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 homogeneous, 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 locomotion and on the motion of escape just described, will be insignificant; 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.
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 structure, 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 relative 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 advantages 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 elements. 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 increase of function may entail either increased thickness of these fibres or increased number of them. The one will arise from inheritance of functionally-produced modifications. The other will arise from inheritance of incidental 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 remain 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 unfavourably. 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 convexity 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 underlying 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 supposed 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>posing, then, that the muscles have here "been modified somewhat in the direction, of multiformit^y, a further specialization 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 issuing 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 repeated actions during this long life; and will be bequeathed with some functionally-produced improvements to posterity.
§ 235. It is not, I think, difficult to see that the same principles continue to hold; and that through successive small stages of analogous kinds, nervous systems may go on complicating. Let us glance at the processes that are likely to occur in a central ganglion that receives and sends out many compound bundles of nerve-fibres.
As implied by what has been said above, when Compound afferent bundles become channels of communication from sense-organs severally composed of many separate elements, the discharges they carry into the central ganglion become very variable in composition, and diffuse themselves through, its plexus in waves that are never twice exactly alike. The fibres composing the optic nerve,, for example, receive different sets of stimulations according to the size of the object, its form,, its direction, its distance. Consequently after a well-established reflex connexion has been formed between the visual impression given by a certain kind of prey in a- certain position, and the muscular adjustment required to seize such prey, it will happen that the excitement of the muscles must be preceded by, and accompanied by, numerous other excitements. For while there are going on those relative motions that end in bringing the prey to the position which calls forth the reflex action, many changing sets of impressions are being made on the optic fibres — some of them conducing to the approaching reflex action and some of them conducing to other actions.
Hence in the central ganglion, numbers of fibres and cells become nascent ly excited before a certain group of them become excited in such way as to cause the appropriate discharge to the muscles. Now the nascent excitations so caused are not lost: they nascently excite multitudinous efferent fibres belonging to various bundles; and through them throw various muscles into states of partial tension. Here, then, is an ever-present opportunity for further specialization of the correspondence. Suppose, for instance, that the reflex action above described is well adjusted to catching a special object seen in a special position, but that no means exists of so modifying the reflex action as to allow for the motion which the object has when it reaches this position. What will happen? As the object approaches this position from different directions, its image will travel over different sets of retinal elements. In passing over any particular set it excites in succession certain groups of optic fibres, certain clusters of fibres and cells in the central ga-nglion, and through them nascently excites many efferent fibres with the muscles they supply. The motions that will be gone through when the reflex action takes place, are sure to be somewhat modified by these states of tension previously given to muscles not immediately concerned. The modification may or may not tend to compensate for the motion which the object had when it reached the point where reflex action was set up. Bat if, by tending to compensate for this motion, the modification is beneficial, the structure producing it will be further developed; and, as before, will be established as an additional adjustment of inner relations to outer relations.
Before proceeding let us note, as bearing on the interpretations given in preceding parts of this work, as well as on interpretations about to be given, that, as above understood, each of these adjustments of inner to outer relations which eventually becomes automatic, passes through stages in which it is not automatic. It begins as a slight tendency 'for an impression or impressions to excite some muscle or muscles more than, the rest. During this stage the passage of the disturbance through the chief nervous centre is slow, hesitating, irregular. The sense-impressions being an appreciable time in the nervous centre before they produce partial motor excitements, remain present there as sense-impressions; and are then the equivalents of what, in higher creatures, we call sensations. Similarly, the nascer.t motor excitements are the equivalents of what, in higher creatures, are the ideas of the contractions to be produced. Gradually as, by repetition in the individual and in the succession of individuals, this additional connexion between impressions and motions becomes more definite, and the sequence more rapid, that link in it which is either consciousness or the homologue of consciousness, becomes shorter, and the process passes into the purely automatic.
CHAPTER V.
THE GENESIS OF DOUBLY-COMPOUND NERVOUS SYSTEMS.
§ 236. When Instead of nerves of touch, proceeding from a dozen or a score of tentacles,, we have to deal with multi tildes of such nerves proceeding from all parts of the skin — when instead of a simple eye., or an eve containing but few reiinal elements, we take an eye having a retina made up of thousands of elements, each of which yields a separate impression— when bundles of afferent fibres from complex organs of hearing, taste,, and smell have to be taken into account — when the stimuli carried in ever-varying amounts and combinations through these recipio-motor structures have to be traced in their effects upon similarly-compounded dinqo-niotor structures; explanations of the kind attempted in the foregoing pages become very difficult if not impossible. But though we cannot hope to interpret specifically the higher complications of nervous development, we may hope to form some general idea of the ways in which the processes traced thus far may work out results still more involved. To facilitate the formation of such general idea, it will be well to contemplate afresh the characters of the evolution we have followed thus far — changing1 somewhat the point of view, re-inforcing some of the conclusions reached, and developing others a stage. We shall then be better able to see where further evolution along the same lines is likely to carry us.
THE GENESIS OF DOUBLY-COMPOUTSTD NERVOUS SYSTEMS.
* § 237. So long as tliere exists but a single afferent nerve and a single mnscle supplied by the accompanying efferent nerve, external stimuli will produce but one kind of action, varying only in degree. Even when the epi-peripheral impressions are received at any or all of numerous points, such as the ends of tentacles, it roust still happen that while the motor apparatus remains quite simple, no modifications in the creature's adjustments can be made beyond the greater or less promptness and strength of the induced contractions.
Such small change as a bifurcation and double insertion of the muscular bundles, makes possible some difference in the kind of effect consequent on difference in the kind of stimulation. And as the nervo-muscular system becomes more complex, it becomes possible for various unlike sets of epi-peripheral impressions to produce various unlike combinations of muscular actions.
But this compounding of stimuli results in the appropriate compounding of movements, only on condition that the nervous centres have become proportionately compounded. The required compounding of them is in principle this: — Tke connexions of their fibres must be such that when any set of external relations to which the acts are to be adjusted, has been impresssd on the senses, the special cluster of stimulations produced, being carried along various afferent nerves, is, in the central plexus, so redistributed that, in passing out again, it discharges itself through particular sets of motor-fibres in particular proportions.
Every further re-distribution of this kind implies additional places for convergence and divergence of the nervewaves — additional ganglionic corpuscles. If a certain group of incoming fibres brings nerve-waves bearing certain proportions to one another, the appropriate group of outgoing fibres cannot have its components affected fti the requisite degrees unless there exists between the two groups a duly adjusted set of convergent and divergent channel^ ill differing in part from ail other sets. A general diffusion through the plexus could cause nothing but a general muscular excitement; and a special diffusion ending in discharges that are special in their directions and quantities is impossible unless by the intermediation of a special structure that is definite in proportion as the co-ordination is definite. ~^ As the case has been thus far stated, there can arise no such more specialized or more complex muscular action._ following the appropriate compound impression, unless i| through some favourable variation in the structure of the ganglionic plexus. But eventually a new cause of development conies into play. There conies a stage at which adjustments of inner to outer relations may not only be indirectly established by the survival of individuals having favourable variations; they may also be directly established by the inheritance of functionally -produced changes. And the direct establishment of them becomes active when there exists a consciousness sufficiently developed to perceive the connexion between a muscular act and its immediate effect; and when the creature is thus rendered capable of making slight modifications in its acts, of establishing these modifications as habits, and of causing correlative modifications in its nervous centres.
Before this process can be understood, it must be premised that as nervous structures become more complex and more integrated, the network of their connexions becomes so close that every special muscular excitement is accompanied by some general muscular excitement. Along with the concentrated discharge to particular muscles, the ganglionic plexuses inevitably carry off a certain diffused discharge to the muscles at large; and this diffused discharge produces on them very variable results. Suppose, now, that in putting out its head to seize prey scarcely within reach, a creature has repeatedly failed. Suppose that along with the group of motor actions approximately adapted to seize prey at TtfE GENESIS OE DOUBLY-COMPOUND NERVOUS SYSTEMS. 545 tins distance, the diffused discharge is, on some occasion, so distributed throughout the muscular system as to cause a slight forward movement of the body. Success will occur instead of failure; and after success will immediately come certain pleasurable sensations with an accompanying large draught Of nervous energy towards the organs employed in eating, &c. That is to say, the lines of nervous communication through which the diffused discharge happened in this case to pass, have opened a new way to certain wide channels of escape; and, consequently, they have suddenly become lines through which a large quantity of molecular motion is drawn, and lines which are so rendered more permeable than before. On recurrence of the circumstances, these muscular movements that were followed by success are likely to be repeated: what was at first an accidental combination of motions will now be a combination having considerable probability. For when on such subsequent occasion the visual impressions have produced nascent tendencies to the acts approximately fitted to seize the object, and when through these there are nascently excited all the states, sensory and motor, which accompany capture, it must happen that among the links in the connected excitations there will be excitations of those fibres and cells through which, on the previous occasion, the diffused discharge brought about the actions that caused success. The tendency for the diffused discharge to follow these lines will obviously be greater than before; and the probability of a successfully modified action will therefore be greater than before. Every repetition of it "will make still more permeable the new channels, and increase the probability of subsequent 'repetitions; until at length the nervous connexions become organized. * One other general fact must be insisted upon. As was pointed out at the close of the last chapter, advancing complexity of nervous organization necessitates an increasing quantity of excitations that do not cause motions. A creature in which the compound impression produced by a special object occupying a special position, arouses the muscular actions effecting capture of the object,, is a creature which, by implication, perpetually receives other compound impressions from objects occupying other positions. Each passing thing, as well as every thing passed, sends into its nervous centres variously- combined waves of disturbance, which course through their fibres and cells in ever-varying combinations, and which, having no special connexions with special motor adjustments, simply diffuse themselves without any more specific effect than that of augmenting the general discharge to the vital organs and muscular system at large. These are what, under their subjective aspects, we call feelings and ideas. And, manifestly, the more extensive and more intricate the central plexus grows, the more detached may these become from the actions — the more may the impressions produced by things and relations reverberate through the nervous system — the more may there arise trains of thought.
§ 238. Thus much premised, let us try to conceive how compound co-ordination passes into doubly-compound coordination. A broad contrast exists between the two;^. and we shall find reasons additional to those before given (§ 22) for assigning the function of doubly-compound co-ordination to the highest nervous centres.
Let Pig. 14 represent, diagrammatically, the chief nervous centre, now considerably evolved, to which afferent fibres bring all orders of epi-peripheral feelings; and fiom which efferent fibres carry to the muscles, the stimuli producing their appropriately-combined *t I TITE GENESIS OF DOUBLY-COMPOUND NEKVOUS SYSTEMS. 547 contractions. And suppose that while other parts of it have for their functions the co-ordination of those epi-peripheral feelings which are least relational, the part A co-ordinates the most relational feelings with one another and with the appropriate motions. Or, to speak specifically., suppose that A is the part of the central plexus where the compound visual impressions joined -with the compound impressions from the eye-muscles, are brought into adjusted relations with those combined muscular feelings and accompanying feelings of touch which are implied by actions of the limbs under guidance of the eyes. Then this part will be the one to which are brought the most involved clusters of feelings in ever-varying proportions through multitudinous fibres; and one from which issue through multitudinous fibres in ever-varying proportions the most complicated motor impulses. By implication, each special adjustment of the muscular motions to the visual impressions must have in this part its co-ordinating plexus of converging and diverging fibres with their points of junction — a plexus which, while having many elements in common with the plexuses that effect other co-ordinations, must have some elements peculiar to itself. Whence it follows that in proportion as these special adjustments becSme more numerous, there must be a multiplication of the elements peculiar to each. Consequently, if some one group of these co-ordinating plexuses takes on a relatively great development, in answer to the relatively immense sphere for new adjustments which certain environing conditions furnish, we may expect one part of this region A, to become protuberant, as at A'. And if these multitudinous new co-ordinating plexuses, growing continually more involved as they grow more numerous, admit of accumulation without limit, we may expect a growth of this protuberance. "We shall soon see that these suppositions and inferences are paralleled by facts.
§ 239. Visual impressions and their concomitants are coordinated with muscular actions and their concomitants in two ways — the one direct,, the other indirect. The direct co-ordinations include such as are possible to a creature by changing the relative positions of its parts without changing its position in space. The indirect co-ordinations include such as are possible only by changing its position in space as well as changing the relative positions of its parts. Let us contrast these two orders.
Without moving from the spot on which I stand, I can explore very completely all things within reach of my hands; and the combined sets of feelings I get have a certain distinctive character of great significance. From each of these things I can derive, simultaneously, four clusters of sensations — those it yields iny retinae, those which come from the specially-adjusted muscles of my eyes and head, those which come from the muscles of the arm and hand by which I lay hold of it, and those given me by its contact with the skin of my fingers. The order of co-ordinations which have this important character in common^ falls into two genera. One extensive genus of quadru ply-clustered sensations I get by exploring the surfaces of my body and limbs. I can adjust my eyes so as to see my hands while they move over my feet; I can use one hand to examine tactually the other hand and arm, and can observe with my eyes, as well as feel with my muscles, the movements I am making. The distinctive trait of quadruplyclustered sensations of this genus, is that each of them contains two sets of tactual sensations — one set coming from tlie parts touched and the other from the parts touching them. Indeed we may say that they are thus characterized by being quintnply-clustered. A much more extensive genus, distinguished by severally containing only a single tactual cluster, I get from all the objects that exist within a range of three feet or so on each side and in front, as well as above and below. I can stoop down to touch a THE GENESIS OF DOUBLY-COMPOUND NEftVOCTS SYSTEMS. 549 thing lying near my feet and see that I touch it. Without changing" my place,, I can successively raise my hand to take down my hat, grasp an umbrella-handle, touch the "back of a hall chair, lay hold of a letter waiting for me. Each of these acts gives me a special group of impressions of colour and form, a special group of muscular feelings from the muscles of the eyes and head, a special group from the muscles of the arm and hand, a special group from the skin of the fingers; and performance of each act implies special co-ordination of the special groups. Thus the region of space occupied by ray body and by things immediately around it, furnishes numerous compound clusters of sensations, severally having the peculiarity that their components can co-exist in consciousness. Within this range the conditions are such that an object which yields me groups of feeling's through the eye and through the eye-muscles, may, if I go through a certain series of muscular feelings, be made to yield me a group of tactual feelings joined with a group of feelings of muscular tension; and these additional groups may be brought into juxtaposition in consciousness with the first groups, without these first groups being in any degree chanyvd.
This immense order of co-ordinations is not absolutely demarcated from the far more immense order to which we shall immediately pass: there is a border region common to the two. While I keep my feet and body quite fixed, there is a definite limit to the range of my hands and therefore to the region •within which objects can yield me clustered feelings co-ordinated in the way described. But by leaning forward or on one side, and still more' by putting out one foot while keeping- the other stationary, I can reach additional objects, and make them yield me combined sets of feelings very much like the preceding ones. They are not quitQ, like however; for each of them contains certain additional elements — the feelings accompanying the partial change of place. These feelings form an intercalated set of links by which tlie visual cluster and its concomitants are brought into a relation of co-existence with the tactual cluster and its concomitants. They become important elements in the cluster in proportion as the objects are difficult to reach without moving from the spot. But while they thus make somewhat indefinite the division between the relatively-simple and direct co-ordinations that have been described, and the relatively-complex and indirect coordinations to be next dealt with., they do not obliterate the broad contrast.
For now mark that beyond the objects I see within easy reach, and beyond the objects I see and can reach by leaning., or by putting out one foot, there are immensely-more numerous objects which I see but canuot reach without locomotion, brief or prolonged. While I stand where I do; the picture on the opposite wall cannot by any actions of mine be made to yield nie tactual impressions: I put out my hands towards it, I bend my body in the same direction, I put one foot forward, all to no purpose. That I may touch it, I must take several steps., with their successive groups of muscular feelings in invleo-s and tactual feelings on the soles of my feet. Thus beyond the small space surrounding my body, there lie successive concentric portions of space containing* objects which after being seen cannot be tactually explored until there have been executed certain movements either of walking or running or leaping, of climbing up or getting down — the concomitant feelings varying with the direction and with the distance. Manifestly, we have here an order of co-ordinations vastly larger in number than the first; %nd manifestly, within this order itself the co-ordinations become increasingly numerous and increasingly complex as the remoteness increases. More than this ^is true. Co-ordinations of this higher order differ from those of the lower order, not only by containing clusters of locomotive feelings which join the visual cluster with the tactual cluster ' they differ in another all-essential THE GENESIS OF DOUBLY-COMPOUND NERVOUS SYSTEMS. 551