localized? May we not reasonably say that the parts of the protoplasmic network which become contractile, will be those of which the contraction is least resisted; and, conversely, that the parts which cannot contract without greater resistance will remain conductive only. So far as ascertained; the facts harmonize with this supposition. Muscular fibres first make their appearance as processes of the epithelial cells forming the surface of the bod}7" (see Bal four's Embryology, vol. II. p. 657); and this is the part in which movement is least resisted, since the inert matter on one side only of the layer, has to be carried along with the contracting elements. Conversely, though the primitive nervous elements, being the recipients of external impressions,, first arise on the outer surface, yet the protoplasmic network connected with them, which lakes on the function of conducting impressions, soon becomes established below the surface: contraction in this pare of the network being opposed by inert tissue imbedding1 the fibres on all sides, and the assumption of conducting function solely being hence favoured.
A lurcher question now presents itself — By what physical process is the functional relation established between these muscular and nervous elements as fast as they differentiate? There is a not unsatisfactory answer. A portion of tissue which, by disturbance of one or other kind, has been made to contract,, is a portion in which molar motion has been produced at the cost of molecular motion: the one implies the other. Contrariwise, a portion of tissue which, by some incident force or stimulus, has been disturbed, but does not undergo contraction, is one in \vhicli the molecular motion generated remains free. In the first place, then, there is caused a deficiency, and in the second place there is caused a surplus. Consequently if, between the two, there is any channel through \vhich an equilibrium can be established, a flow of molecular motion through it may he anticipated. Such a channel exists in the protoplasmic network. Hence, when the iucipiently-differeiitiatiug tissues are disturbed, there will 5:22 PHYSICAL SYNTHESIS.
result a draught from the portion which is beginning to act as a sensorv organ to the portion which is beginning to act as a contractile organ — a draught which, repeated on each occasion, will, in conformity to the principle set forth in the last chapter, tend ever to make the channel more permeable.
And then,, lastly, observe that though there is implied a simultaneity in the production of the deficiency and the surplus in the case described, so that the contraction has ahva rly arisen at the time that the stimulus has been received, yet it is inferable that when the connecting channel has become easily permeable, the reception of a stimulus will cause a flow through the channel, and consequent arrival at the contracting part before this has been otherwise disturbed: the result being that the surplus molecular motion received will initiate a contraction. Sensation, or that which corresponds to it, will produce motion through incipient nerves, § '227. In many coelenterate creatures the contractile sii'1stance is partly differentiated into muscular fibres; which, however,, are distributed in a diffused way. In an Actiniay the average equality of the forces to which the body is exposed all round, is unfavourable to the formation of distinct muscles and a distinct nervous system. There is nothing which tends to bring the contractility to one place; and therefore nothing which causes the waves of molecular disturbance to take special courses. Probably in a sea-anemone, the incipient lines of nervous discharge are as much diffused as the muscular fibres are diffused. Noting only the fact that the contractile tissue which, when it acts, absorbs molecular motion, becomes differentiated before there arise distinct nerve-fibres conveying molecular motion from places where it has been evolved, let us take a hypothetical case fitted to m«tke intelligible the first step in nervous development.
Suppose that the process of continuous gemmation, by which creatures of these low types very generally multiply, is so carried on that the individuals successively produced THE GENESIS OF SIMPLE NERVOUS SYSTEMS.
are interfered witli by the colony more on one side than on the other side. Being unsymnietrically conditioned they will become unsymmetrically developed. (Principles Let Fig. 5 represent a creature of this kind that grows obliquely away from its elder neighbours; and let A B stand for the surface over which the colony is spreading. Then it must happen that when moving objects in the adjacent water, larger than those minute ones serving for prey, come against the creature, first striking its expanded tentacles and then its body, the most exposed part of its body C will be most frequently disturbed. Each time it is disturbed there will be propagated through it that form of molecular change from which contraction results, and there will occasionally be produced more molecules of this same type. (Principles of Biology § 302.) That is to say C will become a place where the contractions ars relatively frequent and decided, and where contractile protoplasm is greater in amount than elsewhere. What further will happen? Mostly when a collision occurs the tentacles are touched before the body; and, for reasons above given, the propagation of molecular change along them is comparatively rapid. Now at the part C, each evolution of mechanical motion is necessarily accompanied by an absorption of molecular motion. Consequently when from the disturbed end of the tentacle D there has been sent a wave of molecular motion, part of which is absorbed in the contraction of each successive portion of the tentacle but a surplus of which passes on, setting up contractions of the portions below, the final surplus when the wave has reached d, will be drafted off to the contractile portion C; since this, being struck the instant after, and made to con -tract, becomes a place where molecular motion is absorbed. But- such an action does not constitute a true nervous action. For the stimulus applied at D is not the cause of the concraction at G. The contraction at C is caused by a collision at 0: and the discharge from d to C cannot take place until after the contraction at C has commenced. Nevertheless, though not a nervous action proper, it may, by frequent repetition, grow into one. If restorations of equilibrium between d and C recur often — if they continually ta,ke the same route through the network of protoplasm — if this becomes a line of less and less resistance that drafts off the molecular motion with rapidity; then, eventually, when an approaching body touches the end of the tentacle D, the impulse conveyed down it and along the incipient nerve from d to C will reach C before the approaching body touches it. Now the contractile colloid at G is capable of having its special molecular transformation set up by various stimuli — by communicated molecular motion as well as by a blow. Hence when a wave of disturbance reaches it before it receives a blow, it will be^iu to contract in anticipation of the blow. A rude touch at the end of the tentacle D, will, by the shrinking it sets up at C, cause withdrawal of the body from the source of danger.
§ 228. To avoid complications of statement, I have presented this primitive nervous action under a simpler forir. than that which actually occurs. For the wave of molecular motion has to be conveyed not to a single point but to a portion of contractile colloid having considerable extension, many parts of which, simultaneously become places where molecular motion is being absorbed. Hence the wave passing to it will somewhere on its way tend to divide according to the respective tensions towards these respective parts. What will result?
Fig, 6 represents the same general distribution as "before, THE G EXE SIS OF SIMPLE NERVOUS SYSTEMS. 525 with the difference that the mass of contractile colloid G, is marked in dotted lines, and that at e the line of nervous communication is shown to take divergent and re-divergent courses towards differento parts of C. For this is the structure implied. The same tendency towards restoration of equilibrium which causes the wave to go from d to C,, will also cause \ \\ it to distribute itself with tolerable even- *&& \ \ ness to all parts of C -y since to any park -^ ^ which by contracting becomes minus molecular motion, the adjacent parts must ever tend to yield some of their relative surplus, and this must find its way along some line of least resistance.
Let us now ask what will happen at the place e. As was shown in the last chapter, the formation of a nerve-thread capable of conveying with facility a wave of molecular i notion, implies a definite line pursued by the wave and a definite adjustment of the molecules to that line;. and, consequently, such adjustment of the molecules as serves for a wave in one direction will not serve for waves in other directions. At the place e, then, where the wave breaks up and its parts diverge, the moieciil.es cannot so arrange themselves as to conduct with facility all parts of the wave. Recurring1 to our old simile, if a regular! v-arrangetl line of bricks on end comes to a place where there is a cluster of bricks on end, from which diverge other lines of regularly-arranged bricks on end, it is clear that when the first line is overthrown at its beginning and delivers its impulse into the cluster, the bricks forming the cluster must be irregularly overthrown — cannot fall in the same directions with all the divergent lines; and no repetitions of the process can adjust the bricks of the cluster into attitudes that will do this. Hence at the point e there will remain some of the nerve-colloid in an amorphous state. Though between the incoming line and the chief outgoing line (ii one carries much more of the wave than the rest) tkero may at last arise a polar arrangement of the molecules, yet this cannot also happen with the minor outgoing lines. But if at e the molecules remain unarranged, the wave of molecular motion brought there will be checked; and by as nmcli as it is checked will tend to cause decompositions among tlie unarranired molecules. As when bricks placed askew fall a train?! one anorher, their angles are more liable to damage than the angles of bricks placed symmetrically; so a nonp..-lar arrangement of the molecules subjects them to destroying forces which they are saved from by a polar arrangement. Xow if decomposition occurs at e, additional molecular motion must be disengaged; so that along the outgoing lines there will be discharged an augmented wave. Thus there will arise at e something having the character o£ a ganglion-corpuscle.
That the structure represented is like no known structure, is true. The most conspicuous deviation from fact is in the wide spreading of the lines between e and 0. And it may be asked — How does their divergence,, which appears a necessity of the argument, become so modified as to correspond with the observed distribution? I reply that tliough the process of direct equilibration will not change this dis-. tribution in the required way, it can be so changed by the process of indirect equilibration. (Principles of Biology § 164.) When in the course of further evolution neighbouring' parts acquire distinct structures, fibres occupying so much space as those between e and C will be in the way. An individual in which the lines as they leave the point e do not diverge so widely, will therefore have an advantage. And gradually, by survival of the fittest, there will result a type that has these once divergent fibres concentrated into a huncy.e, the members of which part company only when they arrive at C.
A more serious objection may be raised. The processes g.ven off by ganglion- cells do not ordinarily continue onwards THE G EXE SIS OF SIMPLE NERVOUS SYSTEMS. 527 as fibres that end in muscles., in the way implied. The hypothesis as above sketched out, is at variance with the drawings of the biologist. Bub this seemingly fatal objection may, I think, be satisfactorily met.
§ 229. For there remains to be introduced a complication which I have, for simplicity sake, omitted; and this complication implies a structure that corresponds with fact.
Tiirougiiout} the exposition we have attended only to the effects caused by the recurring excitations of a single tentacle; and the nervous structure described could arise only in a case of this imaginary simplicity. In reality the excitations are received by many tentacles, each of which sends a wave of disturbance to all parts of the contractile mass C. It does not follow that for every tentacle there must be formed an. independent set of nervous connexions like that shown above. Though each afferent fibre will need some place uf divergence e, yet from each such place of divergence, it is not needful to have a separate nerve-fibre to each of the separate parts of C that have to contract simultaneously. On the contrary, it is inferable that as for each afferent fibre there will be some place of divergence e} whence its wave of molecular motion begins to distribute itself; so, for each efferent fibre communicating with each part of C, there will be an analogous place of convergence, where all the portions of waves going to that part will unite. That the nature of the required structures may be clearly conceived, let us first illustS f trate,, diagrarainatieai'ly, the needful connexions. In Fig. 7, let A stand for half a dozen afjprent fibres, while the dots at a stand for the points of divergence that arise as above explained. Then if, in the muscle to which the wave is distributed, there are half a dozen contractile parts to be independently supplied, it is manifest tliat instead of an independent fibre diverging from each of the points «, aud running to each of these half-dozen con-' tmctile parts,, the same end will be achieved if.there are half a dozen efferent fibres E, setting out from, so many points e* which severally receive fibres from all the points a. Such an arrangement will indeed be more efficient; since along a fibre which conveys a larger wave, composed of many smaller waves, there will arise a greater facility for transmission than would arise along fibres that conveyed the smaller waves separately. A still simpler system of connexions will serve equally well, or — for reasons like those just assigned — still better. To bring any one of the points a into connexion, with all the points e, there does not need a separate fibre all the way to each. The arrangement shown in Fig-. 8, or that shown in Kg. 9, will suffice. Nor must even this more integrated set of connexions be repeated in full for each of the points a. In Fig. 105 each point a is joined with every point e, by a much smaller number of fibres. And since the fibres in this system will be more used than those in any other system, they will become more permeable channels.
Will this kind of structure result from the convergence and divergence of waves of molecular motion following lines of least resistance? We may infer that it will. If to some point a in Pig. 9, there hus been brought by the THE GENESIS OF SIMPLE KERVCDS SYSTEMS. 529 afferent) fibre from a tentacle a wave of molecular motion.,• if all the points e are the beginnings of efferent fibres severally ending in separate portions of a contractile mass,, which by contracting has just become a place where molecular motion is absorbed; if, therefore, between this point -a and all the points e, there arise molecular tensions; then the restoration of equilibrium will be effected by waves of molecular motion which, following a common route for some distance, will break up and diverge on approaching the points e — the numbers and positions of the places of divergence being determined by local conditions. Further, if from another of the points a} a wave has similarly to find its way along lines of least resistance to all the points e, it will do so by passing into some near point of this same plexus. So that between all the points a and all the points e, there will be produced numerous places of converging and diverging communication; each of whicb., for reasons above assigned, will be a place containing unarranged and unstable molecules of nerve-matter, liable to be decomposed when disturbed,, and to pass on in increased amounts the waves that disturb them.
Now if instead of the regularly arranged lines and points, we conceive lines and points irregularly arranged; and if instead of the half-dozen afferent fibres and as many efferent fibres, we suppose a score or more of each (which we must do to correspond with even the simplest observable cases) j and if we porportionately complicate the connecting plexus; we shall have something like a ganglion. Fig 11 represents such a structure. That it is less intricate than an actual ganglion is what might be ex* pected. The conditions presented by a mass of protoplasm out of which a ganglion is evolved, are sure to cause great irregularities; and it is not difficult to see that in the course of its evolution, there are likely to arise many iucipient lines of connexion which do not develop further because others have superseded them. The agreement between inference and observation is, I think, aa close as we can reasonably look for.
It may, indeed^ be objected that an actual ganglion differs from, this hypothetical ganglion in a more serious way — in not displaying a definite network. The microscope discloses an entangled maze of fibres, cells, and branched processes, that are not formed into a distinct plexus of connexions. To this niy reply is, that though I have thus far, for the sake of clearness, spoken of these structures as definite, it is not needful that they should be visibly so. A network of lines of least resistance, is alone requisite; and it may be in part so formed as to be visible and in part so unformed as to be invisible. This qualification must be borne in mind as applying throughout the chapters that are to follow.
§ 230. Let me before closing dispose of a remaining objection. A critical reader may ask — How can a state of molecular tension between two places separated by a great mass of amorphous organic substance, cause transmission along a definite line that divides aad sub-divides in the way described?
Doubtless such a process is not easy to imagine under the conditions we are apt to assume. But the apparent difficulty disappears when, instead of the conditions we are apt to assume, we take the conditions which actually occur. The error naturally fallen into is that of supposing these actions to go on in creatures of considerable bulk; whereas observation warrants us in concluding that they go on in extremely small creatures. The type of nervous system approaching nearest in simplicity to the hypothetical one described, we find among the Polyzoa — creatures of almost microscopic minuteness. The total length of an individual Polyzoon is from a 40th to a 20th of an inch; and if we set down the THE GENESIS OF SIMPLE NEEYOUS SYSTEMS. 531 distance from the roots of the tentacles to the nearest point of the muscle at a 100th of an inch,, we shall be much beyond the mark. When the scale is thus immensely reduced, the physical processes described become comprehensible. The thickness of protoplasm through which these restorations of equilibrium are effected being recognized as about the thickness of stout paper, it is no longer difficult to conceive the molecular tensions,, and transmissions of molecular motion, to take place in the way alleged, with the inferred results.
The structure described having been first formed on this extremely small scale, admits of eventual enlargement to any scale. Conducing to the preservation and growth of the individual; inherited by progeny capable from the aid it yields of growing still larger • and bequeathed with its accumulated increments of size and development to successively higher types, that spread into better habitats and adopt more profitable modes of life; this mere rudiment may, in course of geologic epochs, evolve into a conspicuous nervous apparatus possessed by a creature of large size. And so by this slow indirect method there may be established lines of nervous communication where direct establishment of them would be impossible.
Finally, it may be well to remind the reader that the argument does not necessitate the assertion that the primitive nervous system was formed in this particular way. The essence of the argument is, that to some place of greatest and most frequent contraction, lines of discharge will be formed from places habitually touched before this contraction is set up; and the case I have chosen is one which lent itself most readily for explanation — not one therefore asserted to be actual. With this caveat let us now pass from the.simplest case to more complex: cases. * M CHAPTER IV.
M CHAPTER IV.
§ 231. When contemplating the incipient differentiation of the psychical life from the physical life (§140), it was pointed out that the special senses arise through local modifications of nutrition caused by the special agents responded to. In some of the lowest animals the semitransparent body is coloured green,, red, or brown, by scattered portions of a matter akin to the colouring matter of plants; and the sensitiveness of these creatures to light is doubtless due to the assimilative actions which light sets up in this matter. Higher animals also habitually contain pigment, in cells and scattered granules; and though these are not limited to the superficial tissue, they are ordinarily most abundant in it. Of course the nutrition of deepseated portions of pigment goes on in the absence of light. But though light is certainly not the only cause of the nutrition of pigment, and perhaps not the chief cause, there is evidence that it is a cause; since pigment-grains near the surface commonly increase in size or number or both when much exposed to light. At any rate, we may safely say that in some kinds of pigment produced in animal tissue, f light produces marked molecular changes.
Now the rudimentary eye consists of a few pigmentgrains under the outermost dermal layer; and hence we* may infer that rudimentary vision is constituted by. the wave of disturbance which a sudden change in the states of these pigment-grains propagates through the body.
How such, pigment-grains become concentrated in the particular places they may most advantageously occupy we need not consider at any length. Other tilings equal, they will develop most where most light falls, and where, consequently, variations of light caused by adjacent things are strongest; and since a close cluster of pigment-grains when affected, will send through the body a more efficient wave of disturbance, natural selection will further the concentration — there will be a survival of individuals in which the approximation is greatest, ending in the formation of an integrated patch.
The pre-existence of a simple nervous system, akin to that described in the last chapter, being assumed, let us consider what will happen when incipient vision is added.
§ 232. Suppose /, Fig. 12, to be the cluster of pigment grains constituting the rudimentary eye. And suppose that from, these pigment -grains, when changed by variations in the amounts of light falling on them, there have been propagated waves of disturbance into the mass of organism. Then wherever these waves eventually go, there will arise behind these pigment-grains at g, a plexus of fibres and ganglion-cells. For reasons such as were given in § 229 the separate waves setting out from the separate dis turbed pigment-grains, and pursuing lines of least resistance, will quickly unite; and there will result a cluster of junctions occupied by unstable nerve-matter, whence the aggregate wave will direct itself inwards.
To what place will it tend? As before, to the place where molecular motion is being absorbed, If immediately after molecular motion is liberated at /, molecular motiyn is taken up in the muscle C, a molecular tension will arise between / and C; and motion along the line of least resistance will result. Which will be the line of least resistance? Already there lias been formed a line of easy transmission from the tactual organs to the muscle, along the line d to C; and^ other things equal, the line of least resistance from f to C will be one of which this pre-existing channel forms a part. Hence the tendency will be for the wave of molecular motion to take its coarse from / through the underlying1 plexus g to the pre-established ganglion at e; and gradually to form a connecting fibre.
Yvliat will be the functional effects of this? So long as the nervous communication is incipient., contraction must be set up in the muscle C, before molecular motion disengaged at / can cause a state of tension between / and C; and therefore an impression on the rudimentary eye will not produce a contraction. The only advantage derivable from such a structure in this early stage, would seem, to be that of increasing the amount of contraction otherwise initiated. But as soon as the channel for the transmission of molecular motion from / to the ganglion e becomes tolerably permeable, the molecular motion disengaged by an impression at f, finding its way along this channel, may reach the muscle before the molecular motion set up by touch can reach it; and a consequent contraction of the muscle will withdraw the body in anticipation of touch — the creature will retreat as though alarmed by the approaching object.
§ 233. A nervous system of the type described in the last chapter, or even a nervous system a stage more complex in type, like that just described, can effect none but the simplest adjustments. Small extensions of the correspondence in Space and in Time are alone achievable by it. Muscular contraction is produced by a certain strength of impression on the tentacles, whatever be the nature of the hody.striking them or the direction in which it is moving. Similarly, the rudimentary eye can do no more than convey to the muscle the impression caused by a change in the quantity of incident light; no matter whether that change THE GEXESIS OP COMPOUND NERVOUS SYSTEMS.
be caused by a small body close to or by a large one far off, and no matter whether the motion of the body is or is not such as will presently cause a collision. Nervous systems of these kinds can bring about no special adjustments of the inner acts to special directions and distances of outer objects. Let us consider what further complications will initiate such further adjustments.
More muscles than one are obviously pre-supposed -s otherwise the motion can vary in amount only. And there are obviously pre-supposed more than one place of independent stimulation; otherwise not more than one kind of impulse to contraction can be given. If all the tentacles are similarly connected with the same muscle, or if the channel of communication which each pigment-grain in the eye-speck has with the muscle is like that which every other has, there can be no qualitative distinctions among stimuli, and therefore no specialized motions. A simple locomotive creature (moved of course by muscles and not by cilia) fulfils the requisite conditions. Let us suppose one that is, like most locomotive creatures, bilaterally symmetrical — one having VZF./J ^wo rudimentary eyes and the two muscles, or sets of muscles, which the locomotion of such creatures implies. Suppose that in Fig. 13, a and b are the nerve-threads coming from the two rudimentary eyes to the ganglion e; and that through this, each of these threads is connected with all the threads in each of the two bundles d and /, running to the muscles G and H. Setting out with the least differentiated structure,, we will assume that by means of the plexus at e, each afferent fibre is similarly connected, and equally well connected, with each bundle of efferent fibres. What will in such*case happen? The stimuli continually received through the eye specks as the creature moves through the water, will act indifferently, and equally, through the two motor bundles on 5-50 PHYSICAL SYNTHESIS.