basis of knowledge—perception—constitution of brain—white and grey matter—average size and weight of brains—european, negro, and ape—mechanism of perception—sensory and motor nerves—separate areas of brain—sensory and motor centres—abnormal states of brain—hypnotism—somnambulism—trance—thought-reading—spiritualism—reflex action—ideas how formed—number and space—creation unknowable—conceptions based on perceptions—metaphysics—descartes, kant, berkeley—anthropomorphism—laws of nature.
before entering on the higher subjects of religions and philosophies, it is well to arrive at some precise idea of the limits of human knowledge, and of the boundary line which separates the knowable from the unknowable. the ultimate basis of all knowledge is perception. without an environment to create impressions, and an organ to receive them, we should know absolutely nothing. what is the environment and what the organ of human knowledge? the environment is the whole surrounding universe, or, in the last analysis, the motions, or changes of motion, by which the objects in that universe make impressions on the recipient organ. the organ is the grey matter of that large nervous agglomeration, the brain. but here i must at the outset make two reservations. in the first place i do not define how these impressions are made. in all ordinary[126] cases they are made through the channels of the senses; but it is possible that in certain exceptional cases vibrations in the brain, causing perceptions, may be conveyed to it through the nerves in other ways. in somnambulism, for instance, it seems to be an ascertained fact that a somnambulist with closed eyes securely bandaged can walk in the dark and avoid obstacles as well as if guided by the sight in full daylight. there is a great deal of evidence also that in artificial somnambulism, otherwise called mesmerism or hypnotism, and also in what is called thought-reading, perceptions may be conveyed from one brain to another otherwise than by the usual methods of speech or writing. but these phenomena, however far they may be extended, do not affect the position that impressions on the brain are the essential condition of thought. if the grey matter of the brain is deficient or diseased the mind is affected, and beyond a certain point becomes extinct.
the second and more important reservation is, that although mind and all its qualities are thus indissolubly connected with matter, it by no means follows that they are matter or mere qualities of it. in the case of the atoms and energies, we know absolutely nothing of their real essence, and cannot form even a conception of what they are, how they came there, or what will become of them. it is the same with mind, soul, or self: we feel an instinctive certainty of their existence, as we do of that of matter; and we can trace their laws and manifestations under the conditions in which they are known to us, viz. those of association with matter and motion in the brain. but of their real essence or existence we know nothing, and it is as unscientific to affirm as to deny. directly we pass beyond the boundary[127] of such knowledge as really can be known by human faculty, and stand face to face with the mystery of the great unknown, we can only bow our heads with reverence and say with the poet,
behold, i know not anything.
i hope thus to steer safely between scylla and charybdis—between the arid rocks of materialism and the whirling eddies of spiritualism. materialist and spiritualist seem to me very like two men disputing as to the existence of life in the sun. ‘no,’ argues the former; ‘for the known conditions there are totally inconsistent with any life we can conceive.’ ‘yes,’ says the other; ‘for the belief fits in with many things which i earnestly wish to believe respecting a supreme being and a future existence.’ to the first i say, ignorance is not evidence; to the second, wishes are not proofs. for myself, while not quarrelling with those more favoured mortals who have, or fancy they have, superior knowledge, i can only say that i really know nothing; and this being the case, i see no use in saying that i know, and think it both more truthful and more modest to confess the limitation of my faculties.
with this caution i return to the field of positive knowledge. the brain, spinal marrow, and nerves consist of two substances: one white, which constitutes the great mass consisting of tubes or fibres; the other grey, which is an aggregation of minute cells, so minute that it has been computed that there are several millions of them in a space no larger than a sixpence. the bulk of this grey nerve-tissue is found in the higher animals, and especially in man, in the outside rind which covers the brain, and its amount is greatly increased by the convolutions[128] of that organ giving a greater extent of covering surface. in fact the convolutions of the average human brain give as much grey matter in a head of average size, as would be given by a head of four times the size if the brain were a plane surface. the extent of the convolutions is, therefore, a sure sign of the extent of intellect. they are more numerous and deeper in the european than in the negro; in the negro than in the chimpanzee; in the anthropoid ape than in the monkey or lemur. this grey nerve-tissue is the organ by which impressions from without are turned into perceptions, volitions, and evolutions of nerve force. the white matter is simply the medium of transmission, or we may say the telegraph wires by which the impressions are conveyed to the head office and the answers sent. the cell-tissue of the grey matter is thus emphatically the organ of the mind. in fact, if it did not sound too materialistic, we might call thought a secretion of the grey matter, only in saying so we must bear in mind that it is only a mode of expressing the fact that the two invariably go together; and that if we say with the german philosopher ‘ohne phosphor kein gedank,’ it does not mean that thought and phosphorus are identical, but simply that the condition on which thought depends is that of the existence of a material organ of which phosphorus is an ingredient.
that this grey nerve-tissue is really the organ of thought has been firmly established by numerous experiments both in man and the lower animals. injuries to it, or diseases in it, invariably affect what is called the mind; while considerable portions of the white matter may be removed without affecting the thinking and perceptive powers. a certain amount of it is[129] indispensable for the existence of intellect; the more there is of it as the brain increases in size and the convolutions become deeper, the greater is the intellect; when these fall below certain dimensions intellect is extinguished and we have idiocy. the average brain of the male white european weighs 49? ounces, of the negro a little under 47. the maximum brains which have been accurately weighed and measured, are those of cuvier and daniel webster, the weight of the former being 64? ounces, and the capacity of the latter being 122 cubic inches; while the average capacity of the teutonic race, including english, germans, and americans, is 92 inches, of the negro 83, and of the australian and hottentot 75. the brain of the idiot seldom weighs over 23 ounces, and the minimum weight consistent with a fair degree of intelligence is about 34 ounces.
the mechanism by which correspondence is kept up between the living individual and the surrounding universe is very simple—in reality, as simple as that of any ordinary electric circuit. in the most complex case, that of man, there are a number of nerve-endings, or small lumps of protoplasm, embedded in the tissues all over the body, or highly specialised and grouped together in separate organs such as the eye and ear, from which a nerve-fibre leads direct to the brain, or to the spinal cord and so up to the brain. these nerve-endings receive the different vibrations by which outward energy presents itself, which propagate a current or succession of vibrations of nerve-energy along the nerve-fibre. this nerve-fibre is a round thread of protoplasm covered by a white sheath of fatty matter which insulates it like the wire of a submarine telegraph[130] coated with gutta-percha. this nerve-wire leads up to a nerve-centre, consisting of two corpuscles of protoplasm: the first or sensory, a smaller one, which is connected by branches with the second, a much larger one, called the motor, from which a much larger nerve-fibre or wire proceeds, which terminates in a mass of protoplasm firmly attached to a muscle. thus, a sensation is propagated along the sensory nerve to the sensory nerve-centre, whence it is transmitted to the motor-centre, which acts as an accumulator of stored-up energy, a large flow of which is sent through the large conductor of the motor-nerve to the muscle, which it causes to contract and thus produces motion. it is thus that the simpler involuntary actions are produced by a process which is purely mechanical. in the more complex cases, in which consciousness and will are involved, the process is essentially the same, though more complicated. the message is transmitted to the brain, where it is received by a cluster of small sensory cells or nerve-centres, which are connected with another cluster of fewer and larger motor-centres, often at some distance from them, by a network of interlacing fibres. but it is always a case of a single circuit of wires, batteries, and accumulators, adapted for receiving, recording, and transmitting one sort of vibrations caused by and producing one sort of energy, and one only. the brain does not act as a whole, receiving indiscriminately impressions of light, sound, and heat; but by separate organs for each, located in separate parts of it. it is like a great central office, in one room of which you have a printing instrument reading off and recording messages sent through an electric telegraph; in another a telephone; in a third a self-registering thermometer,[131] and so on. and the same for the motor-centres and nerves. one set is told off to move the muscles of the face, another those of the arms, others for the legs and body, and so forth. this is further complicated by the fact that the brain like the rest of the body has two sides, a right and left, and that in some cases the motor-apparatus is doubled, each working only on one side, while in others the same battery and wires serve for both. as a rule the right hemisphere of the brain works the muscles of the left side of the body, and vice versa, so that an injury to one side of the brain may paralyse the voluntary motion of the limbs on the opposite side, leaving in a perfect condition those on its own side.
in the case of the higher functions involving thought, the upper part of the brain, which performs these functions, seems to be a sort of duplex machine, so that we have two brains capable of thinking, just as we have two eyes capable of seeing. it is a remarkable fact that the areas of the brain which are appropriated to the lowest and most instinctive functions, which appear first, lie lowest, and as the functions rise the position of their nerve-centres rises with them. thus, at the very base of the frontal convolutions at the lowest end of the fissure of rolando, we find the motor areas for the lower part of the face, by which the lowest animals and the new-born infant perform their solitary function of sucking and swallowing. higher up are the centres in the right and left brains for moving the upper limbs, that is, for seizing food and conveying it to the mouth, which is the next function in the ascending scale. next above these are the centres for moving the lower limbs and for co-ordinating the motions of the[132] arms and legs, marking the progression of an organism which can pursue and catch as well as eat its food. and still higher are the centres which regulate the motions of the trunk and body in correspondence with those of the limbs; while highest of all, at the front and hind ends of the enveloping cortex of the brain, come the organs of the intellectual faculties.
it is easy to see that this corresponds with the progression of the individual, for the infant sucks and cries for food from the first day, soon learns to extend its hand and grasp objects, but takes some time to learn to walk, and still longer to perform exercises like dancing or riding, in which the motions of the whole body have to be co-ordinated with those of the limbs. and as the development of the individual is an epitome of the evolution of life from protoplasm, we may well suppose that the brain was developed in this order from its first origin in a swelling at the end of the spinal cord as we find it in the lowest vertebrates.
it is a singular fact that the particular motor area which gives the faculty of articulate speech lies in a small patch of about one and a half square inches on the left side of the lower portion of the first brain. if this is injured, the disease called aphasia is produced, in which the patient loses the power of expressing ideas by connected words. the corresponding area on the right side cannot talk; but in left-handed persons this state of things is reversed, and the right side, which is generally aphasial, can be taught to speak in young people, though not in the aged.
higher up in the cortex, or convoluted envelope of the brain, come the areas for hearing and seeing, the latter being the more extensive. these areas are filled[133] mainly by a great number of sensory nerve-centres or cells, connected with one another in a very complicated network. these seem to be connected with the multitude of ideas which are excited in the brain by perceptions derived from the higher senses, especially that of sight. the simple movements are produced by a few large motor-centres, which have only one idea and do only one thing, whether it be to move the leg or the arm. but a sensation from sight often calls up a multitude of ideas. suppose you see the face of one with whom some fifty years ago you may have had some youthful love passages, but your lives drifted apart, and you now meet for the first time after these long years, how many ideas will crowd on the mind, how many nerve-cells will be set vibrating, and how many nerve-currents set coursing along intricate paths! no wonder that the nerve-corpuscles are numerous and minute, and the nerve-channels many and complicated.
when we come to the seats of the intellectual faculties the question becomes still more obscure. they are probably situated in the hinder and front parts of the surface of the brain, and depend on the grey matter consisting of an immense number of minute sensory cells. it has been computed that there are millions in the area of a square inch, and they are all in a state of the most delicate equilibrium, vibrating with the slightest breath of nervous impression. they depend for their activity entirely on the sensory perceptive centres, for there is no consciousness in the absence of sensory stimulation, as in dreamless sleep. perception, however caused, whether by outward stimulation of real objects, or by former perceptions revived by memory, sends a stream of energy through the sense-area, which[134] expands, like a river divided into numerous channels, fertilising the intellectual area, where it is stored up by memory, giving us the idea of continual individual existence, and by some mysterious and unknown process becoming transformed into consciousness and deliberate thought. and conversely the process is reversed when what we call will is excited, and the small currents of the intellectual area are concentrated by an effort of attention and sent along the proper nerve-channels to the motor-centres, whose function it is to produce the desired movement. this mechanical explanation, it will be observed, leaves entirely untouched the question of the real essence and origin of these intellectual faculties, as to which we know nothing more than we do of the real essence and origin of life, of matter, and of energy.
a very curious light however is thrown on them by phenomena which occur in abnormal states of the brain, as in trance, somnambulism, and hypnotism. in the latter, by straining the attention on a given object or idea, such as a coin held in the hand or a black wafer on a white wall, the normal action of the brain is, in the case of many persons—perhaps one out of every three or four—thrown out of gear, and a state induced in which the will seems to be annihilated, and the thoughts and actions brought into subjection to the will of another person. in this state also a cataleptic condition of the muscles is often induced, in which they acquire enormous strength and rigidity. in somnambulism outward consciousness is in a great measure suspended, and the somnambulist lives for the time in a walking dream which he acts and mistakes for reality. in this state old perceptions, scarcely felt at the time, seem to revive,[135] as in dreams, with such wonderful vividness and accuracy that the somnambulist in acting the dream does things altogether impossible in the waking state. thus an ignorant scotch servant-maid is said to have recited half a chapter of the hebrew version of the old testament: the explanation being that she had been in the service of a scotch minister, who was studying hebrew, and who used to walk about his room reciting this identical passage. it would seem as if the brain were like a very delicate photograph plate, which takes accurate impressions of all perceptions, whether we notice them or not, and stores them up ready to be reproduced whenever stronger impressions are dormant and memory by some strange caprice breathes on the plate.
most wonderful, however, are some of the phenomena of trance. in this case it really seems as if two distinct individuals might inhabit the same body. jones falls into a trance and dreams that he is smith. while the trance lasts he acts and talks as smith, he really is smith, and even addresses his former self jones as a stranger. when he wakes from the trance he has no recollection of it, and takes up the thread of his own life, just as if he had dozed for a minute instead of being in a trance for hours. but if he falls into a second trance, days or weeks afterwards, he takes up his trance life exactly where he dropped it, absolutely forgetting his intermediate real life. and so he may go on alternating between two lives, with two separate personalities and consciousnesses, being to all intents and purposes now jones and now smith. if he died during a trance, which would he be, jones or smith? the question is more easily asked than answered; but it[136] certainly appears as if with one mode of motion in the same brain you might have one mind and personal identity associated with it, and with another mode of motion different ones.
it would take me too far, and the facts are too doubtful, to investigate the large class of cases included under the terms thought-reading, telepathy, psychism, and spiritualism. it may suffice to say that there is a good deal of evidence for the reality of very curious phenomena, but none of any real weight for their being caused by any spiritualistic or supernatural agency. they all seem to resolve themselves into the assertion that under special conditions the perceptions of one brain can be reproduced in another otherwise than by the ordinary medium of the senses, and that in such conditions a special sort of cataleptic energy or psychic force may be developed. the amount of negative evidence is of course enormous, for it is certain that in millions upon millions of cases thought cannot be read, things are not seen beyond the range of vision, and coincidences do not occur between deaths and dreams or visions. neither can tables be turned, nor heavy bodies lifted, without some known form of energy and a fulcrum at which to apply it.
this borderland of knowledge is, therefore, best left to time, which is the best test of truth. that which is real will survive, and be gradually brought within the domain of science and made to fit in with other facts and laws of nature. that which is unreal will pass away, as ghosts and goblins have done, and be forgotten as the fickle fashion changes of superstitious fancy. in the meantime we shall do better to confine ourselves to ascertained facts and normal conditions.
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it is pretty certain that although the brain greatly preponderates as an organ of mind in man and the higher animals, the grey tissue in the spinal marrow and nervous ganglia exercises a limited amount of the same functions proportionate to its smaller quantity. the reflex or automatic actions, such as breathing, are carried on without reference to the brain, and the messages are received and transmitted through the local offices without going to the head office. this is the case with many complicated motions which originated in the brain, but have become habitual and automatic, as in walking, where thought and conscious effort only intervene when something unusual occurs which requires a reference to the head office; and in the still more complex case of the piano-player, who fingers difficult passages correctly while thinking of something else or even talking to a bystander.
indeed, in extreme cases, where experiments on the brain have been tried on lower animals, it is found that it can be entirely removed without destroying life, or affecting many of the actions which require perception and volition. thus, when the brain has been entirely removed from a pigeon, it smoothes its feathers with its bill when they have been ruffled, and places its head under its wing when it sleeps; and a frog under the same conditions, if held by one foot endeavours to draw it away, and if unsuccessful, places the other foot against an obstacle in order to get more purchase in the effort to liberate itself.
so much for the organ of mind; the other factor, that of outward stimulus, is still more obvious. if thought cannot exist without grey nerve-tissue, neither can it without impressions to stimulate that tissue. a[138] perfect brain, if cut off from all communication with the external universe, could no more think and have perceptions, than impressions from without could generate them without the appropriate nerve-tissue. once generated, the mind can store them up by memory, control them by reason, and gradually evolve from them ever higher and higher ideas and trains of reasoning, both in the individual and the species:—in the individual passing from infancy to manhood, partly by heredity from ancestors, and partly by education—using the word in the large sense of influences of all sorts from the surrounding environment; in the species, by a similar but much slower development from savagery to civilisation.
thus the whole fabric of arithmetic, algebra, and the higher calculi are built up from the primitive perception of number. the earliest pal?olithic savage must have been conscious of a difference between encountering one or two cave-bears or mammoths; and some existing races of savages have hardly got beyond this primitive perception. some australian tribes, it is said, have not got beyond three numerals, one, two, and a great number. but by degrees the perceptions of number have become more extensive and accurate, and the number of fingers on each hand has been used as a standard of comparison. thus ten, or two-hand, the number of fingers on the two hands has gradually become the basis of arithmetical numeration, and from this up to sir w. hamilton’s ‘quaternions’ the progression is regular and intelligible. but newton could never have invented the differential calculus and solved the problem of the heavens, if thousands of centuries before some primitive human mind had not received the[139] perception that two apples or two bears were different from one.
in like manner geometry, as its name indicates, arises from primitive perceptions of space, applied to the practical necessity of land-measuring in alluvial valleys like those of the nile and euphrates, where annual inundations obliterated to a great extent the dividing lines between adjoining properties. the first perceptions of space would take the form of the rectangle, or so many feet or paces, or cubits or arm-lengths, forwards, and so many sideways, to give the proper area; but as areas were irregular, it would be discovered that the triangle was necessary for more accurate measurement. hence the science of the triangle, circle, and other regular forms, as we see it developed in euclid and later treatises on geometry, until we see it in its latest development in speculations as to space of four dimensions.
but in all these cases we see the same fundamental principle as prevails throughout the universe under the name of the ‘conservation of energy’; always something out of something, never something out of nothing.
this, therefore, defines the limit of human knowledge, or boundary line between the knowable and the unknowable. whatever is transformation according to existing laws is, whether known or unknown, at any rate, knowable—whatever is creation is unknowable. we have absolutely no faculties to enable us to form the remotest conception of what the essence of these primary atoms and energies really is, how they came there, and how the laws, or invariable sequences, under which they act, came to be impressed on them. we[140] have no faculties, because we have never had any perceptions upon which the mind can work. reason and imagination can no more work without antecedent perceptions than a bird can fly in a vacuum.
thus, for instance, the imagination can invent dragons, centaurs, and any number of fabulous monsters, by piecing together fragments of perceptions in new combinations; but ask it to invent a monster whose head shall be that of an inhabitant of saturn and its body that of a denizen of jupiter, and where is it? of necessity all attempts to define or describe things of which we have never had perceptions, must be made in terms of things of which we have had perceptions, or, in other words, must be anthropomorphic.
so far as science gives any positive knowledge as to the relations of mind to matter, it amounts to this: that all we call mind is indissolubly connected with matter through the grey cells of the brain and other nervous ganglia. this is positive. if the skull could be removed without injury to the living organism, a skilful physiologist could play with his finger on the human brain, as on that of a dog, pigeon, or other animal, and by pressure on different notes, as on the keys of a piano, annihilate successively voluntary motion, speech, hearing, sight, and finally will, consciousness, reasoning power, and memory. but beyond this physical science cannot go. it cannot explain how molecular motions of cells of nerve-centres can be transformed into, or can create, the phenomena of mind, any more than it can explain how the atoms and energies to which it has traced up the material universe were themselves created or what they really are.
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all attempts to further fathom the depths of the unknown follow a different line, that of metaphysics, or, in other words, introspection of mind by mind, and endeavour to explain thought by thinking. on entering into this region we at once find that the solid earth is giving way under our feet, and that we are attempting to fly in an extremely rare atmosphere, if, indeed, we are not idly flapping our wings in an absolute vacuum. instead of ascertained facts which all recognise, and experiments which conducted under the same conditions always give the same results, we have a dissolving view of theories and intuitions, accepted by some, denied by others, and changing with the changing conditions of the age, and with individual varieties of characters, emotions, and wishes. thus, mind and soul are with some philosophers identical, with others mind is a product of soul; with some soul is a subtle essence, with others absolutely immaterial; with some it has an individual, with others a universal, existence; by some it is limited to man, by others conceded to the lower animals; by some located in the brain, by others in the heart, blood, pineal gland, or dura mater; with some it is pre-existent and immortal, with others created specially for its own individual organism; and so on ad infinitum. the greatest philosophers come mostly to the conclusion that we really know nothing about it. thus descartes, after having built up an elaborate metaphysical theory as to a spiritual, indivisible substance independent of the brain and cognisable by self-consciousness alone, ends by honestly confessing ‘that by natural reason we can make many conjectures about the soul, and have flattering hopes, but no assurance.’ kant also, greatest of metaphysicians[142] in demolishing the fallacies of former theories, when he comes to define his ‘noumenon,’ has to use the vaguest of phrases, such as ‘an indescribable something, safely located out of space and time, as such not subject to the mutabilities of those phenomenal spheres, ... and of whose ontological existence we are made aware by its phenomenal projections, or effects in consciousness.’ the sentence takes our breath away, and makes us sympathise with bishop berkeley when he says, ‘we metaphysicians have first raised a dust, and then complain we cannot see.’ it prepares us also for kant’s final admission that nothing can really be proved by metaphysics concerning the attributes, or even the existence, of the soul; though, on the other hand, as it cannot be disproved, its reality may for moral purposes be assumed.
it appears, therefore, that the efforts of the sublimest transcendentalists do not carry us one step farther than the conclusions of the commonest common-sense, viz. that there are certain fundamental conditions of thought, such as space, time, consciousness, personal identity, and freedom of will, which we cannot explain, but cannot get rid of. the sublimest speculations of a plato and a kant bring us back to the homely conclusions of the old woman in the nursery ballad, in whose mind grave questions as to her personal identity were raised by the felonious abstraction of the lower portion of her petticoat.
if i be i, as i think i be,
i’ve a little dog at home, and he’ll know me.
it is a safe ‘working hypothesis’ that when i go home in the afternoon, my wife, children, and little dog will recognise me as being ‘i myself i;’ but why or[143] how i am i, whether i was i before i was born, or shall be so after i am dead, i really know no more than the little dog who wags his tail and yelps for joy when he recognises my personal identity as something distinct from his own, when he sees me coming up the walk.
our conceptions, therefore, are necessarily based on our perceptions, and are what is called anthropomorphic. the term has almost come to be one of reproach, because it has so often been applied to religious conceptions of a deity with human, though often not very humane, attributes; but, if considered rightly, it is an inevitable necessity of any attempt to define such a being or beings. we can only conceive of such as of a magnified man, indefinitely magnified no doubt, but still with a will, intelligence, and faculties corresponding to our own. the whole supernatural or miraculous theory of the universe rests on the supposition that its phenomena are, in a great many cases, brought about, not by uniform law, but by the intervention of some power, which, by the exercise of will guided by intelligent design, alters the course of events and brings about special effects. as long as the theory is confined to knowable transformations of existing things, like those which are seen to be affected by human will, it is not necessarily inconceivable or irrational. inferring like effects from like causes, the hypothesis was by no means unreasonable that thunder and lightning, for instance, were caused by some angry invisible power in the clouds. on the contrary, the first savage who drew the deduction was a natural philosopher who reasoned quite justly from his assumed premises. whether the premises were true or not was a question which could only[144] be determined centuries later by the advance of accurate knowledge.
when do we say we know a thing? not when we know its essence and primary origin, for of these the wisest philosopher is as ignorant as the rudest savage; but when we know its place in the universe, its relation to other things, and can fit it in to that harmonious sequence of events which is summed up in what are called laws of nature. the highest knowledge is when we can trace it up to its earliest origin from existing matter and energy, and follow it downwards so as to be able to predict its results. the force of gravity affords a good illustration of this knowledge, both where it comes up to, and where it falls short of, perfection.
newton’s law leaves nothing to be desired as regards its universal application and power of prediction; but we do not yet fully understand its mode of action or its relation to other forms of energy. it is probable that some day we may be able to understand how the force of gravity appears to act instantaneously at a distance, and how all the transformable forces, gravity, light, heat, electricity, and molecular or atomic forces, are but different manifestations of one common energy. but in the meantime we know this for certain, that the law of gravity is not a local or special phenomenon, but prevails universally from the fixed stars to the atoms, from the infinitely great to the infinitely small. this is a fact to which all other phenomena, which are true facts and not illusions, must conform.
in like manner, if we find in caves or river-gravels, under circumstances implying enormous antiquity, and associated with remains of extinct animals, rude implements so exactly resembling those in use among[145] existing savages, that if the collection in the colonial exhibition of stone celts and arrow-heads used by the bushmen of south africa were placed side by side with one from the british museum of similar objects from kent’s cavern or the caves of the dordogne, no one but an expert could distinguish between them, the conclusion is inevitable that devonshire and southern france were inhabited at some remote period by a race of men not more advanced than the bushmen. any theory of man’s origin and evolution which is to hold water must take account of this fact and square with it. and so of a vast variety of facts which have been reduced to law and become certainly known during the last half-century. a great deal of ground remains unexplored or only partially explored; but sufficient has been discovered to enable us to say that what we know we know thoroughly, and that certain leading facts and principles undoubtedly prevail throughout the knowable universe, including not only that which is known, but that which is as yet partially or wholly unknown. for instance, the law of gravity, the conservation of energy, the indestructibility of matter, and the law of evolution, or development from the simple to the complex.
