilage for so long a time between the epiphysis of bones and the bony masses which they terminate. But there is the reply that the places occupied by this cartilage being places at which the bone lengthens, the non-ossification is in part apparent only—it is rather that new cartilage is formed as fast as the pre-existing cartilage ossifies; and there is the further reply that the slowness of the ultimate ossification of this part, is due to its non-vascularity, and to mechanical conditions which are unfavourable to its acquirement of vascularity. Once more, there is the demurrer that in the epiphyses ossification does not begin at the surface but within the mass of the cartilage. Explanation of this implies ability to follow out the mechanical actions in a resilient substance which, like india-rubber, admits of being distorted in all ways by pressure and recovering its form, and it seems impossible to say how the more superficial and more deep-seated canals traversing it will be respectively affected.

Of course it is not meant that this osseous development by direct equilibration takes place in the individual. Though it is a corollary from the argument that in each individual the process must be furthered and modified by the particular actions to which the particular bones are exposed; yet the leading traits of structure assumed by the bones are assumed in conformity with the inherited type. This, however, is no difficulty. The type itself is to be regarded as the accumulated result of such modifications, transmitted and increased from generation to generation. The actions above described as taking place in the bone of an individual, must be understood as producing their total effect little by little in the corresponding bones of a long series of individuals. Even if but a small modification can be so wrought in the individual, yet if such modification, or a part of it, is inheritable, we may readily understand how, in the course of geologic epochs, the observed structures may arise in the assigned way.

Here may fitly be added a strong confirmation. If we find cases where individual bones, subject in exceptional degrees to the actions described, present in exceptional amounts the modifications attributed to them, we are greatly helped in understanding how there may be produced in the race that aggregate of modifications which the hypothesis implies. Such cases occur in ricketty children. I am indebted to Mr. Busk for pointing out these abnormal formations of dense tissue, that are not apparently explicable as results of mechanical actions and reactions. It was only on tracing out the processes here at work, that there suggested itself the specific interpretation of the normal process, as above set forth. When, from constitutional defect, bones do not ossify with due rapidity, and are meanwhile subject to the ordinary strains, they become distorted. Remembering how a mass which has been made to yield in any direction by a force it cannot withstand, is some little time before it recovers completely its previous form, and usually, indeed, undergoes what is called a “permanent set;” it is inferable that when a bone is repeatedly bent at the same time that the liquid contained in its canals is poor in the materials for forming dense tissue, there will not take place a proportionate strengthening of the parts most strained; and these parts will give way. This happens in rickets. But this having happened, there goes on what, in teleological language, we call a remedial process. Supposing the bone to be one commonly affected—a femur; and supposing a permanent bend to have been caused in it by the weight of the body; the subsequent result is an unusual deposition of cartilaginous and osseous matter on the concave side of the bone. If the bone is represented by a strung bow, then the deposit occurs at the part represented by the space between the bow and the string. And thus occurring where its resistance is most effective, it increases until the approximately-straight piece of bone formed within the arc, has become strong enough to bear the pressure without appreciably yielding. Now this direct adaptation, seeming so like a special provision, and furnishing so remarkable an instance of what, in medical but unscientific language, is called the vis medicatrix naturæ, is simply a result of the above-described mechanical actions and reactions, going on under the exceptional conditions. Each time such a bent bone is subject to a force which again bends it, the severest compression falls on the substance of its concave side. Each time, then, the canals running through this part of its substance are violently squeezed—far more squeezed than they or any other of the canals would have been, had the bone remained straight. Hence, on every repetition of the strain, these canals near the concave surface have their contents forced out in more than normal abundance. The materials for the formation of tissue are supplied in quantity greater than can be assimilated by the tissue already formed; and from the excess of exuded plasma, new tissue arises.[50] A layer of organizable material accumulates between the concave surface and the periosteum; in this, according to the ordinary course of tissue-growth, new vessels appear; and the added layer presently assumes the histological character of the layer from which it has grown. What next happens? This added layer, further from the neutral axis than that which has thrown it out, is now the most severely compressed, and its vessels are the most severely squeezed. The place of greatest exudation and most rapid deposit of matter, is therefore transferred to this new layer; and at the same time that active nutrition increases its density, the excess of organizable material forms another layer external to it: the successive layers so added, encroaching on the space between the concave surface of the bone and the chord of its arc. What limits the encroachment on this space?—what stops the process of filling it up? The answer to this question will be manifest when observing that there comes into play a cause which gradually diminishes the forces falling on each new layer. For the transverse sectional area is step by step increased; and an increase of the area over which the weight borne is distributed, implies a relatively smaller pressure upon each part of it. Further, as the transverse dimensions of the bone increase, the materials composing its convex and concave layers, becoming further from the neutral axis, become better placed for resisting the strains to be borne. So that both by the increased quantity of dense matter and by its mechanically more-advantageous position, the bendings of the bone are progressively decreased. But as they are decreased, each new layer formed on the concave surface has its substance and its vessels less compressed; and the resulting growth and induration are rendered less rapid. Evidently, then, the additions, slowly diminishing, will eventually cease; and this will happen when the bone no longer bends. That is to say, the thickening of the bone will reach its limit when there is equilibrium between the incident forces and the forces which resist them. Here, indeed, we may trace with great clearness the process of direct equilibration—may see how an unusual force, falling on the moving equilibrium of an organism and not overthrowing it, goes on working modifications until the reaction balances the action.

That, however, which now chiefly concerns us, is to note how this marked adaptation supports the general argument. Unquestionably bone is in this case formed under the influence of mechanical stress, and formed just where it most effectually meets the stress. This result, not otherwise explained, is explained by the hypothesis above set forth. And when we see that this special deposit of bone is accounted for by actions like those to which bone-formation in general is ascribed, the probability that these are the actions at work becomes very great.[51]

Of course it is not alleged that osseous structures arise in this way alone. The bones of the skull and various dermal bones cannot be thus interpreted. Here the natural selection of favourable variations appears the only assignable cause—the equilibration is indirect. We know that ossific deposits now and then occur in tissues where they are not usually found; and such deposits, originally abnormal, if they occurred in places where advantages arose from them, might readily be established and increased by survival of the fittest. Especially might we expect this to happen when a constitutional tendency to form bone had been established by actions of the kind described; for it is a familiar fact that differentiated types of tissue, having once become elements of an organism, are apt occasionally to arise in unusual places, and there to repeat all their peculiar histological characters. And this may possibly be the reason why the bones of the skull, though not exposed to forces such as those which produce, in other bones, dense outer layers including less dense interiors, nevertheless repeat this general trait of bony structure. While, however, it is beyond doubt that some bones are not due to the direct influence of mechanical stress, we may, I think, conclude that mechanical stress initiates bone-formation.

§ 302. What is the origin of nerve? In what way do its properties stand related to the properties of that protoplasm whence the tissues in general arise? and in what way is it differentiated from protoplasm simultaneously with the other tissues? These are profoundly interesting questions; but questions to which positive answers cannot be expected. All that can be done is to indicate answers which seem feasible.

That the property specially displayed by nerve, is a property which protoplasm possesses in a lower degree, is manifest. The sarcode of a Rhizopod and the substance of an unimpregnated ovum, exhibit movements that imply a propagation of stimulus from one part of the mass to another. We have not far to seek for a probable origin of this phenomenon. There is good reason for ascribing it to the extreme instability of the organic colloids of which protoplasm consists. These, in common with colloids in general, assume different isomeric forms with great facility; and they display not simply isomerism but polymerism. Further, this readiness to undergo molecular re-arrangement, habitually shows itself in colloids by the rapid propagation of the re-arrangement from part to part. As Prof. Graham has shown, matter in this state often “pectizes” almost instantaneously—a touch will transform an entire mass. That is to say, the change of molecular state once set up at one end, spreads to the other end—there is a progress of a stimulus to change; and this is what we see in a nerve. So much being understood, let us re-state the case more completely.

Molecular change, implying as it does motion of molecules, communicates motion to adjacent molecules; be they of the same kind or of a different kind. If the adjacent molecules, either of the same kind or of a different kind, be stable in composition, a temporary increase of oscillation in them as wholes, or in their parts, may be the only result; but if they are unstable there are apt to arise changes of arrangement among them, or among their parts, of more or less permanent kinds. Especially is this so with the complex molecules which form colloidal matter, and with the organic colloids above all. Hence it is to be inferred that a molecular disturbance in any part of a living animal, set up by either an external or internal agency, will almost certainly disturb and change some of the surrounding colloids not originally implicated—will diffuse a wave of change towards other parts of the organism: a wave which will, in the absence of perfect homogeneity, travel further in some directions than in others. Let us ask next what will determine the differences of distance travelled in different directions. Obviously any molecular agitation spreading from a centre, will go furthest along routes that offer least resistance. What routes will these be? Those along which there lie most molecules that are easily changed by the diffused molecular motion, and which yet do not take up much molecular motion in assuming their new states. Molecules which are tolerably stable will not readily propagate the agitation; for they will absorb it in the increase of their own oscillations, instead of passing it on. Molecules which are unstable but which, in assuming isomeric forms, absorb motion, will not readily propagate it; since it will disappear in working the changes in them. But unstable molecules which, in being isomerically transformed, do not absorb motion, and still more those which, in being so transformed, give out motion, will readily propagate any molecular agitation; since they will pass on the impulse either undiminished, or increased, to adjacent molecules. If then we assume, as we are not only warranted in doing but are obliged to do, that protoplasm contains two or more colloids, either mingled or feebly combined (since it cannot consist of simple albumen or fibrin or casein, or any allied proximate principle); it may be concluded that any molecular agitation set up by what we call a stimulus, will diffuse itself further along some lines than along others, if the components of the protoplasm are not quite homogeneously dispersed, and if some of them are isomerically transformed more easily, or with less expenditure of motion, than others; and it will especially travel along spaces occupied chiefly by those molecules which give out molecular motion during their metamorphoses, if there should be any such. But now let us ask what structural effects will be wrought along a tract traversed by this wave of molecular disturbance. As is shown by those transformations which so rapidly propagate themselves through colloids, molecules that have undergone a certain change of form, are apt to communicate a like change of form to adjacent molecules of the same kind—the impact of each overthrow is passed on and produces another overthrow. Probably the proneness towards isochronism of molecular movements necessitates this. If any molecule has had its components re-arranged, and their oscillations consequently altered, there result movements not concordant with the movements in adjacent untransformed molecules, but which, impressing themselves on the parts of such untransformed molecules, tend to generate in them concordant movements—tend, that is, to produce the re-arrangements involved by these concordant movements. Is this action limited to strictly isomeric substances? or may it extend to substances that are closely-allied? If along with the molecules of a compound colloid there are mingled those of some kindred colloid; or if with the molecules of this compound colloid there are mingled the components out of which other such molecules may be formed; then there arises the question—does the same influence which tends to propagate the isomeric transformations, tend also to form new molecules of the same kind out of the adjacent components? There is reason to suspect that it does. Already when treating of the nutrition of parts (§ 64), it was pointed out that we are obliged to recognize a power possessed by each tissue to build up, out of the materials brought to it, molecules of the same type as those of which it is formed. This building up of like molecules seems explicable as caused by the tendency of the new components which the blood supplies, to acquire movements isochronous with those of the like components in the tissue; which they can do only by uniting into like compound molecules. Necessarily they must gravitate towards a state of equilibrium; such state of equilibrium—moving equilibrium of course—must be one in which they oscillate in the same times with neighbouring molecules; and so to oscillate they must fall into groups identical with the groups around them. If this be a general principle of tissue-growth and repair, we may conclude that it will apply in the case before us. A wave of molecular disturbance passing along a tract of mingled colloids closely-allied in composition, and isomerically transforming the molecules of one of them, will be apt at the same time to form some new molecules of the same type, at any place where there exist the proximate components, either uncombined or feebly combined in some not very different way. And this will be most likely to occur where the molecules of the colloid that are undergoing the isomeric change, predominate, but have scattered through them the other molecules out of which they may be formed, either by composition or modification. That is to say, a wave of molecular disturbance diffused from a centre, and travelling furthest along a line where lie most molecules that can be isomerically transformed with facility, will be likely at the same time to further differentiate this line, and make it more characterized than before by the easy-transformability of its molecules. One additional step, and the interpretation is reached. Analogy shows it to be not improbable that these organic colloids, isomerically transformed by slight molecular impact or increase of molecular motion, will some of them resume their previous molecular structures after the disturbance has passed. We know that what are stable molecular arrangements under one degree of molecular agitation, are not stable under another degree; and there is evidence that re-arrangements of an inconspicuous kind are occasionally brought about by very slight changes of molecular agitation. Water supplies a clear case. Prof. Graham infers that water undergoes a molecular re-arrangement at about 32°—that ice has a colloid form as well as a crystalloid form, dependent on temperature. Send through it an extra wave of the molecular agitation we call heat, and its molecules aggregate in one way. Let the wave die away, and its molecules resume their previous mode of aggregation. And obviously such transformations may be repeated backwards and forwards within narrow limits of temperature. Now among the extremely unstable organic colloids, such a phenomenon is far more likely to happen. Suppose, then, that the nerve-colloid is one of which the molecules are changed in form by a passing wave of extra agitation, but resume their previous form when the wave has passed: the previous form being the most stable under the conditions which then recur. What follows? It follows that these molecules will be ready again to undergo isomeric transformation when there again occurs the stimulus; will, as before, propagate the transformation most along the tract where such molecules are most abundant; will, as before, tend to form new molecules of their own type; will, as before, make the line along which they lie one of easier transfer for the molecular agitation. Every repetition will help to increase, to integrate, to define more completely, the course of the escaping molecular motion—extending its remoter part while it makes its nearer part more permeable—will help, that is, to form a line of discharge, a line for conducting impressions, a nerve.