History of the Intellectual Development of Europe, Volume II (of 2)

So in that collection of substance which constitutes an animal; whatever may be its position, high or low, in the realm of life, there is a perpetual introduction of new material and a perpetual departure of the old. It is a form, rather than an individual, that we see. Its permanence altogether depends on the permanence of the external conditions. If they change, it also changes, and a new form is the result.

Characteristics of animal life. An animal is therefore a form through which material substance is visibly passing and suffering transmutation into new products. In that act of transmutation force is disengaged. That which we call its life is the display of the manner in which the force thus disengaged is expended.

Matter and force. A scientific examination of animal life must include two primary facts. It must consider whence and in what manner the stream of material substance has been derived, in what manner and whither it passes away. And, since force can not be created from nothing, and is in its very nature indestructible, it must determine from what source that which is displayed by animals has been obtained, in what manner it is employed, and what disposal is made of it eventually.

Force is derived from the sun. The force thus expended is originally derived from the sun. Plants are the intermedium for its conveyance. The inorganic material of a saline nature entering into their constitution is obtained from the soil in which they grow, as is also, for the most part, the water they require; but their organic substance is derived from the surrounding atmosphere, and hence it is strictly true that they are condensations from the air.

Mode in which plants obtain material substance. These statements may be sufficiently illustrated, and the relation between plants and animals shown, by tracing the course of any one of the ingredients entering into the vegetable composition, and derived, as has been said, from the air. For this purpose, if we select their chief solid element, carbon, the remarks applicable to the course it follows will hold good for other accompanying elements. It is scarcely necessary to embarrass the brief exposition of vegetable life now to be given by any historical details, since these will come with more propriety subsequently. It is sufficient to mention that the chemical explanations of vegetable physiology rest essentially on the discovery of oxygen gas by Priestley, of the constitution of carbonic acid by Lavoisier, and of water by Cavendish and Watt.

Action of a plant on the air. While the sun is shining, the green parts of plants, especially the leaves, decompose carbonic acid, one of the ingredients of the atmospheric air. This substance is composed of two elements, carbon and oxygen; the former is appropriated by the plant, and enters into the composition of elaborated or descending sap, from which forthwith organic products, such as starch, sugar, wood fibre, acids, and bases are made. The other element, the oxygen, is for the most part refused by the plant, and returns to the air. As the process of decomposition goes on, new portions of carbonic acid are presented through mechanical movements, the trembling of the leaf, breezes, and currents rising from the foliage warmed by the solar beams giving place to other cool currents that set in below.

The action of a plant upon the air is therefore the separation of combustible material from that medium. Carbon is thus obtained from carbonic acid; from water, hydrogen. Plant life is chemically an operation of reduction, for in like manner ammonia is decomposed into its constituents, which are nitrogen and hydrogen; and sulphuric and phosphoric acids, which like ammonia, may have been brought into the plant through its roots in the form of salt bodies, are made to yield up the oxygen with which they had been combined, and their sulphur and phosphorus, combustible elements, are appropriated.

Composition and resolution of matter and force. Every plant, from the humblest moss to the oak of a thousand years, is thus formed by the sun from material obtained from the air – combustible material once united with oxygen, but now separated from that body. It is of especial importance to remark that in this act of decomposition, force, under the form of light, has disappeared, and become incorporated with the combustible, the organizing material. This force is surrendered again, or reappears whenever the converse operation, combination with oxygen, occurs.

Vegetable products thus constitute a magazine in which force is stored up and preserved for any assignable time. Hence they are adapted for animal food and for the procuring of warmth. The heat evolved in the combustion of coal in domestic economy was originally light from the sun appropriated by plants in the Secondary geological times, and locked up for untold ages. The sun is also the source from which was derived the light obtained in all our artificial operations of burning gas, oil, fat, wax, for the purposes of illumination.

Correlation of physical forces. My own experiments have proved that it is the light of the sun, in contradistinction to the heat, which occasions the decomposition of carbonic acid, furnishing carbon to plants and oxygen to the atmosphere. But such is the relation of the so-called imponderable principles of chemistry to each other, and their mutual convertibility, that that which has disappeared in performing its function as light may reappear as heat or electricity, or in the production of some mechanical effect.

The nature of food. Food is used by all animals for the sake of the force it thus contains, the remark applying to the carnivora as well as the herbivora. In both cases the source of supply is the vegetable kingdom, indirectly or directly. The plant is thus indispensable to the animal. It is the collector and preserver of that force the expenditure of which constitutes the special display of animal life.

From this point of view, animals must therefore be considered as machines, in which force obtained as has been described, is utilized. The food they take, or the tissue that has been formed from it, is acted upon by the air they breathe, and undergoes partial or total oxydation, and now emerges again, in part as heat in part as nerve-force, in some few instances in part as light or electricity, the force that originally came from the sun.

Cycle through which matter and force pass. There is, therefore, a cycle or revolution through which material particles suitable for organization incessantly run. At one moment they exist as inorganic combinations in the air or the soil, then as portions of plants, then as portions of animals, then they return to the air or soil again to renew their cycle of movement. The metamorphoses feigned by the poets of antiquity have hence a foundation in fact, and the vegetable and animal, the organic and inorganic worlds are indissolubly bound together. Plants are reducing, animals oxydizing, machines. Plants form, animals destroy.

Thus, by the light of the sun, the carbonic acid of the atmosphere is decomposed – its oxygen is set free, its carbon furnished to plants. The products obtained serve for the food of animals, and in their systems the carbon is re-oxydized by the air they respire, and, resuming the condition of carbonic acid, is thrown back into the atmosphere in the breath, ready to be decomposed by the sunlight once more, and run through the same cycle of changes again. The growth of a plant and the respiration of an animal are dependent on each other.

The duration of matter and imperishability of force. Material particles are thus the vehicles of force. They undergo no destruction. Chemically speaking, they are eternal. And so, likewise, force never deteriorates or becomes lessened. It may assume new phases, but it is always intrinsically unimpaired. The only changes it can exhibit are those of aspect and of distribution; of aspect, as electricity, affinity, light, heat; of distribution, as when the diffused aggregate of many sunbeams is concentrated in one animal form.

It is but little that we know respecting the mutations and distribution of force in the universe. We cannot tell what becomes of that which has characterized animal life, though of its perpetuity we may be assured. It has no more been destroyed than the material particles of which such animals consist. They have been transmuted into new forms – it has taken on a new aspect. The sum total of matter in the world is invariable; so, likewise, is the sum total of force.

Theory of Averroes. These conclusions resemble in many respects those of the philosophy of Averroes, but they are free from the heresy which led the Lateran Council, under Leo X., to condemn the doctrines of the great Spanish Mohammedan. The error of Averroes consisted in this, that he confounded what is here spoken of under the designation of force with the psychical principle, and erroneously applied that which is true for animals to the case of man, who is to be considered as consisting of three essentially distinct parts – a material body, upon which operate various physical forces, guided and controlled by an intelligent soul.

In the following paragraphs the distinction here made is brought into more striking relief.

Anatomical mode of determining position in the animal series. The station of any animal in the organic series may be determined from the condition of its nervous system. To this observation man himself is not an exception. Indeed, just views of his position in the world, of the nature of his intellect and mental operations, can not be obtained except from the solid support afforded by Anatomy. The uselessness of the metaphysical sciences. The reader has doubtless remarked that, in the historical sketch of the later progress of Europe given in this book, I have not referred to metaphysics, or psychology, or mental philosophy. Cultivated as they have been, it was not possible for them to yield any other result than they did among the Greeks. A lever is no mechanical power unless it has a material point of support. It is only through the physical that the metaphysical can be discovered.

Necessity of resorting to Anatomy and Physiology. An exposition of the structure, the physical forces, and the intellectual operations of man must be founded on anatomy. We can only determine the methods of action from the study of the mechanism, and the right interpretation of that mechanism can only be ascertained from the construction of its parts, from observations of the manner in which they are developed, from comparisons with similar structures in other animals, not rejecting even the lowest, and from an investigation of their habits and peculiarities. Believing that, in the present state of science, doctrines in psychology, unless they are sustained by evidence derived from anatomy and physiology, are not to be relied on, I have not thought it necessary to devote much space to their introduction. They have not taken a part in the recent advances of humanity. They belong to an earlier social period, and are an anachronism in ours. I have referred to these points heretofore in my work on Physiology, and perhaps shall be excused the following extract:

"The study of this portion of the mechanism of man brings us therefore in contact with metaphysical science, and some of its fundamental dogmas we have to consider. Nearly all philosophers who have cultivated in recent times that branch of knowledge, have viewed with apprehension the rapid advances of physiology, foreseeing that it would attempt the final solution of problems which have exercised the ingenuity of the last twenty centuries. Solution of psychological questions. In this they are not mistaken. Certainly it is desirable that some new method should be introduced, which may give point and precision to whatever metaphysical truths exist, and enable us to distinguish, separate, and dismiss what are only vain and empty speculations.

Uncertainty of metaphysics. "So far from philosophy being a forbidden domain to the physiologist, it may be asserted that the time has now come when no one is entitled to express an opinion in philosophy unless he has first studied physiology. It has hitherto been to the detriment of truth that these processes of positive investigation have been repudiated. If from the construction of the human brain we may demonstrate the existence of a soul, is not that a gain? for there are many who are open to arguments of this class on whom speculative reasoning or a mere dictum falls without any weight. Why should we cast aside the solid facts presented to us by material objects? In his communications throughout the universe with us, God ever materializes. He equally speaks to us through the thousand graceful organic forms scattered in profusion over the surface of the earth, and through the motions and appearances presented by the celestial orbs. Our noblest and clearest conceptions of his attributes have been obtained from these material things. I am persuaded that the only possible route to truth in mental philosophy is through a study of the nervous mechanism. The experience of 2500 years, and the writings of the great metaphysicians attest, with a melancholy emphasis, the vanity of all other means.

"Whatever may be said by speculative philosophers to the contrary, the advancement of metaphysics is through the study of physiology. What sort of a science would optics have been among men who had purposely put out their own eyes? What would have been the progress of astronomy among those who disdained to look at the heavens? Yet such is the preposterous course followed by the so-called philosophers. They have given us imposing doctrines of the nature and attributes of the mind in absolute ignorance of its material substratum. Necessity of the interpretation of structure. Of the great authors who have thus succeeded one another in ephemeral celebrity, how many made themselves acquainted with the structure of the human brain? Doubtless some had been so unfortunate as never to see one! Yet that wonderful organ was the basis of all their speculations. In voluntarily isolating themselves from every solid fact which might serve to be a landmark to them, they may be truly said to have sailed upon a shoreless sea from which the fog never lifts. The only fact they teach us with certainty is, that they know nothing with certainty. It is the inherent difficulty of their method that it must lead to unsubstantial results. What is not founded on a material substratum is necessarily a castle in the air."

Intellectual relations of man depend on his nervous system. Considering thus that scientific views of the nature of man can only be obtained from an examination of his nervous system, and that the right interpretation of the manner of action of that system depends on the guiding light of comparative anatomy and physiology, I shall, in the following exposition, present the progress of discovery on those principles.

The rudimentary nervous system is automatic. In those low tribes of life which show the first indications of a nervous system, its operation is purely mechanical. An external impression, as a touch, made upon animals of that kind, is instantly answered to by a motion which they execute, and this without any manifestation of will or consciousness. The phenomenon is exactly of the same kind as in a machine of which, if a given lever is touched, a motion is instantly produced.

Two elementary forms of nerve structure. In any nervous system there are two portions anatomically distinct. They are, 1st, the fibrous; 2d, the vesicular. It may be desirable to describe briefly the construction and functions of each of these portions. Their conjoint action will then be intelligible.

Structure of a nerve fibre. 1st. A nerve fibre consists essentially of a delicate thread – the axis filament, as it is called – enveloped in an oil-like substance, which coagulates or congeals after death. This, in its turn, is inclosed in a thin investing sheath or membranous tube. Many such fibres bound together constitute a nerve.

Function of a nerve fibre is conduction. The function of such a nerve fibre is indisputably altogether of a physical kind, being the conveyance of influences from part to part. The axis filament is the line along which the translation occurs, the investing material being for the purpose of confining or insulating it, so as to prevent any lateral escape. Such a construction is the exact counterpart of many electrical contrivances, in which a metallic wire is coated over with sealing-wax or wrapped round with silk, the current being thus compelled to move in the wire without any lateral escape. Of such fibres, some convey their influences to the interior, and hence are called centripetal; some convey them to the exterior, and hence are called centrifugal. No anatomical difference in the structure of the two has, however, thus far been discovered. As in a conducting wire the electrical current moves in a progressive manner with a definite velocity, so in a nerve filament the influence advances progressively at a rate said to be dependent on the temperature of the animal examined. It seems in the cold-blooded to be much slower than in the hot. It has been estimated in the frog at eighty-five feet per second; in man at two hundred feet – an estimate probably too low.

The fibres thus described are of the kind designated by physiologists as the cerebro-spinal; there are others, passing under the name of the sympathetic, characterized by not possessing the investing medullary substance. In colour they are yellowish-gray; but it is not necessary here to consider them further.

Structure of a nerve vesicle. 2nd. The other portion of the nervous structure is the vesicular. As its name imports, it consists of vesicles filled with a gray granular material. Each vesicle has a thickened spot or nucleus upon it, and appears to be connected with one or more fibres. If the connexion is only with one, the vesicle is called unipolar; if with two, bipolar; if with many, multipolar or stellate. Every vesicle is abundantly supplied with blood.

Function of a nerve vesicle. As might be inferred from its structure, the vesicle differs altogether from the fibre in function. I may refer to my "Physiology" for the reasons which have led to the inference that these are contrivances for the purposes of permitting influences that have been translated along or confined within the fibre to escape and diffuse themselves in the gray granular material. They also permit influences that are coming through many different channels into a multipolar vesicle to communicate or mix with one another, and combine to produce new results. Moreover, in them influences may be long preserved, and thus they become magazines of force. Combined together, they constitute ganglia or nerve centres, on which, if impressions be made, they do not necessarily forthwith die out, but may remain gradually declining away for a long time. Thus is introduced into the nervous mechanism the element of time, and this important function of the nerve vesicle lies at the basis of memory.

It has been said that the vesicular portion of the nerve mechanism is copiously supplied with blood. Indeed, the condition indispensably necessary for its functional activity is waste by oxydation. Arterial vessels are abundantly furnished to insure the necessary supply of aerated blood, and veins to carry away the wasted products of decay. Also, through the former, the necessary materials for repair and renovation are brought. Physiological condition of nerve action is nerve waste. There is a definite waste of nervous substance in the production of a definite mechanical or intellectual result – a material connexion and condition that must never be overlooked. Hence it is plain that unless the repair and the waste are synchronously equal to one another, periodicities in the action of the nervous system will arise, this being the fundamental condition connected with the physical theories of sleep and fatigue.

The statements here made rest upon two distinct forms of evidence. In part they are derived from an interpretation of anatomical structure, and in part from direct experiment, chiefly by the aid of feeble electrical currents. The registering or preserving action displayed by a ganglion may be considered as an effect, resembling that of the construction known as Ritter's secondary piles.

It will not suit my purpose to offer more than the simplest illustration of the application of the foregoing facts. When an impression, either by pressure or in any other way, is made on the exterior termination of a centripetal fibre, the influence is conveyed with a velocity such as has been mentioned into the vesicle to which that fibre is attached, and thence, going forth along the centrifugal fibre, may give rise to motion through contraction of the muscle to which that fibre is distributed. Reflex action of the nervous system. An impression has thus produced a motion, and to the operation the designation of reflexion is commonly given. This reflexion takes place without consciousness. The three parts – the centripetal fibre, the vesicle, and the centrifugal fibre – conjointly constitute a simple nervous arc.

Gradual complexity of the nervous system. A repetition of these arcs, each precisely like all the others, constitutes the first step toward a complex nervous system. Their manner of arrangement is necessarily subordinated to the general plan of construction of the animals in which they occur. Thus, in the Radiates it is circular; in the Articulates, linear, or upon an axis. But, as the conditions of life require consentaneousness of motion in the different parts, these nerve arcs are not left isolated or without connexion with each other. As it is anatomically termed, they are commissured, nerve fibres passing from each to its neighbours, and each is thus brought into sympathy or connexion with all the others.

First appearance of special ganglia. The next advance is a very important one, for it indicates the general plan on which the nervous system is to be developed: it is the dedication of special nerve arcs to special duties. Thus, in the higher articulates and molluscs, there are such combinations expressly for the purpose of respiration and deglutition. Their action is altogether of the reflex kind; it takes place without consciousness. These ganglia are commissured for the sake of sympathetic action, and frequently several of them are coalesced for the sake of package.

This principle of dedication to special uses is carried out in the introduction of ganglia intended to be affected by light, or sounds, or odours. The impressions of those agencies are carried to the ganglion by its centripetal fibres. Such ganglia of special action are most commonly coalesced together, forming nervous masses of conspicuous size; they are always commissured with those for ordinary motions, the action being reflex, as in the preceding case, though of a higher order, since it is attended with consciousness.

They are automatic mechanisms. Such being the elementary construction of a nervous system, it is plain that animal tribes in which it exists in no higher degree of complexity must be merely automata. In this remark many insects must be included, for the instinct they display is altogether of a mechanical kind, and, so far as they are concerned, without design. Their actions are uniformly alike; what one does under given circumstances, under the same circumstances another will certainly do. They are incapable of education, they learn nothing by experience, and the acts they are engaged in they accomplish as well at the first trial as ever after.

Of parts like those described, and of others of a higher order, as will be presently seen, the most complex nervous system, even that of man, is composed. Evidence to be used in these investigations. It might, perhaps, be expected that for the determination of the duty of each part of such complex system the physiologist must necessarily resort to experiment, observing what functions have been injured or destroyed when given portions have been removed by his knife. At the best, however, evidence of that kind must be very unsatisfactory on account of the shock the entire system receives in vivisections, and accordingly, artificial evidence can, for the most part, be used only in a corroborative way. But, as Cuvier observed, the hand of Nature has prepared for us these very experiments without that drawback. The animal series, as we advance upward from its lowest members, proves to us what is the effect of the addition of new parts in succession to a nervous system, as also does any individual thereof in its successive periods of development. It is one of the most important discoveries of modern physiology that, as respects their nervous system, we can safely transfer our reasonings and conclusions from the case of the lowest to that of the highest animal tribes.

The articulata present structures and a mode of action illustrating in a striking manner the nervous system of man. Lengthwise upon their ventral region is laid a double cord, with ganglia, like a string of beads; sometimes the cords are a little distance apart, but more generally they are coalesced, each pair of ganglia being fused into one. First introduction of governing ganglia. To every segment of the body a pair is supplied, each pair controlling its own segment, and acting toward it automatically, each also acting like any of the others. But in the region of the head there is a special pair, the cephalic ganglia, receiving fibres from the eyes and other organs of sense. From them proceed filaments to the ventral cord, establishing communications with every segment. So every part has two connexions, one with its own ventral ganglia, and one with the cephalic.

It is not difficult to determine experimentally the functions of the ventral ganglia and those of the cephalic. If a centipede be decapitated, its body is still capable of moving, the motion being evidently of a reflex kind, originating in the pressure of the legs against the surface on which they rest. But thus far actions are only instinctive. The ventral cord, with its ganglia, is hence purely an automatic mechanism. But if, in making the decapitation, we leave a portion of the body in connexion with the head, we recognize very plainly that the cephalic ganglia are exercising a governing power. In the part from which they have been cut off the movement is forward, regardless of any obstacle; in that to which they are attached there are modifications in the motions, depending on sight or other special senses; obstacles are avoided, and a variety of directions pursued. Yet still the actions are not intelligent, only instinctive. The general conclusion therefore is, that the cephalic ganglia are of a higher order than the ventral, the latter being simply mechanical, the former instinctive; but thus far there is no trace of intelligence.

Nervous anatomy of vertebrates, as man. In man these typical parts are all present, and discharge the functions specified. His spinal cord answers to the ventral cord of the articulates. It has its lateral communications in the same way, and each segmental portion presents the same reflex action. Toward its upper part it dilates to form the medulla oblongata, sending forth nerves for respiration and deglutition. Their automatic apparatus. Of these the action is still reflex, as is proved by the involuntary movements of respiration and deglutition. A portion of food being placed in the pharynx, contraction instantly occurs, the will having no kind of control over the act of swallowing. Their instinctive apparatus. Above or in front of this enlargement is a series of ganglia, to which converge the nerves of special sense – of hearing, sight, smell; these are, therefore, the equivalents of the cephalic ganglia of insects, their function being also the same. In the lowest vertebrates, as in the amphioxus, the nervous system consists of nothing more. It may therefore be said to have only two parts – the cord and the sensory ganglia, and to have two functions – the automatic, attributable to the former, and the instinctive, attributable to the latter.

But as we advance from the low vertebrates upward in the animal scale, we begin to detect new organs; on the medulla oblongata a cerebellum, and on the sensory ganglia a cerebrum. Their intellectual apparatus. From this moment the animal displays reasoning powers, its intelligence becoming more strikingly marked as the development of the new organs is greater.

Functions of the brain. It remains to determine with exactness the function of one of these new parts, the cerebrum; the other portion, the cerebellum, being of minor interest, and connected, probably, with the locomotive apparatus. For the same reason it is unnecessary to speak of the sympathetic nerve, since it belongs to the apparatus of organic life. Confining our attention, therefore, to the true brain, or cerebrum, we soon recognize that the intelligence of an animal is, in a general manner, proportional to the relative size of this organ as compared with the sensory ganglia. We are also struck with the fact that the cerebrum does not send forth to other portions any independent fibres of its own, nor does it receive any from them, its only means of communication being through the parts that have been described – that is to say, through the sensory and automatic apparatus. Its relations to the instinctive and automatic portions. The cerebrum is therefore a mechanism of a higher order, and its relationship with the thalami optici and corpora striata indicate the conditions of its functions. It can only receive impressions which have come through them, and only act upon the body through their intermedium. Its secondary and tertiary lobes. Moreover, as we ascend the animal scale, we find that these cerebral parts not only increase in size, but likewise, in their turn, give rise to offshoots; secondary lobes emerging posteriorly on the primary ones, and, in due season, tertiary lobes posteriorly on the secondary. To these, in human anatomy, the designations of anterior, middle, and posterior lobes have been respectively given. In proportion, as this development has proceeded, the intellectual qualities have become more varied and more profound.

Action of the spinal cord alone. The relation of the cerebrum to the cranio-spinal axis is manifested by the circumstance that the latter can act without the former. In sleep the cerebrum is, as it were, torpid, but respiration, deglutition, and other reflex actions go on. If we touch the palm of a sleeping infant our finger is instantly grasped. Conjoint action of the brain and cord. But, though the axis can work without the cerebrum, the cerebrum can not work without the axis. Illustrations of these truths may be experimentally obtained. An animal from which the cerebrum has been purposely removed may be observed to perform actions automatic and instinctive, but never intelligent; and that there is no difference between animals and man in this respect is demonstrated by the numerous instances recorded in the works of medicine and surgery of injuries by accident or disease to the human nervous system, the effects corresponding to those artificially produced in experiments on animals. This important observation, moreover, shows that we may with correctness use the observations made on animals in our investigations of the human system.

Three distinct parts of the nervous system of man. In the nervous system of man our attention is therefore especially demanded by three essentially distinct parts – the spinal cord, the sensory ganglia, and the cerebrum. They are the automatic, the instinctive, the intellectual. Of the first, the spinal cord, the action is automatic; by its aid we can walk, from place to place, without bestowing a thought on our movements; by it we swallow involuntarily; by it we respire unconsciously. The second portion, the sensory ganglia, is, as we have seen, the counterpart of the cephalic ganglia of invertebrates; it is the place of reception of sensuous impressions and the seat of consciousness. To these ganglia instinct is to be referred. Their function is not at all impaired by the cerebrum superposed upon them. The third portion, the cerebrum, is anatomically distinct. It is the seat of ideas. It does not directly give rise to motions, being obliged to employ for that purpose its intermediate automatic associated apparatus. Dominating control of the latter. In this realm of ideas thoughts spring forth suggestively from one another in a perpetual train or flux, and yet the highest branch of the nervous mechanism still retains traces of the modes of operation of the parts from which it was developed. Its action is still often reflex. Reason is not always able to control our emotions, as when we laugh or weep in spite of ourselves, under the impression of some external incident. Nay, more; the inciting cause may be, as we very well know, nothing material – nothing but a recollection, an idea – and yet it is enough. But these phenomena are perhaps restricted to the first or anterior lobes of the brain, and, accordingly, we remark them most distinctly in children and in animals. As the second and third lobes begin to exercise their power, such effects are brought under control.

Progressive nervous development in the animal series. There is, therefore, a regular progression, a definite improvement in the nervous system of the animal series, the plan never varying, but being persistently carried out, and thus offering a powerful argument for relationship among all those successively improving forms, an observation which becomes of the utmost interest to us in its application to the vertebrates. In the amphioxus, as has been said, the cranio-spinal axis alone exists; the Cyclostome fishes are but a step higher. In fishes the true cerebrum appears at first in an insignificant manner, a condition repeated in the early embryonic state both of birds and mammals. An improvement is made in reptiles, whose cerebral hemispheres are larger than their optic lobes. As we advance to birds, a further increase occurs; the hemispheres are now of nearly sufficient dimensions to cover over those ganglia. In the lower mammals there is another step, yet not a very great one. But from the anterior lobes, which thus far have constituted the entire brain, there are next to be developed the middle lobes. In the Rodents the progress is still continued, and in the Ruminants and Pachyderms the convolutions have become well marked. It attains its maximum in man. In the higher carnivora and quadrumana the posterior or tertiary lobes appear. The passage from the anthropoid apes to man brings us to the utmost development thus far attained by the nervous system. The cerebrum has reached its maximum organization by a continued and unbroken process of development.

The same progressive development occurs in each individual man. This orderly development of the nervous system in the animal series is recognized again in the gradual development of the individual man. The primitive trace, as it faintly appears in the germinal membrane, marks out the place presently to be occupied by the cranio-spinal axis, and, that point of development gained, man answers to the amphioxus. Not until the twelfth week of embryonic life does he reach the state permanently presented by birds; at this time the anterior lobes are only perceptible. In four or six weeks more the middle lobes are evolved posteriorly on the anterior, and, finally, in a similar manner, the tertiary or posterior ones are formed. And thus it appears that, compared with the nervous system of other animals, that of man proceeds through the same predetermined succession of forms. Theirs suffers an arrest, in some instances at a lower, in some at a higher point, but his passes onward to completion.

It occurs again in the entire life of the globe. But that is not all. The biography of the earth, the life of the entire globe, corresponds to this progress of the individual, to this orderly relation of the animal series. Commencing with the oldest rocks that furnish animal remains, and advancing to the most recent, we recognize a continual improvement in construction, indicated by the degree of advancement of the nervous system. The earliest fishes did not proceed beyond that condition of the spinal column which is to be considered as embryonic. The Silurian and Devonian rocks do not present it in an ossified state. Fishes, up to the Carboniferous epoch, had a heterocercal tail, just as the embryos of osseous fishes of the present time have up to a certain period of their life. There was, therefore, an arrest in the old extinct forms, and an advance to a higher point in the more modern. The buckler-headed fishes of the Devonian rocks had their respiratory organs and much of their digestive apparatus in the head, and showed an approximation to the tadpoles or embryos of the frog. The crocodiles of the oolite had biconcave vertebræ, like the embryos of the recent ones which have gained the capability of making an advance to a higher point. In the geological order, reptiles make their appearance next after fishes, and this is what we should expect on the principle of an ascending nervous development. Not until long after come birds, later in date and higher in nervous advancement, capable not only of instinct, but also of intelligence. Of mammals, the first that appear are what we should have expected – the marsupials; but among the tertiary rocks, very many other forms are presented, the earlier ones, whether herbivorous or carnivorous, having a closer correspondence to the archetype than the existing ones, save in their embryonic states, the analogies occurring in such minor details as the possession of forty-four teeth. Absolute necessity of admitting transmutation of forms. The biography of the earth is thus, on the great scale, typical of individual life, even that of man, and the succession of species in the progress of numberless ages is the counterpart of the transmutation of an individual from form to form. As in a dissolving view, new objects emerge from old ones, and new forms spontaneously appear without the exercise of any periodical creative act.

Life of man from infancy to maturity in accordance with his anatomy. For some days after birth the actions of the human being are merely reflex. Its cranio-spinal axis alone is in operation, and thus far it is only an automaton. But soon the impressions of external objects begin to be registered or preserved in the sensory ganglia, and the evidences of memory appear. The first token of this is perhaps the display of an attachment to persons, not through any intelligent recognition of relationship, but merely because of familiarity. This is followed by the manifestation of a liking to accustomed places and a dread of strange ones. At this stage the infant is leading an instinctive life, and has made no greater advance than many of the lower mammals; but they linger here, while he proceeds onward. He soon shows high powers of memory, the exercise of reason in the determinations of judgment, and in the adaptation of varied means to varied ends.