[Footnote 2: Since this was written (in 1857) the advance of paleontological discovery, especially in America, has shown conclusively, in respect of certain groups of vertebrates, that higher types have arisen by modifications of lower; so that, in common with others, Prof. Huxley, to whom the above allusion is made, now admits, or rather a.s.serts, biological progression, and, by implication, that there have arisen more heterogeneous organic forms and a more heterogeneous a.s.semblage of organic forms.]

[Footnote 3: For detailed proof of these a.s.sertions see essay on "Manners and Fashion."]

[Footnote 4: The argument concerning organic evolution contained in this paragraph and the one preceding it, stands verbatim as it did when first published in the _Westminster Review_ for April, 1857. I have thus left it without the alteration of a word that it may show the view I then held concerning the origin of species. The sole cause recognized is that of direct adaptation of const.i.tution to conditions consequent on inheritance of the modifications of structure resulting from use and disuse. There is no recognition of that further cause disclosed in Mr.

Darwin's work, published two and a half years later--the indirect adaptation resulting from the natural selection of favourable variations. The multiplication of effects is, however, equally ill.u.s.trated in whatever way the adaptation to changing conditions is effected, or if it is effected in both ways, as I hold. I may add that there is indicated the view that the succession of organic forms is not serial but proceeds by perpetual divergence and re-divergence--that there has been a continual "divergence of many races from one race": each species being a "root" from which several other species branch out; and the growth of a tree being thus the implied symbol.]

[Footnote 5: "Personal Narrative of the Origin of the Caoutchouc, or India-Rubber Manufacture in England." By Thomas Hanc.o.c.k.]

TRANSCENDENTAL PHYSIOLOGY.

[_First published in_ The National Review _for October,_ 1857_, under the t.i.tle of "The Ultimate Laws of Physiology". The t.i.tle "Transcendental Physiology", which the editor did not approve, was restored when the essay was re-published with others in_ 1857.]

The t.i.tle Transcendental Anatomy is used to distinguish that division of biological science which treats, not of the structures of individual organisms considered separately, but of the general principles of structure common to vast and varied groups of organisms,--the unity of plan discernible throughout mult.i.tudinous species, genera, and orders, which differ widely in appearance. And here, under the head of Transcendental Physiology, we purpose putting together sundry laws of development and function which hold not of particular kinds or cla.s.ses of organisms, but of all organisms: laws, some of which have not, we believe, been hitherto enunciated.

By way of un.o.btrusively introducing the general reader to biological truths of this cla.s.s, let us begin by noticing one or two with which he is familiar. Take first, the relation between the activity of an organ and its growth. This is a universal relation. It holds, not only of a bone, a muscle, a nerve, an organ of sense, a mental faculty; but of every gland, every viscus, every element of the body. It is seen, not in man only, but in each animal which affords us adequate opportunity of tracing it. Always providing that the performance of function is not so excessive as to produce disorder, or to exceed the repairing powers either of the system at large or of the particular agencies by which nutriment is brought to the organ,--always providing this, it is a law of organized bodies that, other things equal, development varies as function. On this law are based all maxims and methods of right education, intellectual, moral, and physical; and when statesmen are wise enough to see it, this law will be found to underlie all right legislation.

Another truth co-extensive with the organic world, is that of hereditary transmission. It is not, as commonly supposed, that hereditary transmission is exemplified merely in re-appearance of the family peculiarities displayed by immediate or remote progenitors. Nor does the law of hereditary transmission comprehend only such more general facts as that modified plants or animals become the parents of permanent varieties; and that new kinds of potatoes, new breeds of sheep, new races of men, have been thus originated. These are but minor exemplifications of the law. Understood in its entirety, the law is that each plant or animal produces others of like kind with itself: the likeness of kind consisting not so much in the repet.i.tion of individual traits as in the a.s.sumption of the same general structure. This truth has been made by daily ill.u.s.tration so familiar as nearly to have lost its significance. That wheat produces wheat,--that existing oxen are descended from ancestral oxen,--that every unfolding organism ultimately takes the form of the cla.s.s, order, genus, and species from which it sprang; is a fact which, by force of repet.i.tion, has a.s.sumed in our minds the character of a necessity. It is in this, however, that the law of hereditary transmission is princ.i.p.ally displayed; the phenomena commonly named as exemplifying it being quite subordinate manifestations. And the law, as thus understood, is universal. Not forgetting the apparent, but only apparent, exceptions presented by the strange cla.s.s of phenomena known as "alternate generation," the truth that like produces like is common to all types of organisms.

Let us take next a universal physiological law of a less conspicuous kind. To the ordinary observer, it seems that the multiplication of organisms proceeds in various ways. He sees that the young of the higher animals when born resemble their parents; that birds lay eggs, which they foster and hatch; that fish deposit sp.a.w.n and leave it. Among plants, he finds that while in some cases new individuals grow from seeds only, in other cases they also grow from tubers; that by certain plants layers are sent out, take root, and develop new individuals; and that many plants can be reproduced from cuttings. Further, in the mould that quickly covers stale food, and the infusoria that soon swarm in water exposed to air and light, he sees a mode of generation which, seeming inexplicable, he is apt to consider "spontaneous." The reader of popular science thinks the modes of reproduction still more various. He learns that whole tribes of creatures multiply by gemmation--by a development from the body of the parent of buds which, after unfolding into the parental form, separate and lead independent lives. Concerning microscopic forms of both animal and vegetal life, he reads that the ordinary mode of multiplication is by spontaneous fission--a splitting up of the original individual into two or more individuals, which by and by severally repeat the process. Still more remarkable are the cases in which, as in the _Aphis_, an egg gives rise to an imperfect female, from which other imperfect females are born viviparously, grow, and in their turns bear other imperfect females; and so on for eight, ten, or more generations, until finally, perfect males and females are viviparously produced. But now under all these, and many more, modified modes of multiplication, the physiologist finds complete uniformity. The starting-point, not only of every higher animal or plant, but of every clan of organisms which by fission or gemmation have sprung from a single organism, is always a spore, seed, or ovum. The millions of infusoria or of aphides which, by sub-division or gemmation, have proceeded from one individual; the countless plants which have been successively propagated from one original plant by cuttings or tubers; are, in common with the highest creature, primarily descended from a fertilized germ. And in all cases--in the humblest alga as in the oak, in the protozoon as in the mammal--this fertilized germ results from the union of the contents of two cells. Whether, as among the lowest forms of life, these two cells are seemingly identical in nature; or whether, as among higher forms, they are distinguishable into sperm-cell and germ-cell; it remains throughout true that from their combination results the ma.s.s out of which is evolved a new organism or new series of organisms. That this law is without exception we are not prepared to say; for in the case of the _Aphis_ certain experiments are thought to imply that under special conditions the descendants of an original individual may continue multiplying for ever, without further fecundation. But we know of no case where it _actually is_ so; for although there are certain plants of which the seeds have never been seen, it is more probable that our observations are in fault than that these plants are exceptions. And until we find undoubted exceptions, the above-stated induction must stand. Here, then, we have another of the truths of Transcendental Physiology: a truth which, so far as we know, _transcends_ all distinctions of genus, order, cla.s.s, kingdom, and applies to every living thing.

Yet another generalization of like universality expresses the process of organic development. To the ordinary observer there seems no unity in this. No obvious parallelism exists between the unfolding of a plant and the unfolding of an animal. There is no manifest similarity between the development of a mammal, which proceeds without break from its first to its last stage, and that of an insect, which is divided into strongly-marked stages--egg, larva, pupa, imago. Nevertheless it is now an established fact, that all organisms are evolved after one general method. At the outset the germ of every plant or animal is relatively h.o.m.ogeneous; and advance towards maturity is advance towards greater heterogeneity. Each organized thing commences as an almost structureless ma.s.s, and reaches its ultimate complexity by the establishment of distinctions upon distinctions,--by the divergence of tissues from tissues and organs from organs. Here, then, we have yet another biological law of transcendent generality.

Having thus recognized the scope of Transcendental Physiology as presented in its leading truths, we are prepared for the considerations that are to follow.

And first, returning to the last of the great generalizations above given, let us inquire more nearly how this change from the h.o.m.ogeneous to the heterogeneous is carried on. Usually it is said to result from successive differentiations. This, however, cannot be considered a complete account of the process. During the evolution of an organism there occur, not only separations of parts, but coalescences of parts.

There is not only segregation, but aggregation. The heart, at first a simple pulsating blood-vessel, by and by twists upon itself and becomes integrated. The bile-cells const.i.tuting the rudimentary liver, do not merely diverge from the surface of the intestine in which they at first form a simple layer; but they simultaneously consolidate into a definite organ. And the gradual concentration seen in these and other cases is a part of the developmental process--a part which, though more or less recognized by Milne-Edwards and others, does not seem to have been included as an essential element in it.

This progressive integration, manifest alike when tracing up the several stages pa.s.sed through by every embryo, and when ascending from the lower organic forms to the higher, may be most conveniently studied under several heads. Let us consider first what may be called _longitudinal integration_.

The lower _Annulosa_--worms, myriapods, &c.--are characterized by the great numbers of segments of which they respectively consist, reaching in some cases to several hundreds; but as we advance to the higher _Annulosa_--centipedes, crustaceans, insects, spiders,--we find these numbers greatly reduced, down to twenty-two, thirteen, and even fewer; and accompanying this there is a shortening or integration of the whole body, reaching its extreme in crabs and spiders. Similarly with the development of an individual crustacean or insect. The thorax of a lobster, which, in the adult, forms, with the head, one compact box containing the viscera, is made up by the union of a number of segments which in the embryo were separable. The thirteen distinct divisions seen in the body of a caterpillar, become further integrated in the b.u.t.terfly: several segments are consolidated to form the thorax, and the abdominal segments are more aggregated than they originally were. The like truth is seen when we pa.s.s to the internal organs. In the lower annulose forms, and in the larvae of the higher ones, the alimentary ca.n.a.l consists either of a tube that is uniform from end to end, or else bulges into a succession of stomachs, one to each segment; but in the developed forms there is a single well-defined stomach. In the nervous, vascular, and respiratory systems a parallel concentration may be traced. Again, in the development of the _Vertebrata_ we have sundry examples of longitudinal integration. The coalescence of several segmental groups of bones to form the skull is one instance of it. It is further ill.u.s.trated in the _os coccygis_, which results from the fusion of a number of caudal vertebrae. And in the consolidation of the sacral vertebrae of a bird it is also well exemplified.

That which we may distinguish as _transverse integration_, is well ill.u.s.trated among the _Annulosa_ in the development of the nervous system. Leaving out those simple forms which do not present distinct ganglia, it is to be observed that the lower annulose animals, in common with the larvae of the higher, are severally characterized by a double chain of ganglia running from end to end of the body; while in the more advanced annulose animals this double chain becomes a single chain. Mr.

Newport has described the course of this concentration in insects; and by Rathke it has been traced in crustaceans. In the early stages of the _Astacus fluviatilis_, or common cray-fish, there is a pair of separate ganglia to each ring. Of the fourteen pairs belonging to the head and thorax, the three pairs in advance of the mouth consolidate into one ma.s.s to form the brain, or cephalic ganglion. Meanwhile out of the remainder, the first six pairs severally unite in the median line, while the rest remain more or less separate. Of these six double ganglia thus formed, the anterior four coalesce into one ma.s.s; the remaining two coalesce into another ma.s.s; and then these two ma.s.ses coalesce into one.

Here we see longitudinal and transverse integration going on simultaneously; and in the highest crustaceans they are both carried still further. The _Vertebrata_ exhibit this transverse integration in the development of the generative system. The lowest of the mammalia--the _Monotremata_--in common with birds, have oviducts which towards their lower extremities are dilated into cavities severally performing in an imperfect way the function of a uterus. "In the _Marsupialia_, there is a closer approximation of the two lateral sets of organs on the median line; for the oviducts converge towards one another and meet (without coalescing) on the median line; so that their uterine dilatations are in contact with each other, forming a true 'double uterus.' ... As we ascend the series of 'placental' mammals, we find the lateral coalescence becoming gradually more and more complete.... In many of the _Rodentia_, the uterus still remains completely divided into two lateral halves; whilst in others, these coalesce at their lower portion, forming a rudiment of the true 'body'

of the uterus in the Human subject. This part increases at the expense of the lateral 'cornua' in the higher Herbivora and Carnivora; but even in the lower Quadrumana, the uterus is somewhat cleft at its summit."[6] And this process of transverse integration, which is still more striking when observed in its details, is accompanied by parallel though less important changes in the opposite s.e.x. Once more; in the increasing commissural connexion of the cerebral hemispheres, which, though separate in the lower vertebrata, become gradually more united in the higher, we have another instance. And further ones of a different order, but of like general implication, are supplied by the vascular system.

Now it seems to us that the various kinds of integration here exemplified, which are commonly set down as so many independent phenomena, ought to be generalized, and included in the formula describing the process of development. The fact that in an adult crab, many pairs of ganglia originally separate have become fused into a single ma.s.s, is a fact only second in significance to the differentiation of its alimentary ca.n.a.l into stomach and intestine. That in the higher _Annulosa_, a single heart replaces the string of rudimentary hearts const.i.tuting the dorsal blood-vessel in the lower _Annulosa_, (reaching in one species to the number of one hundred and sixty), is a truth as much needing to be comprised in the history of evolution, as is the formation of a respiratory surface by a branched expansion of the skin. A right conception of the genesis of a vertebral column, includes not only the differentiations from which result the _chorda dorsalis_ and the vertebral segments imbedded in it; but quite as much it includes the coalescence of numerous vertebral processes with their respective vertebral bodies. The changes in virtue of which several things become one, demand recognition equally with those in virtue of which one thing becomes several. Evidently, then, the current statement which ascribes the developmental progress to differentiations alone, is incomplete. Adequately to express the facts, we must say that the transition from the h.o.m.ogeneous to the heterogeneous is carried on by differentiations and accompanying integrations.

It may not be amiss here to ask--What is the meaning of these integrations? The evidence seems to show that they are in some way dependent on community of function. The eight segments which coalesce to make the head of a centipede, jointly protect the cephalic ganglion, and afford a solid fulcrum for the jaws, &c. The many bones which unite to form a vertebral skull have like uses. In the consolidation of the several pieces which const.i.tute a mammalian pelvis, and in the anchylosis of from ten to nineteen vertebrae in the sacrum of a bird, we have kindred instances of the integration of parts which transfer the weight of the body to the legs. The more or less extensive fusion of the tibia with the fibula and the radius with the ulna in the ungulated mammals, whose habits require only partial rotations of the limbs, is a fact of like meaning. And all the instances lately given--the concentration of ganglia, the replacement of many pulsating blood-sacs by fewer and finally by one, the fusion of two uteri into a single uterus--have the same implication. Whether, as in some cases, the integration is merely a consequence of the growth which eventually brings into contact adjacent parts performing similar duties; or whether, as in other cases, there is an actual approximation of these parts before their union; or whether, as in yet other cases, the integration is of that indirect kind which arises when, out of a number of like organs, one, or a group, discharges an ever-increasing share of the common function, and so grows while the rest dwindle and disappear;--the general fact remains the same, that there is a tendency to the unification of parts having similar duties.

The tendency, however, acts under limiting conditions; and recognition of them will explain some apparent exceptions. In the human foetus, as in the lower vertebrata, the eyes are placed one on each side of the head. During evolution they become relatively nearer, and at birth are in front; though they are still, in the European infant as in the adult Mongol, proportionately further apart than they afterwards become. But this approximation shows no signs of further increase. Two reasons suggest themselves. One is that the two eyes have not quite the same function, since they are directed to slightly-different aspects of each object looked at; and, since the resulting binocular vision has an advantage over monocular vision, there results a check upon further approach towards ident.i.ty of function and unity of structure. The other reason is that the interposed structures do not admit of any nearer approach. For the orbits of the eyes to be brought closer together, would imply a decrease in the olfactory chambers; and as these are probably not larger than is demanded by their present functional activity, no decrease can take place. Again, if we trace up the external organs of smell through fishes,[7] reptiles, ungulate mammals and unguiculate mammals, to man, we perceive a general tendency to coalescence in the median line; and on comparing the savage with the civilized, or the infant with the adult, we see this approach of the nostrils carried furthest in the most perfect of the species. But since the septum which divides them has the function both of an evaporating surface for the lachrymal secretion, and of a ramifying surface for a nerve ancillary to that of smell, it does not disappear entirely: the integration remains incomplete. These and other like instances do not however militate against the hypothesis. They merely show that the tendency is sometimes antagonized by other tendencies. Bearing in mind which qualification, we may say, that as differentiation of parts is connected with difference of function, so there appears to be a connexion between integration of parts and sameness of function.

Closely related to the general truth that the evolution of all organisms is carried on by combined differentiations and integrations, is another general truth, which physiologists appear not to have recognized. When we look at the organic world as a whole, we may observe that, on pa.s.sing from lower to higher forms, we pa.s.s to forms which are not only characterized by a greater differentiation of parts, but are at the same time more completely differentiated from the surrounding medium. This truth may be contemplated under various aspects.

In the first place it is ill.u.s.trated in _structure_. The advance from the h.o.m.ogeneous to the heterogeneous itself involves an increasing distinction from the inorganic world. In the lowest _Protozoa_, as some of the Rhizopods, we have a h.o.m.ogeneity approaching to that of air, water, or earth; and the ascent to organisms of greater and greater complexity of structure, is an ascent to organisms which are in that respect more strongly contrasted with the relatively structureless ma.s.ses in the environment.

In _form_ again we see the same truth. A general characteristic of inorganic matter is its indefiniteness of form, and this is also a characteristic of the lower organisms, as compared with the higher.

Speaking generally, plants are less definite than animals, both in shape and size--admit of greater modifications from variations of position and nutrition. Among animals, the _Amoeba_ and its allies are not only almost structureless, but are amorphous; and the irregular form is constantly changing. Of the organisms resulting from the aggregation of amoeba-like creatures, we find that while some a.s.sume a certain definiteness of form, in their compound sh.e.l.ls at least, others, as the Sponges, are irregular. In the Zoophytes and in the _Polyzoa_, we see compound organisms, most of which have modes of growth not more determinate than those of plants. But among the higher animals, we find not only that the mature shape of each species is quite definite, but that the individuals of each species differ very little in size.

A parallel increase of contrast is seen in _chemical composition_. With but few exceptions, and those only partial ones, the lowest animal and vegetal forms are inhabitants of the water; and water is almost their sole const.i.tuent. Dessicated _Protophyta_ and _Protozoa_ shrink into mere dust; and among the acalephes we find but a few grains of solid matter to a pound of water. The higher aquatic plants, in common with the higher aquatic animals, possessing as they do much greater tenacity of substance, also contain a greater proportion of the organic elements; and so are chemically more unlike their medium. And when we pa.s.s to the superior cla.s.ses of organisms--land plants and land animals--we find that, chemically considered, they have little in common either with the earth on which they stand or the air which surrounds them.

In _specific gravity_, too, we may note the like. The very simplest forms, in common with the spores and gemmules of the higher ones, are as nearly as may be of the same specific gravity as the water in which they float; and though it cannot be said that among aquatic creatures superior specific gravity is a standard of general superiority, yet we may fairly say that the superior orders of them, when divested of the appliances by which their specific gravity is regulated, differ more from water in their relative weights than do the lower. In terrestrial organisms, the contrast becomes extremely marked. Trees and plants, in common with insects, reptiles, mammals, birds, are all of a specific gravity considerably less than the earth and immensely greater than the air.

We see the law similarly fulfilled in respect of _temperature_. Plants generate but an extremely small quant.i.ty of heat, which is to be detected only by delicate experiments; and practically they may be considered as being in this respect like their environment. Aquatic animals rise very little above the surrounding water in temperature: that of the invertebrata being mostly less than a degree above it, and that of fishes not exceeding it by more than two or three degrees, save in the case of some large red-blooded fishes, as the tunny, which exceed it by nearly ten degrees. Among insects, the range is from two to ten degrees above that of the air: the excess varying according to their activity. The heat of reptiles is from four to fifteen degrees more than that of their medium. While mammals and birds maintain a heat which continues almost unaffected by external variations, and is often greater than that of the air by seventy, eighty, ninety, and even a hundred degrees.

Once more, in greater _self-mobility_ a progressive differentiation is traceable. Dead matter is inert: some form of independent motion is our most general test of life. Pa.s.sing over the indefinite border-land between the animal and vegetable kingdoms, we may roughly cla.s.s plants as organisms which, while they exhibit the kind of motion implied in growth, are not only without locomotive power, but in nearly all cases are without the power of moving their parts in relation to one another; and thus are less differentiated from the inorganic world than animals.

Though in those microscopic _Protophyta_ and _Protozoa_ inhabiting the water--the spores of algae, the gemmules of sponges, and the infusoria generally--we see locomotion produced by ciliary action; yet this locomotion, while rapid relatively to their sizes, is absolutely slow.

Of the _Coelenterata_, a great part are either permanently rooted or habitually stationary, and so have scarcely any self-mobility but that implied in the relative movements of parts; while the rest, of which the common jelly-fish serves as a sample, have mostly but little ability to move themselves through the water. Among the higher aquatic _Invertebrata_,--cuttle-fishes and lobsters, for instance,--there is a very considerable power of locomotion; and the aquatic _Vertebrata_ are, considered as a cla.s.s, much more active in their movements than the other inhabitants of the water. But it is only when we come to air-breathing creatures that we find the vital characteristic of self-mobility manifested in the highest degree. Flying insects, mammals, birds, travel with velocities far exceeding those attained by any of the lower cla.s.ses of animals; and so are more strongly contrasted with their inert environments.

Thus, on contemplating the various grades of organisms in their ascending order, we find them more and more distinguished from their inanimate media in _structure_, in _form_, in _chemical composition_, in _specific gravity_, in _temperature_, in _self-mobility_. It is true that this generalization does not hold with regularity. Organisms which are in some respects the most strongly contrasted with the inorganic world, are in other respects less contrasted than inferior organisms. As a cla.s.s, mammals are higher than birds; and yet they are of lower temperature, and have smaller powers of locomotion. The stationary oyster is of higher organization than the free-swimming medusa; and the cold-blooded and less heterogeneous fish is quicker in its movements than the warm-blooded and more heterogeneous sloth. But the admission that the several aspects under which this increasing contrast shows itself bear variable ratios to one another, does not negative the general truth enunciated. Looking at the facts in the ma.s.s, it cannot be denied that the successively higher groups of organisms are severally characterized, not only by greater differentiation of parts, but also by greater differentiation from the surrounding medium in sundry other physical attributes. It would seem that this peculiarity has some necessary connexion with superior vital manifestations. One of those lowly gelatinous forms which are some of them so transparent and colourless as to be with difficulty distinguished from the water they float in, is not more like its medium in chemical, mechanical, optical, thermal, and other properties, than it is in the pa.s.sivity with which it submits to all the actions brought to bear on it; while the mammal does not more widely differ from inanimate things in these properties than it does in the activity with which it meets surrounding changes by compensating changes in itself. Between these two extremes, we see a tolerably constant ratio between these two kinds of contrast. In proportion as an organism is physically like its environment it remains a pa.s.sive partaker of the changes going on in its environment; while in proportion as it is endowed with powers of counteracting such changes, it exhibits greater unlikeness to its environment.

Thus far we have proceeded inductively, in conformity with established usage; but it seems to us that much may be done in this and other departments of biologic inquiry by pursuing the deductive method. The generalizations at present const.i.tuting the science of physiology, both general and special, have been reached _a posteriori_; but certain fundamental data have now been discovered, starting from which we may reason our way _a priori_, not only to some of the truths that have been ascertained by observation and experiment, but also to some others. The possibility of such _a priori_ conclusions will be at once recognized on considering some familiar cases.

Chemists have shown that a necessary condition to vital activity in animals is oxidation of certain matters contained in the body either as components or as waste products. The oxygen requisite for this oxidation is contained in the surrounding medium--air or water, as the case may be. If the organism be minute, mere contact of its external surface with the oxygenated medium achieves the requisite oxidation; but if the organism is bulky, and so exposes a surface which is small in proportion to its ma.s.s, any considerable oxidation cannot be thus achieved. One of two things is therefore implied. Either this bulky organism, receiving no oxygen but that absorbed through its integument, must possess but little vital activity; or else, if it possesses much vital activity, there must be some extensive ramified surface, internal or external, through which adequate aeration may take place--a respiratory apparatus. That is to say, lungs, or gills, or branchiae, or their equivalents, are predicable _a priori_ as possessed by all active creatures of any size.

Similarly with respect to nutriment. There are _entozoa_ which, living in the insides of other animals, and being constantly bathed by nutritive fluids, absorb a sufficiency through their outer surfaces; and so have no need of stomachs, and do not possess them. But all other animals, inhabiting media that are not in themselves nutritive, but only contain ma.s.ses of food here and there, must have appliances by which these ma.s.ses of food may be utilized. Evidently mere external contact of a solid organism with a solid portion of nutriment, could not result in the absorption of it in any moderate time, if at all. To effect absorption, there must be both a solvent or macerating action, and an extended surface fit for containing and imbibing the dissolved products: there must be a digestive cavity. Thus, given the ordinary conditions of animal life, and the possession of stomachs by all creatures living under these conditions may be deductively known.

Carrying out the train of reasoning still further, we may infer the existence of a vascular system or something equivalent to it, in all creatures of any size and activity. In a comparatively small inert animal, such as the hydra, which consists of little more than a sac having a double wall--an outer layer of cells forming the skin, and an inner layer forming the digestive and absorbent surface--there is no need for a special apparatus to diffuse through the body the aliment taken up; for the body is little more than a wrapper to the food it encloses. But where the bulk is considerable, or where the activity is such as to involve much waste and repair, or where both these characteristics exist, there is a necessity for a system of blood-vessels. It is not enough that there be adequately extensive surfaces for absorption and aeration; for in the absence of any means of conveyance, the absorbed elements can be of little or no use to the organism at large. Evidently there must be channels of communication.

When, as in the _Medusae_, we find these channels of communication consisting simply of branched ca.n.a.ls opening out of the stomach and spreading through the disk, we may know, _a priori_, that such creatures are comparatively inactive; seeing that the nutritive liquid thus partially distributed throughout their bodies is crude and dilute, and that there is no efficient appliance for keeping it in motion.

Conversely, when we meet with a creature of considerable size which displays much vivacity, we may know, _a priori_, that it must have an apparatus for the unceasing supply of concentrated nutriment, and of oxygen, to every organ--a pulsating vascular system.

It is manifest, then, that setting out from certain known fundamental conditions to vital activity, we may deduce from them sundry of the chief characteristics of organized bodies. Doubtless these known fundamental conditions have been inductively established. But what we wish to show is that, given these inductively-established primary facts in physiology, we may with safety draw certain general deductions from them. And, indeed, the legitimacy of such deductions, though not formally acknowledged, is practically recognized in the convictions of every physiologist, as may be readily proved. Thus, were a physiologist to find a creature exhibiting complex and variously co-ordinated movements, and yet having no nervous system; he would be less astonished at the breach of his empirical generalization that all such creatures have nervous systems, than at the disproof of his unconscious deduction that all creatures exhibiting complex and variously co-ordinated movements must have an "internuncial" apparatus by which the co-ordination may be effected. Or were he to find a creature having blood rapidly circulated and rapidly aerated, but yet showing a low temperature, the proof so afforded that active change of matter is not, as he had inferred from chemical data, the cause of animal heat, would stagger him more than would the exception to a constantly-observed relation. Clearly, then, the _a priori_ method already plays a part in physiological reasoning. If not ostensibly employed as a means of reaching new truths, it is at least privately appealed to for confirmation of truths reached _a posteriori_.

But the ill.u.s.trations above given go far to show, that it may to a considerable extent be safely used as an independent instrument of research. The necessities for a nutritive system, a respiratory system, and a vascular system, in all animals of size and vivacity, seem to us legitimately inferable from the conditions to continued vital activity.

Given the physical and chemical data, and these structural peculiarities may be deduced with as much certainty as may the hollowness of an iron ball from its power of floating in water.

It is not, of course, a.s.serted that the more _special_ physiological truths can be deductively reached. The argument by no means implies this. Legitimate deduction presupposes adequate data; and in respect to the _special_ phenomena of organic growth, structure, and function, adequate data are unattainable, and will probably ever remain so. It is only in the case of the more _general_ physiological truths, such as those above instanced, where we have something like adequate data, that deductive reasoning becomes possible.

And here is reached the stage to which the foregoing considerations are introductory. We propose now to show that there are certain still more general attributes of organized bodies, which are deducible from certain still more general attributes of things.

In an essay on "Progress: its Law and Cause," elsewhere published,[8] we have endeavoured to show that the transformation of the h.o.m.ogeneous into the heterogeneous, in which all progress, organic or other, essentially consists, is consequent on the production of many effects by one cause--many changes by one force. Having pointed out that this is a law of all things, we proceeded to show deductively that the multiform evolutions of the h.o.m.ogeneous into the heterogeneous--astronomic, geologic, ethnologic, social, &c.,--were explicable as consequences. And though in the case of organic evolution, lack of data disabled us from specifically tracing out the progressive complication as due to the multiplication of effects; yet, we found sundry indirect evidences that it was so. Now in so far as this conclusion, that organic evolution results from the decomposition of each expended force into several forces, was inferred from the general law previously pointed out, it was an example of deductive physiology. The particular was concluded from the universal.

We here propose in the first place to show, that there is another general truth closely connected with the above; and in common with it underlying explanations of all progress, and therefore the progress of organisms--a truth which may indeed be considered as taking precedence of it in respect of time, if not in respect of generality. This truth is, that _the condition of h.o.m.ogeneity is a condition of unstable equilibrium_.

The phrase _unstable equilibrium_ is one used in mechanics to express a balance of forces of such kind, that the interference of any further force, however minute, will destroy the arrangement previously existing, and bring about a different arrangement. Thus, a stick poised on its lower end is in unstable equilibrium: however exactly it may be placed in a perpendicular position, as soon as it is left to itself it begins, at first imperceptibly and then visibly, to lean on one side, and with increasing rapidity falls into another position. Conversely, a stick suspended from its upper end is in stable equilibrium: however much disturbed, it will return to the same position. Our meaning is, then, that the state of h.o.m.ogeneity, like the state of the stick poised on its lower end, is one that cannot be maintained; and that hence results the first step in its gravitation towards the heterogeneous. Let us take a few ill.u.s.trations.

Of mechanical ones the most familiar is that of the scales. If accurately made and not clogged by dirt or rust, a pair of scales cannot be perfectly balanced: eventually one scale will descend and the other ascend--they will a.s.sume a heterogeneous relation. Again, if we sprinkle over the surface of a liquid a number of equal-sized particles, having an attraction for one another, they will, no matter how uniformly distributed, by and by concentrate irregularly into groups. Were it possible to bring a ma.s.s of water into a state of perfect h.o.m.ogeneity--a state of complete quiescence, and exactly equal density throughout--yet the radiation of heat from neighbouring bodies, by affecting differently its different parts, would soon produce inequalities of density and consequent currents; and would so render it to that extent heterogeneous. Take a piece of red-hot matter, and however evenly heated it may at first be, it will quickly cease to be so: the exterior, cooling faster than the interior, will become different in temperature from it. And the lapse into heterogeneity of temperature, so obvious in this extreme case, is ever taking place more or less in all cases. The actions of chemical forces supply other ill.u.s.trations. Expose a fragment of metal to air or water, and in course of time it will be coated with a film of oxide, carbonate, or other compound: its outer parts will become unlike its inner parts. Thus, every h.o.m.ogeneous aggregate of matter tends to lose its balance in some way or other--either mechanically, chemically, thermally or electrically; and the rapidity with which it lapses into a non-h.o.m.ogeneous state is simply a question of time and circ.u.mstances. Social bodies ill.u.s.trate the law with like constancy. Endow the members of a community with equal properties, positions, powers, and they will forthwith begin to slide into inequalities. Be it in a representative a.s.sembly, a railway board, or a private partnership, the h.o.m.ogeneity, though it may continue in name, inevitably disappears in reality.