-^ ^ 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 ar- range 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 im- pulse into the cluster, the bricks forming the cluster must be irregularly overthrown — cannot fall in the same direc- tions 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 526 PHYSICAL SYNTHESIS.
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 un- arranired 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 non- p..-lar arrangement of the molecules subjects them to destroy- ing forces which they are saved from by a polar arrange- ment. Xow if decomposition occurs at e, additional mole- cular 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 struc- ture, 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 corre- spond 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 in- dividual 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 hypo- thesis as above sketched out, is at variance with the draw- ings 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 com- plication 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 ten- tacle; and the nervous structure described could arise only in a case of this imaginary simplicity. In reality the excita- tions 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 por- tions of waves going to that part will unite. That the nature of the required structures may be clearly concei- ved, let us first illustS f tS f trate,, diagrarainati- eai'ly, the needful con- nexions. 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 PHYSICAL SYNTHESIS.
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 sepa- rate fibre all the way to each. The arrange- ment 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 sys- tem 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 mole- cular 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 di- vergence 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 un- arranged 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 observ- able cases) j and if we porportionately complicate the con- necting plexus; we shall have something like a ganglion. Fig 11 represents such a structure. That it is less intri- cate than an actual gang- lion 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 530 PHYSICAL SYNTHESIS.
many iucipient lines of connexion which do not develop further because others have superseded them. The agree- ment 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 dis- closes an entangled maze of fibres, cells, and branched pro- cesses, that are not formed into a distinct plexus of con- nexions. 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 un- formed 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 observa- tion 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 con- ceive the molecular tensions,, and transmissions of mole- cular 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 estab- lished lines of nervous communication where direct estab- lishment of them would be impossible.
Finally, it may be well to remind the reader that the argument does not necessitate the assertion that the primi- tive 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 contrac- tion is set up; and the case I have chosen is one which lent itself most readily for explanation — not one therefore as- serted to be actual. With this caveat let us now pass from the.simplest case to more complex: cases. * M M CHAPTER IV.
THE GENESIS OF COMPOUND NERVOUS SYSTEMS.
§ 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 re- sponded to. In some of the lowest animals the semi- transparent 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 deep- seated 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 pigment- grains 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.
THE GENESIS OF COMPOUND NERVOUS SYSTEMS. 533 How such, pigment-grains become concentrated in the parti- cular 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 pro- pagated 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 in- wards.
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 534 PHYSICAL SYNTHESIS.
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 perme- able, 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 correspond- ence 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 other- wise 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 simi- larly 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 there- fore 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 con- nected 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 con- nected, 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.
the two sets of muscles — the alternating contractions of these supplying an instance of the rhythm inevitably generated by antagonistic energies. Only one specialization of the movements will be effected. So long as the changes in the visual stimuli arising from objects which the creature passes, or which pass it, are moderate, the muscles will be excited to moderate contractions. But the approach of a large object, causing sudden and strong impressions on the rudimentary eyes., will send to the muscles sudden and strong discharges, making them violently contract so as to produce a dart — a dart which, though made at random,, will usually decrease the chance of being caught, if the approaching body is a predatory animal.. But now, however much alike the connexions of the two afferent fibres with the two bundles of efferent fibres may have originally been, it must happen in virtue of the universal law of the instability of the homo- geneous, that they will become in some, or rather in most, individuals of the species, slightly unequal. Let the cells, processes and fibres of the ganglion e, be congenitally developed in such ways that the fibre a has somewhat easier communication with the bundle d than with the bundle /j or vice versa; and let the connexions of the fibre b similarly deviate from complete equality. The effects on ordinary loco- motion and on the motion of escape just described, will be in- significant; but there will occur under certain circumstances modified motions of great significance. Suppose that on the side A, an adjacent small object produces in the eye-speck, and sends through the optic fibre, a moderate disturbance. If the connexions of this fibre with the efferent bundle /are better than its connexions with the efferent bundle d, the muscle on the opposite side of the body will contract most; and the body (supposing it to bend like that of a fish) will be turned away from the object which produced the impression. If, contrariwise, its connexions with the bundle on its own side are the best, the body will be turned towards the object. Now in many cases the object is one that will serve for food.
THE GENESIS OF COMPOUND NERVOUS SYSTEMS. 537 If; then, this congenital variation in the nervous connexions is such that a moderate stimulus on the eye-speck makes the body turn away from the object yielding the stimulus, the individual will lose rather than gain by the incipient vision; and will therefore disappear. A contrary variation of struc- ture, entailing a contrary effect,, will conduce to the welfare of the individual on every occasion when the object towards which the body is turned is food. Each discharge thus sent in excess towards the one set of muscles, will increase the rela- tive permeability of the one set of channels over the other; making the one-sidedness of the next discharge greater still. And since the more decided this tendency becomes the more decidedly the welfare of the individual will be farthered,, the creature7 s life will, on the average of cases, be longer, and the number of progeny left will be greater than is usual in the species. I need scarcely add that among descendants inheriting this modification, functionally increased during the entire life of the parent, the same causes will insure not simply continuance of it but progressive development.
§ 234. A further step may now be taken. The ad- vantages derivable from rudimentary eyes such as are above supposed, will increase as the eyes are evolved, whether in size or in structure. A larger sensitive area will, other things equal, render the creature impressible b]j smaller objects and by remoter objects, thereby conducing to its welfare; so that survival of the fittest will favour the growth of visual spots made up of numerous sensitive ele- ments. As this multiplication of sensitive elements progresses the ganglionic plexus underneath the eye-speck will develop, and there will fall an additional amount of function on the fibres connecting it with the central ganglion. This in- crease of function may entail either increased thickness of these fibres or increased number of them. The one will arise from inheritance of functionally-produced modi- fications. The other will arise from inheritance of in- 538 PHYSICAL SYNTHESIS.
cidental variations; since we have clear proof that in a cluster of homologous parts there occasionally arises a mem - her in excess of the normal number. Assuming that a bundle of nerve-fibres connecting the enlarged eye with the central.sranglion has been thus established,, let us ask what will happen. From the instability of the homogeneous it follows, as before, that however completely alike may at first have been the connexions of these fibres with the different parrs of the central ganglion, their connexions eamiot re- main alike. And, as before, it is clear that- while some variations in their connexions will affect the movements of the creature favourably others will affect them unfa- vourably. What are the favourable variations likely to be? If over the visual surface, now composed of a considerable number of sensitive elements, the transparent epidermis has, by survival of the fittest, acquired that con- vexity usually observable, the impressions received will fall on the whole patch of sensitive elements onlv when the objects producing them ai^e opposite to the patch — an object muck in advance or behind, muck above or below, will cast a vfurae image on one portion of the patch only. Hence if the rlbres composing the afferent bundle are not related with absolute; equality to all parts of the nervous plexus under- lying the patch of sensitive elements (and mere differences of position must entail inequality) it will happen that when, out of the patch of sensitive elements, one group is affected more than ike rest, some members of the afferent bundle will carry larger waves of molecular disturbance than the rest. In cases where the muscular system consists, as sup- posed in the last section, of but two contractile masses capable of acting only as wholes, this somewhat increased ketrogeneiry of the reeipw-motor structures will produce no definite effects. But it is an indue lively -established fact that there frequently occur variation? in the numbers and attachments of muscular bundles: even in so specific a type as the human, such variations are not uncommon. Sut>pos- THE GENESIS OF COMPOUND NERVOUS SYSTEMS. 539 ing, then, that the muscles have here "been modified some- what in the direction, of multiformit^y, a further specializa- tion of movements becomes possible. For a discharge carried more largely by some fibres of the incipient optic nerve than by others, will, on arriving at the central ganglion, diffuse itself not quite in the same way as one brought by all the fibres in equal amounts. Hence two somewhat different discharges taking somewhat unlike courses through the central plexus of fibres and cells,, and issu- ing in their multiplied amounts through a bundle of efferent fibres, will severally affect this in diverse ways — some fibres of the bundle taking more of the one discharge and some more of the other. So that if the masses of contractile substance to which this bundle of efferent fibres is distributed are capable of any separateness in their actions,, the two discharges will work on them unlike effects, and the motions produced will not be the same. Now the differences in the produced motions, relatively to the objects causing these special impressions, are almost certain to be advantageous or disadvantageous. And, as before, the structures producing motions that are on the average advantageous will conduce to the long life of the individual; will Le developed by their