" Life," according to De Blainville, " is the two-fold internal movement of composition and decomposition, at once general and continuous." This conception is in some respects too narrow, and in other respects too wide. On the one hand, while it expresses what physiologists distinguish as vegetative life, it excludes those nervous and muscular functions which form the most conspicuous and distinctive classes of vital phenomena. On the other hand, it describes not only the integrating and disintegrating processes going on in a living body, but it equally well describes those going on in a galvanic battery; which also exhibits a " two-fold internal movement of composition and decomposition, at once general and continuous."
Elsewhere, I have myself proposed to define Life as " the co-ordination of actions; "* and I still incline towards this definition as one answering to the facts with tolerable precision, * See Westminster Eevieio for April, 1852. — Art. IV. "A Theory of Population."
PROXIMATE DEFINITION OF LIFE. Gi It includes all organic changes, alike of the viscera, the limbs, and the brain. It excludes the great mass of inorganic changes; which display little or no co-ordination. By making co-ordination the specific characteristic of vitality, it involves the truths, that an arrest of co-ordination is death, and that imperfect co-ordination is disease. Moreover, it harmonizes with our ordinary ideas of life in its different gradations: seeing that the organisms which we rank as low in their degree of life, are those which display but little co-ordination of actions; and seeing that from t?iese up to man, the recognized increase in degree of life corresponds with an increase in the extent and complexity of co-ordination. But, like the others, this definition includes too much; for it may be said of the Solar System, with its regularlyrecurring movements and its self-balancing perturbations, that it, also, exhibits co-ordination of actions. And however plausibly it may be argued that, in the abstract, the motions of the planets and satellites are as properly comprehended in the idea of life, as the changes going on in a motionless, unsensitive seed; yet, it must be admitted that they are foreign to that idea as commonly received, and as here to be formulated.
It remains to add the definition since suggested by Mr G. H. Lewes — '' Life is a series of definite and successive changes, both of structure and composition, which take place within an individual without destroying its identity." The last fact which this statement has the merit of bringing into view — the persistence of a living organism as a whole, in spite of the continuous removal and replacement of its parts — is important. But otherwise it may be argued, that since changes of structure and composition, though probably the causes of muscular and nervous actions, are not the muscular and nervous actions themselves, the definition excludes the more visible movements Avith which our idea of life is most associated; and further, that in describing vital changes as a serieSy it scarcelv includes the fact that manv of them, as Nutrition, Circulation, Kespiration, and Secretion, in their many subdivisions, go on simultaneously.
Thus, however well each of these definitions expresses the phenomena of life under some of its aspects, no one of them is more than approximately true. It may turn out, that to find a formula which will bear every test is impossible. Meanwhile, it is possible to frame a more adequate formula than any of the foregoing. As we shall presently find, these all omit an essential peculiarity of vital changes in general — a peculiarity which, perhaps more than any other, distinguishes them from non- vital changes. Before specifying this peculiarity, however, it wdll be well to trace our way, step by step, to as complete an idea of Life as may be reached from our present stand- point: by doing which, we shall both see the necessity for each limitation as it is made, and ultimately be led to feel the need for a further limitation.
And here, as the best mode of determining what are those general characteristics which distinguish vitality from nonvitality, we shall do well to compare the two most unlike kinds of vitality, and see in what they agree. Manifestly, that which is essential to Life must be that which is common to Life of all orders. And manifestly, that which is common to all forms of Life, will most readily be seen on contrasting those forms of Life which have the least in common, or are the most uAlike.* § 25. Choosing assimilation, then, for our example of bodily life, and reasoning for our example of that life known as intelligence; it is first to be observed, that they are both processes of change. Without change, food cannot be taken into the blood nor transformed into tissue: without * This paragraph replaces a sentence that, in The Frinciples of Fsi/choloffi/, referred to a preceding chapter on "Method;" in which the mode of procedure here indicated, was set forth as a mode to be systematically pursued in the choice of hypotheses. Should opportunity ever permit, this chapter on Method will be embodied, along with other matter on the same topic, in a General Introduction to lu-iit Frinci2)lcs.
niOXIMATE DEFINITION OF LIFE. 63 change, there can be no getting from premisses to conclusion. And it is this conspicuous manifestation of change, which forms the substratum of our idea of Life in generah Doubtless we see innumerable changes to which no notion of vitality attaches: inorganic bodies are ever undergoing changes of temperature, changes of colour, changes of aggregation. But it will be admitted that the great majority of the phenomena displaj^ed by inorganic bodies, are statical and not dynamical; that the modifications of inorganic bodies are mostly slow and unobtrusive; that on the one hand, when we see sudden movements in inorganic bodies, we are apt to assume living agencj^, and on the other hand, when we see no movements in organic bodies, we are apt to assume death. From all which considerations it is manifest, that be the requisite qualifications what they may, a definition of Life must be a definition of some kind of change or changes.
On further comparing assimilation and reasoning, with a view of seeing in what respect the change displayed in both differs from non- vital change, we find that it differs in being not simple change, but change made up of successive changes. The transformation of food into tissue, involves mastication, deglutition, chj^mification, chylification, absorption, and those various actions gone through after the lacteal ducts have poured their contents into the blood. Carrying on an argument necessitates a long chain of states of consciousness; each implying a change of the preceding state. Inorganic changes, however, do not in any considerable degree exhibit this peculiarity. It is true that from meteorologic causes, inanimate objects are daily, sometimes hourty, undergoing modifications of temperature, of bulk, of hygrometric and electric condition. Not only, however, do these modifications lack that conspicuousness and that rapidity of succession which vital ones possess, but vital ones form an additional series. Living as well as not-living bodies are affected by atmospheric influences; and beyond the changes which these produce, living bodies exhibit other changes, more numerous and more marked. So that though organic change is not rigorously distinguished from inorganic change by presenting successive phases — though some inanimate objects, as watches, display phases of change both quick and numerous— though all objects are ever undergoing change of some kind, visible or invisible — though there is scarcely any object which does not, in the lapse of time, undergo a considerable amount of change that is fairly divisible into phases; yet, vital change so greatly exceeds other change in its display of varying phases, that we may consider this as practically one of its characteristics. Life, then, as thus roughly differentiated, may be regarded as change presenting successive phases; or otherwise, as a series of changes. And it should be observed, as a fact in harmony with this conception, that the higher the life the more conspicuous the variations. On comparing inferior with superior organisms, these last will be seen to display more rapid changes, or a more lenorthened series of them, or both.
Contemplating afresh our two typical phenomena, we may see that vital change is further distinguished from nonvital change, by being made up of many simultaneous changes. Assimilation is not simply a series of actions, but includes many actions going on together. During mastication the stomach is busy with the food already swallowed; on which it is both pouring out solvent fluids and expending muscular efforts. While the stomach is still active, the intestines are performing their secretive, contractile, and absorbent functions; and at the same time that one meal is being digested, the nutriment obtained from a previous meal is undergoing that transformation into tissue which constitutes the final act of assimilation. So also is it, in a certain sense, with mental changes. Though the states of consciousness which make up an argument occur in series, yet, as each of these states is complex — implies the simultaneous excitement of those many faculties by which the perception of any object or relation has been effected; it is obvious that each such change in niOXlMATE DEFINITION OF LIFE. 65 consciousness implies many component ch-anges. In this respect too, however, it must be admitted that the distinction between animate and inanimate is not precise. No mass of dead matter can have its temperature altered, without at the same time undergoing an alteration in bulk, and sometimes also in hygrometric state. An inorganic body cannot be oxidized, without being at the same time changed in weight, colour, atomic arrangement, temperature, and electric condition. And in some vast and mobile aggregates like the sea, the simultaneous as well as the successive changes displayed, outnumber those going on in an animal. Nevertheless, speaking generally, a living thing is distinguished from a dead thing, by the multiplicity of the changes at any moment taking place in it. Add to which, that by this peculiarity, as by the previous one, not only is the vital more or less clearly marked off from the non- vital; but creatures possessing high vitality are marked off from those possessing low vitality. It needs but to contrast the many organs co-operating in a mammal, with the few in a polype, to see that the actions which are progressing together in the body of the first, as much exceed in number the actions pro* gressing together in the body of the last, as these do those in a stone. As at present analyzed, then, Life consists of simultaneous and successive changes.
Continuing the comparison, we next find that vital changes, both visceral and cerebral, differ from other changes in their heterogeneity. Neither the simultaneous acts nor the serial acts, which together constitute the process of digestion, are at all alike. The states of consciousness comprised in any ratiocination are not repetitions of each other, either in composition or in modes of dependence. Inorganic processes, on the other hand, even when like organic ones in the number of the simultaneous and successive changes they involve, are unlike them in the homogeneity of these changes. In the case of the sea, just referred to, it is observable that countless as are the actions at any moment going on, they are mostly mechanical actions tliat are to a great degree similar; and in this respect widely differ from the actions at any moment taking place in an organism: which not only belong to the several classes, mechanical, chemical, thermal, electric, but present under each of these classes, innumerable unlike actions.
Even where life is nearly simulated, as by the working of a steam-engine, we may see that considerable as is the number of simultaneous changes, and rapid as are the successive ones, the regularity with which they soon recur in the same order and degree, renders them unlike those varied changes exhibited by a living creature. Still, it will be found that this peculiarity, like the foregoing ones, does not divide the two classes of changes with precision; inasmuch as there are inanimate things which exhibit considerable heterogeneity of change: for instance, a cloud. The variations of state which this undergoes, both simultaneous and successive, are many and quick; and they differ widely from each other both in quality and quantity. At the same instant there may occur in a cloud, change of position, change of form, change of size, change of density, change of colour, change of temperature, change of electric state; and these several kinds of change are continuously displayed in different degrees and combinations. Yet notwithstanding this, when we consider that very few inorganic objects manifest heterogeneity of change in a marked manner, while all organic objects manifest it; and further, that in ascending from low to high forms of life, we meet with an increasing variety in the kinds and amounts of changes displaj^ed; we see that there is here a further leading distinction between organic and inorganic actions. According to this modified conception, then. Life is made up of heterogeneous changes both simultaneous and successive.
If now we look for some point of agreement between the assimilative and logical processes, by which they are distinguished from those inorganic processes that are most like them iu the heterogeneity of the simultaneous and successive PROXIMATE DEFINITION OF LIFE. 07 cluanges they comprise, we discover that they are distinguished by the combination subsisting among their constituent changes. The acts that make up digestion are mutually dependent. Tliose composing a train of reasoning are in close connection. And generally, it is to be remarked of vital changes, that each is made possible by all, and all are affected b}^ each. Respiration, circulation, absorption, secretion, in their many sub-divisions, are bound up together. Muscular contraction involves chemical change, change of temperature, and change in the excretions. Active thought influences the operations of the stomach, of the heart, of the kidneys. But we miss this union among inorganic processes. Life-like as may seem the action of a volcano in respect of the heterogeneity of its many simultaneous and successive changes, it is not lifelike in respect of their combination. Though the chemical, mechanical, thermal, and electric phenomena exhibited, have some inter-dependence; yet the emission of stones, mud, lava, flame, ashes, smoke, steam, usually takes place irregularly in quantity, order, intervals, and mode of conjunction. Even here, however, it cannot be said that inanimate things present no parallels to animate ones. A glacier may be instanced as showing nearly as much combination in its changes as a plant of the lowest organization. It is ever growing and ever decaying; and the rates of its composition and decomposition preserve a tolerably constant ratio. It moves; and its motion is in immediate dependence on its thawing. It emits a torrent of water, which, in common with its motion, undergoes annual variations, as plants do. During part of the year the surface melts and freezes alternately; and on these changes are dependent the variations in movement, and in efflux of water. Thus we have growth, decay, changes of temperature, changes of consistence, changes of velocity, changes of excretion, all going on in connexion; and it may be as truly said of a glacier as of an animal, that by ceaseless integration and disintegration it gradually undergoes an entire change of substance without losing its individuality.
This exceptional instance, however, will scarcely be held to obscure that broad distinction from inorganic processes, which organic processes derive from the combination among their constituent changes. And the reality of this distinctiou becomes yet more manifest when we find that, in common with previous ones, it not only marks off the living from the not-living, but also things which live little from things which, live much. For while the changes going on in a plant or a zoophyte are so imperfectly combined that they can continue after it has been divided into two or more pieces, the combination among the changes going on in a mammal is so close that no part cut off from the rest can live, and any considerable disturbance of one function causes a cessation of the others. Life, therefore, as we now regard it, is a combination of heterogeneous changes, both simultaneous and successive.
Once more looking for a characteristic common to these two kinds of vital action, we perceive that the combinations of heterogeneous changes which constitute them, differ from the few combinations which they otherwise resemble, in respect of dcfiniteness. The associated changes going on in a glacier, admit of indefinite variation. Under a conceivable alteration of climate, its thawing and its progression may be stopped for myriads of years, without disabling it from again displaying these phenomena under appropriate conditions. By a geological convulsion, its motion may be arrested without an arrest of its thawing; or by an increase in the inclination of the surface it slides over, its motion may be accelerated without accelerating its rate of dissolution. Other things remaining the same, a more rapid deposit of snow may cause an indefinite increase of bulk; or, conversely the accretion may entirely cease, and yet all the other actions continue until the mass disappears. Here, then, the combination has none of that dcfiniteness which, in a plant, marks the mutual dependence of assimilation, respiration, and circulation; mucli less has it that dcfiniteness seen in the I PllOXIMATF- DEFLNITION OF MFE. 69 mutual (lopcndence of the chief animal functions: no one of which can be varied without varying the rest: no one of which can go on unless the rest go on. It is this definitcness of combination which distinguishes the changes occurring in a living body from those occurring in a dead one. Decomposition exhibits both simultaneous and successive changes, which are to some extent heterogeneous, and in a sense combined; but they are not combined in a definite manner. They vary according as the surrounding medium is air, water, or earth. They alter in nature with the temperature. If the local conditions are unlike, they progress differentlj^ in difTerent parts of the mass, without mutual influence. They may end in producing gases, or adipocire, or the dry substance of which mummies consist. They may occupy a few days, or thousands of years. Thus, neither in their simultaneous nor in their successive changes, do dead bodies display that definiteness of combination which characterizes living ones. It is true that in some inferior creatures the cycle of successive changes admits of a certain indefiniteness — that it may be apparently suspended for a long period by desiccation or freezing; and may afterwards go on as though there had been no breach in its continuity. But the circumstance that only a low order of life permits the cycle of its changes to be thus modified, serves but to suggest that, like the pre^ vious characteristics, this characteristic of definiteness in its combined changes, distinguishes high vitality from low vitality, as it distinguishes low vitality from inorganic processes. Hence, our formula as further amended reads thus: — life is a definite combination of heterogeneous changes, both simultaneous and successive.
Finally, we shall still better express the facts, if, instead of saying a definite combination of heterogeneous changes, we say the definite combination of heterogeneous changes. As it at present stands, the definition is defective both in allowing that there may be other definite combinations of heterogeneous changes, and in directing attention to the heteroro geneous changes rather than to the definiteness of their combination. Just as it is not so much its chemical elements which constitute an organism, as it is the arrangement of them into special tissues and organs; so it is not so much its heterogeneous changes which constitute Life, as it is the definite combination of them. Observe what it is that ceases when life ceases. In a dead bod}^ there are going on heterogeneous changes, both simultaneous and successive. What then has disappeared? The definite combination has disappeared. Mark, too, that however heterogeneous the simultaneous and successive changes exhibited by an inorganic object, as a volcano, we much less tend to think of it as living, than we do a watch or a steam-engine, which, though displaying homogeneous changes, displays them definitely combined. So dominant an element is this in our idea of Life, that even when an object is motionless, yet, if its parts be definitely combined, we conclude either that it has had life, or has been made by something having life. Thus then, we conclude that Life is — the definite combination of heterogeneous changes, both simultaneous and successive.
§ 26. Sucli is the conception at which we arrive without changing our stand-point. It is, liowever, an incomplete conception. This ultimate formula (which is to a considerable extent identical with one above given — " the co-ordination of actions; *' seeing that " definite combination " is synonymous with " co-ordination," and " changes both simultaneous and successive" are comprehended under the term " actions; " but which differs from it in specifying the fact, that the actions or changes are " heterogeneous ") — this ultimate formula, I say, is after all but proximately correct. It is true that it does not fail by including the growth of a crystal; for the successive changes this implies cannot be called heterogeneous. It is true that the action of a galvanic battery is not comprised in it; since liere, too, heterogeneity is not exhibited by the successive changes. It is true that by rilOXlMATE DEFINITION OF I.IFK. 71 this same qualification the motions of the Solar System are excluded; as are also those of a watch and a steam-engine. It is true, moreover, that while, in virtue of their heterogeneity, the actions going on in a cloud, in a volcano, in a glacier, fulfil the definition; they fall short of it in lacking definiteness of combination. It is further true that this definiteness of combination, distinguishes the changes taking place in an organism during life, from those which commence at death. And beyond all this it is true that, as well as serving to mark off, more or less clearly, organic actions from inorganic actions, each member of the definition serves to mark off the actions constituting high vitality from those constituting low vitality; seeing that life is high in proportion to the number of successive changes occurring between birth and death; in proportion to the number of simultaneous changes; in proportion to the heterogeneity of the changes; in proportion to the combination subsisting among the changes; and in proportion to the definiteness of their combination. Nevertheless, answering though it does to so many requirements, this definition is essentially defective.
It does not convey a complete idea of the thing contemplated. The definite combination of heterogeneous changes, both simtdtaneous and successive^ is a formula which fails to call up an adequate conception. And it fails from omitting the most distinctive peculiarity — the peculiarity of which we have the most familiar experience, and with which our notion of Life is, more than with any other, associated. It remains now to supplement the definition by the addition of this peculiarity.
CHAPTEE V.
THE CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES.
§ 27. We habitually distinguish between a live object and a dead one, by observing whether a change w^hich we make in the surrounding conditions, or one w^hich Nature makes in them, is or is not followed by some perceptible change in the object. By discovering that certain things shrink when touched, or fly away when approached, or start when a noise is made, the child first roughly discriminates between the living and the not-living; and the man when in doubt whether an animal he is looking at is dead or not, stirs it wijth his stick; or if it be at a distance, shouts, or throws a stone at it. Vegetal and animal life are alike primarily recognized by this process. The tree that puts out leaves when the spring brings a change of temperature, the flower which opens and closes with the rising and setting of the sun, the plant that droops when the soil is dry, and re-erects itself when watered, are considered alive because of these induced changes; in common with the zoophyte which contracts on the passing of a cloud over the sun, the worm that comes to the surface when the ground is continuously shaken, and the hedgehog that rolls itself up when attacked.
Not only, however, do we habitually look for some response when an external stimulus is applied to a living organism, but we perceive a fitness in the response. Dead as well as living things display changes under certain changes of con- CORRESroXDENCF. BETWEEN LIFE AND IT."=; CIRCUMSTANCES. 73 dition: instance, a lump of carbonate of soda that effervesces when dropped into sulphuric acid; a cord that contracts when wetted; a piece of bread that turns brown when hold near the fire. But in these cases, we do not see a connexion between the changes undergone, and the preservation of the things that undergo them; or, to avoid any teleological implication— the changes have no apparent relations to future external events which are sure or likely to take place. In vital changes, however, such relations are manifest. Light being necessary to vegetal life, we see in the action of a plant which, when much shaded, grows towards the unshaded side, an appropriateness which we should not see did it grow otherwise. Evidently the proceedings of a spider, which rushes out when its web is gently shaken and stays within when the shaking is violent, conduce better to the obtain ment of food and the avoidance of danger than were they reversed. The fact that we feel surprise when, as in the case of a bird fascinated by a snake, the conduct tends towards self-destruction, at once shows how generally we have observed an adaptation of living changes to changes in surrounding circumstances.
Note further the kindred truth, rendered so familiar by infinite repetition that we forget its significance, that there is invariably, and necessarily, a conformity between the vital functions of any organism, and the conditions in which it is placed — between the processes going on inside of it, and the processes going on outside of it. We know that a fish cannot live in air, or a man in water. An oak growing in the ocean, and a seaweed on the top of a hill, are incredible combinations of ideas. We find that every animal is limited to a certain range of climate; every plant to certain zones of latitude and elevation. Of the marine flora and fauna, each species is found exclusively between such and such depths. Some blind creatures flourish only in dark caves; the limpet only where it is alternately covered and uncovered by the tide; the red-snow alga rarely elsewhere than in the arctic regions or among alpine peaks.
74 THE DATA OF I^IOLOGY.
Grouping together the cases first named, in which a particular change in the circumstances of an organism is followed by a particular change in it, and the cases last named, in which the constant actions occurring within an organism imply some constant actions occurring without it; we see that in both, the changes or processes displayed by a living body are specially related to the changes or processes in its environment. And here we have the needful supplement to our conception of Life. Adding this all-important characteristic, our conception of Life becomes — The definite combination of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences. That the full significance of this addition may be seen, it will be necessary to glance at the correspondence under some of its leading aspects.* § 28. Neglecting minor requirements, tne actions going * Speaking of "the general idea of /*/<?," M. Comte says: — " Cette idee suppose, en effct, non-seulement celle d'un etre organise de maniere a comporter I'etat vital, mais aiissi celle, non moins indispensable, d'un certain ensemble d'inflaences exterieure propres a son accomplissement. Une telle harmonie entre I'etre vivant et le milieu correspondant, caracterise evidemment la condition fondamentale de la vie." Commenting on de Blainville's definition of life, which he adopts, he says: — ** Cette lumineuse definition ne me parait laisser rien d'important a desirer, si ce n'est une indication plus directe et plus explicite de ces deux conditions fondamentales co-relatives, necessairement inseparables de I'etat vivant, un organisme determine et un milieu convenable." It is strange that M. Comte should have thus recognized the necessity of a harmony between an organism and its environment, as a condition essential to life, and should not have seen that the continuous maintenance of such inner actions as will counterbalance outer actions, constitutes life. It is the more strange that he should have been so near this truth and yet missed it, since, besides his wide range of thought, M. Comte is often remarkable for his clear intuitions. Lest by saying this, I should deepen a misconception into which some have fallen, let me take the opportunity of stating, that though I believe some of M. Comte's minor generalizations to be true, and thouo-h I recognize the profundity of many