If to these general questions we discover a satisfactory general answer, we shall do all that is needful. If from a corollary to the Persistence of Force, we can legitimately draw the conclusion that, under certain conditions, lines of nervous communication will arise, and, having arisen, will become lines of more and more easy communication, in proportion to the numbers and strengths of the discharges propagated through them; we shall have found a physical interpretation which completes the doctrine of psychical evolution, as set forth in the last two parts. It will be made manifest how the experience of an external relation produces a corresponding internal relation — how, as experiences of tlie external relation become more numerous,, the internal relation becomes more coherent — how perpetual repetitions of the one cause indissolubleness of the other — how outer persistences that are almost or quite absolute, establish, in the course of generations., inner cohesions that are automatic or organic; and thus the interpretation of instincts and forms of thought will be assimilated to that of the ordinary phenomena of association.* * The general doctrine elaborated- in the succeeding chapters, was prefigured in the first edition of this work, in a note on page 544 — the verbal form, however, being such as I should not now use. I made a more definite statement of it in an article published in the Medico-CJururgical Review for January, 1859.
THE GENESIS OF NERVES.
§ 223. In First Principles, Part II., Cliap. IX., we found that in all cases, motion " follows the line of greatest traction, or the line of least resistance, or the resultant of the two." We also saw " that motion once set up along any line "becomes itself a cause of subsequent motion along that line" — equally when the motion is that of matter through space, that of matter through matter, and that of molecular undulations through an aggregate of molecules.
In the section dealing with nervous actions (§ 79), it was contended that the mode of motion we distinguish as a nervous discharge, conforms to this law. " Supposing the various forces throughout an organism to "be previously in equilibrium, then any part which becomes the seat of a further force, added or liberated, must be one from which the force, being resisted by smaller forces around, will initiate motion towards some other part of the organism. If elsewhere in the organism there is a point at which force is being expended, and which so is becoming minus a force which it before had, instead of plus a force which it before had not, and thus is made a point at which the re-action against surrounding forces is diminished; then, manifestly, a motion taking place between the first and the last of these points is a motion along the line of least resistance. Now a sensation implies a force added to, or evolved in, that part of the organism which is its seat; while a mechanical move- M2 PHYSICAL SYNTHESIS.
ment implies an expenditure or loss of force in that part of the organism which is its seat. * * * When there is anything in the circumstances of an animal's life,, involving that a sensation in one particular place is habitually followed by a contraction in another particular place — when there is thus a frequently-repeated motion through the organism between these places • what must be the result as respects the line along which the motions take place? Eestoration of equilibrium, between the points at which the forces have been increased and decreased, must take place through borne channel. If this channel is affected by the discharge — if the obstructive action of the tissues traversed, involves any reaction upon them, deducting from their obstructive power; then a subsequent motion between these two points will meet with less resistance along this channel than the previous motion met with; and will consequently take this channel still more decidedly."
In the Pr'iYu'lpltjS of Biology, § 302, tnis general proposition was further elaborated. It there became needful to indicate a possible process by which, among other tissues, nerve-tissue arises out of that protoplasm composing the nndifferentiated organism. Here, in an abbreviated form., is the argument which was used: — (C 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 THE GENESIS OF NERVES. 6 I'd molecular motion in assuming their new states. * * * Unstable molecules which, in being isornerically 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 undiininished, or increased, to adjacent molecules. * * * 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 mingled colloids forming " 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. * * * As is shown by those transformations that 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 over throw. * * * Is this action limited to strictly isomerio substances? or may it extend to substances that are closely allied? # # # There is reason to suspect that it does.
Already when treating of the nutrition of parts, 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. * * * 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 proxiuiate components, either uncombined or L L feebly combined in some not very different way. * * * That is to say, a ware of molecular disturbance diffused from a centre, and travelling furthest along a line where lie most molecules that can be isomerically transformed with facilitv, will be likelv at the same time to further differentiate this line, and make it more characterized than before by the easv-transforinability of its molecules/' Referring1 the reader to the Principles of Biology for the details and conclusion of this abridged argument/, it may be well to remind him that in the first part of this work, the interpretations of nerve-structure and nerve-function were grounded on a conception which is a corollary from the conception 'recalled above j and that sundry verifications were there found. We saw that the quantity of effect produced by irritated nerve-fibre, increases with tiie distance between the place of irritation and the place of discharge; and this accumulation of force we found to be just that winch would result from a wave of isomeric transformation through matter of the required kind (§ 19). We saw, too, that the ultimate nitrogenous nerve-threads are severally sheathed in a peculiar substance, which, judging by its unequalled molecular complexity, is less capable than any other known substance of transferring molecular motion, and therefore best fitted to prevent lateral loss of that growing wave of molecular motion which a nerve-fibre transmits. And we further saw that a close analogy exists between this assumed propagation of isomeric change along a nerve-fibre, and certain observed propagations of like changes along fibres of other substances (§ 34). To which let me here add the fact that protoplasm and its derivatives are distinguished by the great number of their isomeric forms, and the great facility with which these are changed by Very various agents; so that in regarding* a nervous discharge as a wave of isomeric transformation, we are re°tarding it as one out of the many such transformations which living matter continually undergoes.
THE GENESIS OF NEB FES. 515 . § 224. Another preliminary step remains. We Have to observe the possible modes in which a line of nervous communication may be improved. When, through undif- : ferentiated tissue, there has passed -for the first time a wave ^ of disturbance from some place where molecular motion is f* liberated to some place where it is absorbed, the line of least resistance followed must be an indefinite and irregular one. Fully to understand the genesis of nerve, then, we must understand the physical actions which change this vague course into a definite channel* that becomes ever more permeable as it is more used.
Several actions conduce to this result. The first is that already described, by which, along a line of discharge, there is a genesis of the matter most capable of comii lamenting the discharge. Every time an incipient nerve is traversed by another wave of molecular motion, there is apt to be a further formation of the molecules which are isomerically transformed by the wave and pass it on in being transformed. This process acts with continually -increasing power, for two reasons. One is that progressing limitation of the wave to a well-marked line, enables it to produce more decided effects along that line. An illustra-' tion will here help us. When a body of water flows over a surface offering no distinct course,, it thins out into wide--; spread shallows near its margin, where it is almost motionless; and it has but little motion even along its central J - deepest parts. But if the inundation is long continued, the j abraiding action of the current along these central deepest t parts where it moves fastest,, tends to deepen its channel ] thei-e more than elsewhere. A secondary result is a retreat ; of the water from the shallows— the current becomes more 1 concentrated. In proportion as it becomes more concentrated the force of its central part becomes greater still, and the deepening more rapid; which entails a further drawing in of the margins and a further addition to the excavating force. So that the growing definiteness of the current brings a growing power of making its channel quite definite. Now though, in the case "before us we have not a motion of matter over matter, but} a transfer of molecular motion from molecules to molecules,, the parallel holds. Any greater effect produced by the transfer along one part of its originally-broad course, similarly tends to concentrate the transfer along this part, and thus to intensify the action which makes this part a precisely-marked channel. A further facilitation results from an absolute increase in the amount of the nervous discharge. The more permeable the line of molecules becomes, the greater becomes the initial quantity of molecular motion it draughts off. As with water, the formation of a definite channel not only makes the transfer easier and adds to the excavatinopower of the current, supposing its volume be constant, but also "(if the reservoir can supply more) augments the volume carried away, which again adds to the excavating power; so the formation of a better line of nervous communication is followed by an increase of the wave that sets out to traverse it, and a consequent increase in the channel-makin enaction. Once more, every addition to the molecular motion transmitted, adds to the effectiveness of each discharge in overcoming an obstacle. Suppose the greater part of its channel has become tolerably permeable, but that at some place in it the colloidal matter is less transformed than elsewhere into the fit type. Then the more the rest of its channel increases in permeability, the more powerful must be the wave of molecular motion brought to bear on the untransforrned part, and the greater must be the tendency to transform it. Hence the channel will progress towards a state of uniform permeability.
There is another possible, and I think probable, way in whiph the passage of a nervous discharge is made easier. The molecules of the peculiar colloid composing a nerve, may be either irregularly arranged or regularly arranged; and if irregularly arranged they will transmit a wave of TEE GENESIS OF SEKVES. 517 molecular motion less readily tlian if regularly arranged. Now when a thread of molecules capable of the required easy isomeric transformation is first formed, the probabilities are infinity to one that adjacent molecules will be unsvmmetrically placed with respect to one another — they will not stand in polar order. Molecules that are highly coiaplex and massive,, either do not crystallize at all or crystallize with great difficulty. Either their colloidal, non-polar arrangement is a permanent one, or it is one out of which they pass into a polar arrangement very slowly, under special conditions. Nevertheless, molecules of every type have a form of distribution in which their polar forces are in equilibrium. Towards this they must ever tend, however feebly j and towards this every slight molecular disturbance enables them to • approach. Hence, if through a line of colloidal molecules wholly out- of polar arrangement, there pass successive waves of molecular motion, each will help adjacent molecules towards polar arrangement, or state of equilibrium. Let us consider the concomitants.
To aid our conceptions we will as before (§ 19) take the rude analogy furnished by a row of bricks on end, which overthrow one another in succession. If such bricks on end have been adjusted so that their faces are all at right angles to the line of the series, the change will be propagated along them with the least hindrance; or, under certain conditions, with the greatest multiplication of the original impulse, For when so placed, the impact each brick gives to the next, being exactly in the line of the series, will be wholly effective y but when they are otherwise placed it- will not. If the bricks stand with their faces variously askew, each in falling will have a motion more or less diverging from, the line of the series; and hence only a part of its momentum will impel the next in the required direction. Now though in the case of a series of molecules the action can be by no means so simple, yet the same principle holds. The isomeric change of a molecule must diffuse a wave which is greater In some one direction than in all others. If so, there are certain relative positions of molecules such that each will receive the greatest amount of this wave from its predecessor, and will so receive it as most readily to produce a like change in itself. A series of molecules thus placed must stand in symmetrical relations to one another — polar relations. And it is not difficult to see that, as in the case of the bricks,, any deviation from symmetrical or polar relations will involve a proportionate deduction from the efficiency of the shock, and a diminution in the quantity of molecular motion given out at the far end. But now, what is the indirect result when a wave of change passes along a line of molecules thus unsymmetrically placed? The indirect result is that the motion which is not passed on by the unsymmetrically-placed molecules,, goes towards placing them symmetrically. Let us again consider what happens with our row of bricks. When one of these in falling comes against tho next, standing askew, its impact is given to the nearest angle of this next,, and so tends to give this next a motion round its axis. Further, when the next thus moved delivers its motion to its successor, it does this not through the angle on the side that was struck, but through the diagonallyopposite angle; and, consequently, the reaction, of its impact on its successor adds to the rotatory motion already received. Hence the amount of force which it does not pass on, is the amount of force absorbed in turning it towards parallelism with its neighbours. Similarly with the molecules. Each in falling into its new isomeric attitude, and passing on the shock to its successor, gives to its successor. a motion which is all passed on if the successor stands in polar relation towards it, but which, if the relation is not polar,is only partially passed on — some of it being taken up in moving the successor towards a polar relation. One more consequence is to be observed. Every approach of the molecules towards symmetrical arrangement, increases the amount of molecular motion transferred from one end of the THE GENESIS OF NERVES. 5] 9 series to the other. Suppose that the row of bricks, which were at first very much out of parallelism, have fallen, and that part of the motion given by each to tho next has gone towards bringing their faces nearer to parallelism; and suppose that, without further changing the positions of their bases, the bricks are severally restored to their vertical attitudes; then it will happen that if the serial overthrow of them is repeated, the actions, though the same as before in their kinds, will not be the same as before in their degrees. Each brick, falling as it now does more in the line of the series, will deliver more of its momentum to the next; and less momentum will be taken up in moving the next towards parallelism with its neighbours. If, then, the analogy holds, it must happen that in the series of isomerically-changing molecules, each transmitted wave of molecular motion is expended partly in so altering the molecular attitudes as to render the series more permeable to future waves, and partly in setting up changes at the end of the series; that in proportion as less of it is absorbed in working this structural change, more of it is delivered at the far end and greater effect produced there; and that the final state i& one in which the initial wave of molecular motion is transmitted without deduction — or rather, with the addition of the molecular motion given out by the successive molecules of the series in their isomeric falls.
§ 225. From, beginning to end, therefore, the development of nerve results from the passage of motion along the line of least resistance, and the reduction of it to a line of less and less resistance continually. The first opening of a route along which equilibrium is restored between a place where molecular motion is in excess and a place where it is in defect, comes within this formula. The production^of a more continuous line of that peculiar colloid best fitted to transmit the molecular motion, also comes within this formula; as does likewise the making of this line thicker and more even. And the formula also covers that final process by which the line, having been formed, lias its molecules brought Into the polar order which least resists, and indeed facilitates, the transmission of the wave.
§ 225a. S-mie qualifications of the foregoing exposition must now be made. Instead of changing it throughout to meet criticisms,, I have thought it best to repeat the exposition as given in the second, third, and fourth editions of this work and then to indicate the needful modifications.
At the meeting of the British Association held in Belfast in 1874, Prof: Clerk-Maxwell objected to the hypothesis that the nerve-current consists of successive waves of isomeric change, on the ground that the implied conception was that of a "heat-machine," and that a heat-machine is impossible in the absence of difference of temperature. In reply,! contended that my hypothesis is not at variance with this law of thermo-dynamics, since it supposes that the falling; of eac'i molecule from one isomeric form to another is accompanied by absorption of heat, and that the nerve-fibre, thereupon rendered of lower temperature than the surrounding matters, instantly takes up from them heat sufficient to cause the molecules to resume their previous form: the implication being that the nerve-current is at the cost of the heat yielded by the imbedding tissues. The discussion which ensued failed to draw from Prof. Clerk-Maxwell the admission that my reply was adequate, and failed also to make me understand his difficulty. This difficulty, as since explained to me by Lord Rayleigh, is that, being a lower form of molecular motion, heat cannot reproduce that higher form of molecular motion implied by the hypothesis. Here I have no alternative but to accept the dicta of these "two distinguished physicists. It is true that the heat supplied by a sitting hen apparently suffices to build up a variety of complex compounds, some of which, as protagon, are more complex than any of those contained in the unorganized materials of the THE GENESIS OF NERVES. 52G& egg; and it miglit seern a fair infereDce that such being the case, heat roust be capable of raising a molecule of protein from a lower isomeric form to a higher. But I suppose there must be some lack of parallelism between the two cases, and that this rhythmical isomeric change in nerve-fibre implies some further physical process which ultimate physical principles negative.
"What qualification then must the hypothesis undergo to render it tenable? Apparently we must conclude that in nerve-fibre, as in the tissues at large, performance of function is accompanied by molecular disintegration, and that fitness for subsequent performance of function is to be gained only by re-integration. The implication would seem to be that the molecular change must be, to some extent, a chemical change; and that each molecule "which has undergone modification, has to be repaired by absorption of needful matter from the nutrient liquid with which it is bathed.
This supposition appears congruous with the fact that the axis-cylinder of a nerve-fibre, small in diameter though it is, consists of a bundle of still more minute fibres. It seems not unlikely that such component fibres as have conveyed a wave of nerve-change, and become thereby temporarily incapacitated, have their functions undertaken by another cluster of component fibres, which carry the next wave, and these, again,, leave the function to be afterwards performed by a third cluster, and so on: the fibres of the first cluster having meanwhile refitted themselves for activity. If so, there is a parallelism between the action of nerves and the action of muscles; in which last the sets t,f fibres take their turns of action and rest, while the muscle as a whole continues in a state of contraction.
CHAPTER III.
THE G-EXE3I3 OF SIMPLE ]NTEETOUS SYSTEMS/3' 5 226, Careful and extended observations have necessitated changes in the cell-doctrine as originally propounded. The statement that all organisms of sensible sizes are made up of minute nucleated bodies,, completely distinct from one another, has to be much qualified.
Among botanists a wide change of view resulted from the discovery that in the tissues of plants the protoplasm within each cell is united to that within adjacent cells by threads of protoplasm which pass through the respective cell- walls: a discovery that at once makes more comprehensible various plant-movements. Implying a kindred structure, Prof. Sedgwick writes: — "It is becoming more and more clear * In his Inaugural Address to the Section of Anatomy and Physiology, at the meeting of the British Association in 1880, Prof. Balfour, after indicating certain new lights thrown on the evolution of nervous systems, remarked concerning the contents of this chapter: — " These hypotheses of Herbert Spencer, which have been widely adopted in this country, are, it appears to me, not borne out by the discoveries to which I have called your attention to-day." Being, as I considered, bound to accept Prof. Balfour's representations, I was about to change essentially the first part of this chapter, when my attention was drawn to an opinion since published by Prof. Adam Seclgwiet, the successor of Prof. Balfour in the same chair at Cambridge. The opinion in question is contained in A Monograph of the Development of Peripatus Capen&is, p. 49, and is expressed as follows: — " Herbert Spencer's view of the origin of the nervous system may perhaps not be so far from the mark aS at first sight appeared." Taking advantage of the recent results of histological researches, I have been led, by the criticism and counter-criticism above quoted, to re-cast the early part of this chapter, and to give the contained hypothesis a more satisfactory form, I think, than previously seemed possible.
THE GENESIS OP SIMPLE NERVOUS SYSTEMS. 520cs ereiy day that the cells composing the tissues of animals are not isolated units, but that they are connected with one another. I need only refer to the connection known to exist between connective tissue cells, cartilage cells, epithelial cells, &c. And not only may the cells of one tissue be continuous with each other, but they may also be continuous with the cells of other tissues?> fpp. 47-8).
The revised conception to which we are thus introduced, is that throughout those aggregations of Protozoa by which Metazoa have been formed, there has been an incompleteness of those spontaneous fissions which, if complete, would have multiplied the Protozoa: the units have remained connected by prolongations homologous with pseudopodia. As the members of a compound Rhizopod, say one of the Foraminifera, are not wholly cut off from one another, but maintain some continuity of substance through perforations in the septa — as the living units which make up a Volvos or a Raphldiopkrys are held together by threads of protoplasm which traverse their respective limiting membranes; so it appears that segmentation in a fertilized ovum does not absolutely isolate the contained matter of each segment. In the ovum of Peripalus, at any rate, which is exceptionally adapted for displaying the early changes, there results a network of protoplasm which unites the cell-masses with one another. And Prof. Sedgwick, saying that In Peripatus "the connection of cell widi cell is not a secondary feature acquired late in development, but is primary," leans to the conclusion that " the continuity in the various cells of the adult:> is " due to a primitive continuity which has never been broken" (p. 49).
Thus, then, we must conceive of animal tissue as having from the beginning consisted of a matrix of. relatively inert substances throughout which there runs a nucleated network of living and active protoplasm.
§ 226$. As shown in the actions of a Rhizopod, protoplasm displays at once the properties of nerve and muscle: it 52CM PHYSICAL SYNTHESIS.
conducts and it contracts. These united properties still characterize it when it assumes the form of an imbedded network. That it continues to possess them when permeating vegetal tissues is proved by such actions as those of the Sensitive Plant and the Dioncsa — actions which show us both the conveyance of a disturbance and the production of movement at a distance. And that the protoplasmic network of animal tissue lias the like combined traits we see in such simple types as the Hydra.
Observe, next, that these properties are most markedly displayed where the protoplasm exists in an elongated form. If the tentacle of a polype is touched it contracts with tolerable promptness — with greater promptness than the body contracts. Among the oceanic Hydrozoa which,, floating or swimming, have long pendant tentacles, such as Diphyes and Physalia, the threads of nucleated sarcode thus trailing behind or hanging down, are quickly drawn up when struck by small creatures serving for prey. These traits are in great measure cause and consequence. Molecular change set up at the end of a thread-shaped portion of substance is necessarily limited to the line formed by the substance. It cannot be lost by diffusion through a large mass like that of the body, but must be concentrated within the channel formed by the sides of the thread.
This much premised, let us now ask what will result in the body of a creature which is as yet but little differentiated?
what will happen to the nucleated network of protoplasm diffused through it? From the general law of the instability of the homogeneous, it is to be inferred that the originallyunited properties of protoplasm will not remain the same throughout. We may expect a specialization such as will restrict the contractile power to some parts of it and leave the power of conduction to other parts. How will this differentiation be likely to arise? Suppose the incipientlyorganized mass to be from time to time rudely disturbed by passing bodies— How will the effects of the shocks be TEE GENESIS OP SIMPLE NERVOUS SYSTEMS. 521