systems of sncli creatures have gained their complex structures and functions little by little; then, necessarily, the involved forms of consciousness which are the correlatives of these complex structures and functions must have arisen, by degrees. And as it is impossible truly to comprehend the organization, of the body in general, or of the nervous system in particular, without tracing its successive stages of complication; so it must be impossible to comprehend mental organization without similarly tracingits stages.
Here, then, we commence the study of Mind as objectively manifested in its ascending gradations through the various types of sentient beings.
§ 130. From what point are we likely to obtain the widest view of this evolution? How shall we guide ourselves towards a conception general enough to include the entire range of menial manifestations, up from creatures that yield but the faintest traces of feeling to creatures having intellects and emotions like our own? 3 In pursuance of the method of choosing hypotheses, we I must compare mental phenomena with the phenomena most | like them, and observe what character, presented by 110 • other phenomena, they both present.* A generalization uniting two different but allied classes of facts, necessarily unites all the facts contained in either class. Hence, if we find a formula which along with mental evolution includes the evolution nearest akin to it, we shall, by implication, find a formula comprehending the entire process of mental evolution. It may afterwards be needful so to limit this formula that mental evolution alone is expressed by it. But we shall best fulfil the requirements of clear exposition by first exhibiting mental evolution as it may be most gene- * Reference is here made to tlie omitted chapter on Method, named in the preceding note.
LIFE AND 3ITND AS CORRESPONDENCE. 293 rally conceived, and subsequently specializing the eonception.
The.. phenomena which those of Mind resemble in the greatest degree are those of bodily Jjfe. While these classes of phenomena are intimately related to one another, they are related to other classes of phenomena in comparatively remote ways. Oar question, therefore, becomes — What is it that mental life and bodily life have in common? And this amounts to the question — What distinguishes Life in general?
§ 131. Thus, in looking for a conception of mental evolution sufficientlv large to take in all the facts, we are led back to the definition of Life reached at the outset of the Principles of Biolo<jij.
In Part I., Chap. IV. of that work, the proximate idea we arrived at was that Life is "the definite combination of heterogeneous changes, both simultaneous and successive/* In the next chapter It was shown that to develop this proximate idea into a complete idea, it is needful to recognize the connexion between these actions going on within an organism and the actions going on without it. We saw that Life is adequately conceived only when we think of it, as:rthe definite combination of heterogeneous changes, j both simultaneous and successive, in correspondence with, external co-existences and sequences." Afterwards this definition was found to be reducible to the briefer definition— " The continuous adjustment of internal relations to external relations -/' and though, by leaving out the characteristic of heterogeneity, this definition is rendered somewhat too wide, so that it includes a few non- vital phenomena which simulate vitality, yet practically no error is likely to result from its use.
That Life consists in the maintenance.of inner actions corresponding' with outer actions, was confirmed on further observing how the degree of Life varies as the degree of correspondence. It was pointed out that, beginning witli tlie low life of plants and of rudimentary animals, the progress to life of liigher and higher kinds essentially consists in a continual improvement of the adaptation "between organic processes and processes which environ the organism. We observed how along with complexity of organization there goes an increase in the number, in the range, in the speciality, in the complexity, of the adjustments of inner relations to outer relations. And in tracing up the increase we found ourselves passing without break from the phenomena of bodily life to the phenomena of mental life.
We have now to start afresh, and to develop the general truth there briefly indicated into a combination of more special truths.
§ 182. In doing this it will be needful to begin with the life of forms almost too simple to be called organisms, that we may note the first traces of differentiation between the vital actions we class as physical and the vital actions we class as psychical. Though throughout we shall continue to regard these two classes of actions as falling within the one class marked out by our definition, yet, as we follow under each of its several aspects the progress of the correspondence between the organism and its environment, the reader will not fail to observe how we pass from the physical to the psychical the moment we rise above the correspondences that are few, simple, and immediate.
THE CORRESPONDENCE AS DIRECT AND HOMOGENEOUS.
§ 133. The lowest life is found in environments of unusual simplicity. Most environments present both coexistences and sequences; but there are some which, for a short time, present co-existences only; and in these, during* this short time, occur the least-developed organic forms. Of those classed with the vegetal kingdom, may be instanced the Yeast-plant, and the Protococeus nivalis or red snow alga. Of those held to be of animal nature, the Gregarina, and the Hydatid may be taken as samples.
The life of each of these organisms consists, almost wholly, of a few contemporaneous processes adjusted to the co-existent properties of the medium, which surrounds it. The yeast-plant has for its habitat a fluid consisting of water holding in solution certain oxy-hydro-carbons, some nitrogenous matter, oxygen, and probably other elements in minor proportions. That it may flourish, the water must be neither very hot nor very cold; and light must be excluded. The conditions being fulfilled, the yeast-plant displays what we call vital changes, in correspondence with chemical changes among the substances bathing its surface. The cell grows and multiplies; the fluid ferments; and while the fluid continues to supply the needful materials under the needful conditions, the cell continues to manifest the same phenomena. But let the temperature be consider- 296 GENERAL SYNTHESIS ably raised, or some of the ingredients exhausted, and the actions cease. The life, limited in length to the brief period during1 which the environment remains practically uniform, exhibits no successive changes such as those by which a shrub responds to the alternations of day and night, and of the seasons. Excluding those modifications of form and size which are the necessary concomitants of continued assimilation, the only successive changes exhibited by the yeast-plant in common with the higher plants, are those which end in the formation of spores. Determined as they probably are by the diminishing quantities of the materials needful for growth, these generative actions may be regarded as successive changes in the organism corresponding with successive changes in the environment; and most likely there is no organism but what, in addition to the simultaneous processes taking place in it, undergoes a serial process of this character. Evidently, however, the two orders of changes, answering in this case to the two allessential functions of assimilation and reproduction, exist under their simplest forms in correspondence with the simplest relations in the environment; and ending as they do with that new state of the environment soon arising, the life is as short as it is incomplex.
It is needless to present in detail each of the other cases named. The P-rotococcus nivalis exists in snow — a medium simple and constant in chemical character, and restricted in its variations of temperature. Reddening by its rapid multiplication large tracts in the arctic regions in a single night, during which the circumstances must remain almost uniform, this minute organism exhibits vital processes corresponding only to surrounding co-existences; and can undergo scarcely any changes corresponding to surrounding sequences. To a new state in its medium, it does not adapt itself but dies: the snow melts and it disappears. Similarly with the Gfegarina — a single-celled creature which inhabits the intestines of certain insects; which is there bathed by THE CORRESPONDENCE AS DIRECT AND HOMOGENEOUS. 297 nutritive liquid; which, is kept at a tolerably constant temperature; and which exists no longer than its special environment exists. In these and other such cases the peculiarities to be noted are: — first, that the actions in the organism are immediately dependent on the affinities of the elements touching it on all sides; and., second, that the internal changes proceed uniformly, or nearly so,, because, during the brief time that the life lasts, the external relations remain uniform,, or nearly so. The correspondence is at once direct and homogeneous. The disintegrating matter and the matter to be integrated, being everywhere diffused through the environment, it results that all the agents to which the vital changes stand related, are not only in contact with the organism, but continually in contact with it. And hence there need neither those motions nor locomotions, which, where they occur, involve more or less heterogeneity in the correspondence.
§ 134. In strictness, no other forms of life than those conditioned as.above described, can be said to exhibit a correspondence at once direct and homogeneous. But the transition to higher forms being gradual, it is impossible to make divisions in such way as entirely to avoid incongruities; and on the whole, it seenis best to notice here a class of organisms which, while they exhibit motion, either absolute or relative, do so with comparative uniformity. The simplest of the ciliated animalcules; the most regular of the compound ciliated organisms, like the Volvo v glolator; together with the Sponges and their allies; may be instanced as displaying life of this order.
Water, either fresh or salt, being in all these cases the medium inhabited, the general fact to be observed is, that the incipient multiformity of the vital actions is in correspondence with the incipient multiformity of the environment. Though, from a human point of view, the liquids in which the yeast-plant and the Gregarina live are far more 298 GIETEBAL SYNTHESIS.
heterogeneous than the water, either of the sea or of a pond; yet, relatively to these contained organisms, they are less heterogeneous. For every portion of the wort bathing tho cell-wall of the yeast-plant, and every portion of the nutritive emulsion surrounding the Gregarina, presents the matter to be assimilated; but every portion of the water in which a Protozoan swims, though it presents oxygen, does not present nutriment. Evenly diffused as the food of the first is, and irregularly scattered as is that of the last, the external relations must be more homogeneous to the one than to the other. And manifestly, an organism whose medium, though unceasingly disintegrating it, is not unceasingly supplying it with integrable matter, but presents only dispersed atoms of integrable matter, must either traverse its medium with such velocity as shall bring it in contact with the requisite quantity of integrable matter, or must cause the medium to move past it with such velocity — must have either an absolute motion, as the infusory animalcule, or a relative motion, as the sponge towards the water it draws in and expels. Thus then, the addition of mechanical changes to the changes displayed by motionless organisms, is the addition of new internal relations in correspondence with new external relations.
It is, however, to be remarked, that the processes by which movements of this order are effected, are themselves in direct and nearly homogeneous correspondence with almost everpresent properties of the environment. The fact that the ciliary action of fresh-water creatures ceases when they are put into sea-water, and that of sea-water creatures when they are put into fresh- water; the fact that when creatures displaying it have been killed, the ciliary action on uninjured parts, and even on parts that have been cut off, continues for a long time; and the further fact, discovered by Virchow, that ciliary motion which his ceased may be re-excited by a solution of caustic potash; unite to show that the motion of these microscopic hairs is caused by the immediate con- THE CORRESPONDENCE AS DIRECT AND HOMOGENEOUS. 299 tact of something in the environment — consists of a succession of minute internal changes, in correspondence with those minute recurring actions of the medium which the waving of the cilia themselves involve. And the occasional suspensions of the motion may possibly result from local deficiencies in the medium., of those materials or conditions that determine it; in which case this slight heterogeneity in the mechanical changes answers to a slight heterogeneity in the environment.
certain special changes, corresponding with special changes;in it. Though to the chemical, thermal, and hygrometric actions affecting the whole mass of its medium, the actions going on in the plant slowly respond, they do not respond to surrounding mechanical actions; as those of a wireworm gnawing its roots, or a herbivore browsing on its leaves. But the most conspicuous of a zoophyte's actions are those which follow the touching of its expanded tentacles. To a relation of co-existence between tangible and other properties, presented in a particular part of the environment, there corresponds, in the organism, a relation of sequence between certain tactual impressions and certain contractions. Here are several facts to be noticed. First, that being a stationary creature whose medium does not supply matter to be integrated so uniformly as it supplies disintegrating matter, the zoophyte must obtain matter to be integrated by arresting those portions here and there moving through its medium; and doing this presupposes sensitiveness and contractility connected in the manner seen. Second, that the ability to respond, not simply to the co-existences and sequences presented by the whole mass of the environment, but to the co-existences and sequences presented by particular bodies in it, is an advance in the correspondence; which is also rendered more heterogeneous by the addition.
§ 138. Of all these cases however, as of those in the last chapter, it is to be remarked, that the correspondence between internal and external relations extends only to external relations which have one or both terms in contact?with the organism. The processes going on in the yeastplant cease unless its cell- wall is bathed by the saccharine and other matters on whose affinities they depend. The tree must have its carbonic acid, water, earthy salts, ammonia, and the rest, applied directly to its surface in the presence of light and heat: until they are thus applied it TRE CORRESPONDENCE AS DIRECT BUT HETEROGENEOUS. 303 remains inert. And so too among the lowest animals, the substances to be assimilated must come in collision with the organism before any correspondence between inner and outer changes is shown. Alike in those forms of life whose environments perpetually present the disintegrating and integrable matters under the requisite conditions; in those whose environments perpetually present them, but under variable conditions • in those whose environments, though not full of integrable matter, yet contain it in such abundance that mere random locomotion brings them in contact with a sufficiency; and in those whose environments contain it in moving masses so numerous that,, though themselves stationary, chance brings them as many as they want — alike in all these forms of life, there is an absence of that correspondence between internal relations and distant external relations^ which characterizes more highly-endowed forms.
CHAPTER IY.
§ 139. ON ascending from the lowest types of life, in which the adjustment of inner relations to outer relations is thus limited, one marked manifestation of the heightening correspondence, is the increasing disWtf.ee at which co-existences and sequences in the environment produce adapted changes in the organism. This progress accompanies the development of the senses of smell, sight, hearing, &c., and the subsequent development of the intellect.
There is reason to believe that the susceptibilities to odours, colours, and sounds, arise by degrees out of that irritability which animal tissue, in its lowest forms, possesses. The saying of Democritus that all the senses are ''modifications of touch, modern science goes far to confirm. Smelling obviously implies the contact of dispersed particles with a specially-modified part of the organism — implies that these particles are so carried by a current of air or water as to impinge on this modified part. Hearing results when we feel the vibrations of the air lying in contact with our bodies. As the skin at large is sensitive to a succession of mechanical impulses given by dense matter; so certain external auditory structures, easily moved, are sensitive to a far more rapid succession of mechanical impulses given by matter of great tenuity. The organ of jight, again, is one through which the pulses or undulations of a yet more delicate THE OOKRESPONDE3TCE AS EXTENDING IN SPACE. 305 medium are impressed on us — undulations incomparably faster in a medium incomparably rarer. Here however, as before, contact of the undulating medium with an adapted part of the surface, is the pre-requisite. So that in every case the sensation produced in us by something in the. environment, involves mechanical action on some part of our periphery. In every case, therefore, touch, of a coarse or refined order, is implied. Not only do the conclusions of physicists support this doctrine which Demo critus taught y but thejconclusipns L_pf biologists do the like. The organs of the special senses are every one of them developed from the dermal system — are modifications of that same tissue in which the tactual sense in general is seated. Nor is this all. It is a remarkable fact that the eye and the ear are, in their types of structure, morphologically identical with the vilrissce, or most perfect organs of touch. (Principles of Biology, § 295.)
The hypothesis of Evolution implies that the senses in general have a yet deeper basis in those primordial properties of organic matter which distinguish it from inorganiq matter. And many facts point to the conclusion that seiisi Mitjr of all kinds takes its rise out of those fundamental processes of nutrition and waste — integration and disintegration— in which Lif<^ in its primitive form, consists.* Though these facts 'do not suffice to establish such a conclusion, and though it is not necessary to the general argument that they should be here given, yet they form so appropriate an introduction to the subject of the chapter that it will be well to devote a section to them.
§ 140. In the lowest animals, which are so little organized \ as to be almost, if not quite, homogeneous, all the j^ital! functions are diffused throughout the whole body. Every? part exhibits more or less of that contractility which in); higher creatures is confined to the muscles; that sensitive-; ness which they show only in the nerves; that ability to ( absorb nutriment which is eventually confined to the alimentary canal; that excretory action afterwards divided among the lungs, skin, and kidneys; that reproductive power which with them is localized. Where, as in the lowest creatures of all, the body consists of nothing more than a structureless substance, and where, as in somewhat higher and larger creatures, the body is little else than an ."aggregation of like units of nucleated protoplasm, there is an almost complete community of functions throughout; and only as fast as this originally-uniform tissue becomes I differentiated, does each part lose the power of sub- ; "serving other processes than its habitual one. (Principles But this specialization of functions does not altogether obliterate the original community of functions. Even where I " the physiological division of labour" has beeu carried ;l furthest, many of the tissues retain certain powers of ful-' filling one another's duties. In man, skin can discharge the ': office of mucous membrane, and mucous membrane of skin. Lungs and kidneys can to some extent supply each other's shortcomings. When the liver fails, biliary matter is got rid of through both skin and kidneys. In salivation, the glands of the mouth become supplementary excreting.organs. And the skin, while having mainly the function of ejecting perspirable matter, yet remains, to some extent, both a respiratory surface and a surface through which nutriment can be absorbed.
Bearing in mind this general fact, that throughout the life made up of unintelligent organic processes, heterogeneity of function arises out of a primordial homogeneity, the traces of which are never entirely lost, we shall be prepared for a parallelism of method and results in the evolution of that other division of life consisting of the * sensory and motor actions. Here, too, we may look for a I certain community of function throughout the whole organ-\ ism — a possession by the whole organism of those suscepti- THE CORRESPONDENCE AS EXTENDING Itf SPACE, 807 Imities which are ultimately located and developed in eyes, ejLrs,_nose, and the rest. The nucleated protoplasm which, by one process of differentiation and integration,, gives origin to the internal and external systems — the visceral and nervo-muscular organs — must have,, to some extent, the powers of the last as well as those of the first. Not only the fundamental division into vegetative and animal functions, "but the subdivisions of each of these, must be regarded as specializations of the various properties which every part of the elemental tissue possesses in some slight degree. Let us glance at the genesis of the several senses from this point of view.
Between touch and assimilation there exists, in the lowest creatures, an intimate connexion. In many Ehizopods the tactual surface and the absorbing surface are co-extensive. The Amcela, a speck of jelly having no constant form, sends out, in this or that direction, prolongations of its substance. One of these meeting with, and attaching itself to, some relatively fixed object, becomes a temporary limb by which the body of the creature is drawn for ward • but if this prolongation meets with some relatively small portion of organic matter, it slowly expands its extremity round this, slowly contracts, and slowly draws the nutritive morsel into the mass of the body, which collapses round it and presently dissolves it. That is to say, the same portion of tissue is at once arm, hand, mouth, and intestine — shows us the tactual and absorbent functions united in one. And if we assume, as we may fairly do, that the behaviour of this protruded part when its end touches assimilable matter, arises from some molecular action set up between the two — is caused by a commencing absorption of the assimilable matter, we shall see a still closer relation between the primordial sense and the primordial vegetative function.
In the same phenomena we may trace a nascent sense of; taste. The ability to discriminate between organic and1 inorganic matter, appears to. be. possessed. in some degree even by the.simplest animals. lUnzopods do nob absorb indiscriminately all fragments of available sizo; nor do tlio tentacles of polypes commonly bohavo in the same wny when touched by inorganic bodies as \vheu touched by organic bodies. And bearing in mind that to creatures living in water, the inorganic or mnutritivo matters are, generally speaking, the insoluble, while the organic or nutritive are the soluble; it may be inferred that the selective power which they possess is due,, as above implied, to the setting up of an assimilative process when assimilable, matter is brought in contact/ with them, and to tlio abseueo of that process when the matter presented is not assimilable. So that this selective power, which is an. incipient sense of taste, is, primarily, one aspect of that integrating action which mainly constitutes the life. For thus interpreting the facts we have the warrant that, even in. its highest developments, tasting forms one link in the chain of assimilative actions. The mouth is part of the alimentary canal, which secretes digestive fluids and takes up dissolved substances. The mouth does both these: its saliva, is a digestive fluid, and in the act of tasting, some of tlio dissolved substances are absorbed through the mucous membrane of the tongue and palate.
Smell has the same root with taste, and remains throughout closely associated with it. In aquatic creatures the two senses can be but degrees of the same: the 0110 responding •to a more dilute solution of nutritive substance, and tho other to a more concentrated solution. As tlio solublo matters which surround a fragment of animal tissue arc not confined to its actual surface, but are diil'used in tlio surrounding water with an abundance that decreases as tho distance increases, it is obvious that a greater susceptibility will render the fragment perceptible before there is absolute contact; and that so, taste must pass gradually into smell. The mtimate connection of taste with smell, and of both with touch, "is displayed even in man. Tho nerves of both are spread out under a membrane that is continuous with, and but a slight modification of, the skin; they lie under adjacent parts of this membrane, near its junction with the skin; the sensations they give are so closely allied that, knowing the smell of a substance, we can frequently form, an approximately true judgment of its taste; and to both, the substances to be perceived must be presented in solution— the sapid must be either already dissolved, or dissolvable by the saliva, and the odorous must be condensed by the film of moisture covering the membrane which lines the olfactory chambers. Thus, the difference is less between the modes in which the sensations are ultimately produced,, than between the forms under which the substances producing them originally exist — liquid or solid in the one case; gaseous or vaporous in the other. Further, the relationship of the sense of smell to the fundamental organic actions, is directly traceable even in ourselves. The nostrils and the olfactory chambers which open out of them, are simply divergent branches of the alimentary canal_, from which, in the embryo, they are not separate,* and absorbing into the system, as they do, some of the floating particles given off by the food that is being eaten, or is about to be eaten, their action, too, is but an evanescent form of nutrition. Add to which, that in so far as the olfactory action is not nutritive it is respiratory; and thus, in a sense, lies between the two primary vital processes.
Again, in its initial stages even the faculty of sigh/ is implicated with the functions of organic life. The organisms which occupy the border land between the animal and vegetal kingdoms, share with plants the ability to decompose carbonic acid under the influence of light. Water containing Protozoa gives off oxygen on exposure to the sun;s rays. The link between the two great divisions of living forms, which these lowest creatures present in structure, development, and chemical character, they appear to present in their nutritive action also* Naturally, then, we may expect that on passing from them to vegetal and animal organisms respectively,, we shall on the one hand find the ability to decompose carbonic acid by the agency of light more and more developed, and on the other hand more and more wanting. Recently disclosed facts answer to the expectation. In the first place, the researches of Sdmlfczc go to establish an identity between the chlorophyll of plants,, and the colouring matter of sundry low types of animals,, as for instance the Hydra. In the second place, the ][ydr<t habitually shuns the light — habitually chooses the dark side of the vessel in which it is placed. May we not infer that the sensitiveness to light which the Hydra exhibits, results from, the action of light on its contained chlorophyll; that as in plants, this action is one through which the components of chlorophyll are assimilated; and that thus, the power which the primordial tissue possesses to distinguish light, from darkness— a power which forms the germ of the visual faculty — is due to a modification produced by light OH the general vital processes? Any doubt that may be felt respecting this hypothesis, will, I think, disappear on remembering jthat even in ourselves the general surface of the body I resins a physiological sensitiveness to light. The darkening of the skin caused by long exposure to sunshine, imnlies a modified assimilation in the tissue penetrated by light- — a change in the absorption of materials supplied by the blood, In transparent and semi-transparent creatures, any such photogenic effect must pervade the whole body; and if so it is easy to understand how light may produce marked changes in such creatures.
That hearing has, like the other senses, a root in the primitive vital functions, there is little if any direct evidence. But for suspecting that it, too, is differentiated from thorn, we have the reason that to sound, as to light, tho whole animal organism in its simplest forms possesses a feeble susceptibility. A slight tap, causing a vibration to pass through the vessel containing them, is responded to by