We have before found our conceptions of vital processes made clearer by studying analogous social processes. In societies there is a mutual dependence of functions, essentially like that which exists in organisms; and there is also an essentially like reaction of functions on structures. From the laws of adaptive modification in societies, we may therefore hope to get a clue to the laws of adaptive modification in organisms. Let us suppose, then, that a society has arrived at a state of equilibrium analogous to that of a mature animal—a state not like our own, in which growth and structural development are rapidly going on, but a state of settled balance among the functional powers of the various classes and industrial bodies, and a consequent fixity in the relative sizes of such classes and bodies. Further, let us suppose that in a society thus balanced there occurs something which throws an unusual demand on one industry—say an unusual demand for ships (which we will assume to be built of iron) in consequence of a competing mercantile nation having been prostrated by famine or pestilence. The immediate result of this additional demand for iron ships is the employment of more workmen, and the purchase of more iron, by the ship-builders; and when, presently, the demand continuing, the ship-builders find their premises and machinery insufficient, they enlarge them. If the extra requirement persists, the high interest and high wages bring such extra capital and labour into the business as are needed for new ship-building establishments. But such extra capital and labour do not come quickly; since, in a balanced community, not increasing in population and wealth, labour and capital have to be drawn from other industries, where they are already yielding the ordinary returns. Let us now go a step further. Suppose that this iron-ship-building industry, having enlarged as much as the available capital and labour permit, is still unequal to the demand; what limits its immediate further growth? The lack of iron. By the hypothesis, the iron-producing industry, like all theother industries throughout the community, yields only as much iron as is habitually required for all the purposes to which iron is applied: ship-building being only one. If, then, extra iron is required for ship-building, the first effect is to withdraw part of the iron habitually consumed for other purposes, and to raise the price of iron. Presently, the iron-makers feel this change and their stocks dwindle. As, however, the quantity of iron required for ship-building forms but a small part of the total quantity required for all purposes, the extra demand on the iron-makers can be nothing like so great in proportion as is the extra demand on the ship-builders. Whence it follows that there will be much less tendency to an immediate enlargement of the iron-producing industry; since the extra quantity will for some time be obtained by working extra hours. Nevertheless if, as fast as more iron can be thus supplied, the ship-building industry goes on growing—if, consequently, the iron-makers experience a permanently-increased demand, and out of their greater profits get higher interest on capital, as well as pay higher wages; there will eventually be an abstraction of capital and labour from other industries to enlarge the iron-producing industry: new blast-furnaces, new rolling-mills, new cottages for workmen, will be erected. But obviously, the inertia of capital and labour to be overcome before the iron-producing industry can grow by a decrease of certain other industries, will prevent its growth from taking place until long after the increased ship-building industry has demanded it; and meanwhile, the growth of the ship-building industry must be limited by the deficiency of iron. A remoter restraint of the same nature meets us if we go a step further—a restraint which can be overcome only in a still longer time. For the manufacture of iron depends on the supply of coal. The production of coal being previously in equilibrium with the consumption; and the consumption of coal for the manufacture of iron being but a small part of the total consumption; it follows that aconsiderable extension of the iron manufacture, when it at length takes place, will cause but a comparatively small additional demand on the coal-owners and coal-miners—a demand which will not, for a long period, suffice to cause enlargement of the coal-trade, by drawing capital and labour from other investments and occupations. And until the permanent extra demand for coal has become great enough to draw from other investments and occupations sufficient capital and labour to sink new mines, the increasing production of iron must be restricted by the scarcity of coal, and the multiplication of ship-yards and ship-builders must be checked by the want of iron. Thus, in a community which has reached a state of moving equilibrium, though any one industry directly affected by an additional demand may rapidly undergo a small extra growth, yet a growth beyond this, requiring as it does the building-up of subservient industries, less directly and strongly affected, as well as the partial unbuilding of other industries, can take place only with comparative slowness. And a still further growth, requiring structural modifications of industries still more distantly affected, must take place still more slowly.
On returning from this analogy, we see more clearly the truth that any considerable member of an animal organism, cannot be greatly enlarged without some general reorganization. Besides a building up of the primary, secondary, and tertiary groups of the subservient parts, there must be an unbuilding of sundry non-subservient parts; or, at any rate, there must be permanently established a lower nutrition of such non-subservient parts. For it must be remembered that in a mature animal, or one which has reached a balance between assimilation and expenditure, there cannot (supposing general conditions to remain constant) be an increase in the nutrition of some organs without a decrease in the nutrition of others; and an organic establishment of the increase implies an organic establishment of the decrease—implies more or less change in the processes and structuresthroughout the entire system. And here, indeed, is disclosed one reason why growing animals undergo adaptations so much more readily than adult ones. For while there is surplus nutrition, it is possible for specially-exercised parts to be specially enlarged without any positive deduction from other parts. There is required only that negative deduction implied in the diminished growth of other parts.
§ 70. Pursuing the argument further, we reach an explanation of the third general truth; namely that organisms, and species of organisms, which, under new conditions, have undergone adaptive modifications, soon return to something like their original structures when restored to their original conditions. Seeing, as we have done, how excess of action and excess of nutrition in any part of an organism, must affect action and nutrition in subservient parts, and these again in other parts, until the re-action has divided and subdivided itself throughout the organism, affecting in decreasing degrees the more and more numerous parts more and more remotely implicated; we see that the consequent changes in the parts remotely implicated, constituting the great mass of the organism, must be extremely slow. Hence, if the need for the adaptive modification ceases before the great mass of the organism has been much altered in its structure by these ramified but minute reactions, we shall have a condition in which the specially-modified part is not in equilibrium with the rest. All the remotely-affected organs, as yet but little changed, will, in the absence of the perturbing cause, resume very nearly their previous actions. The parts that depend on them will consequently by and by do the same. Until at length, by a reversal of the adaptive process, the organ at first affected will be brought back almost to its original state. Reconsidering the above-drawn analogy between an organism and a society, will enable us better to recognize this necessity. If, in the case supposed, the extra demand for iron ships, after causingthe erection of some additional ship-yards and the drawing of iron from other manufactures, were to cease; the old dimensions of the ship-building trade would be quickly returned to: discharged workmen would seek fresh occupations, and the new yards would be devoted to other uses. But if the increased need for ships lasted long enough, and became great enough, to cause a flow of capital and labour from other industries into the iron-manufacture, a falling off in the demand for ships, would much less rapidly entail a dwindling of the ship-building industry. For iron being now produced in greater quantity, a diminished consumption of it for ships would cause a fall in its price, and a consequent fall in the cost of ships: thus enabling the ship-builders to meet the competition which we may suppose led to a decrease in the orders they received. And since, when new blast-furnaces and rolling-mills, &c., had been built with capital drawn from other industries, its transference back into other industries would involve great loss; the owners, rather than transfer it, would accept unusually low interest, and an excess of iron would continue to be produced; resulting in an undue cheapness of ships, and a maintenance of the ship-building industry at a size beyond the need. Eventually, however, if the number of ships required still diminished, the production of iron in excess would become very unremunerative: some of the blast-furnaces would be blown out; and as much of the capital and labour as remained available would be re-distributed among other occupations. Without repeating the steps of the argument, it will be clear that were the enlargement of the ship-building industry great enough, and did it last long enough to cause an increase in the number of coal-mines, the ship-building industry would be still better able to maintain itself under adverse circumstances; but that it would, though at a more distant period, end by sinking down to the needful dimensions. Thus our conclusions are:—First, that if the extra growth caused by extra activity in a particular industry has lasted long enoughonly to remodel the proximately-affected industries; it will dwindle away again after a moderate period, if the need for it disappears. Second, that a long period must be required before the re-actions produced by an enlarged industry can cause a re-construction of the whole society, and before the countless re-distributions of capital and labour can again reach a state of equilibrium. And third, that only when such a new state of equilibrium is eventually reached, can the adaptive modification become a permanent one. How, in animal organisms the like argument holds, need not be pointed out. The reader will readily follow the parallel.
That organic types should be comparatively stable, might be anticipated on the hypothesis of Evolution. The structure of any organism being a product of the almost infinite series of actions and reactions to which ancestral organisms have been exposed; any unusual actions and reactions brought to bear on an individual, can have but an infinitesimal effect in permanently changing the structure of the organism as a whole. The new set of forces, compounded with all the antecedent sets of forces, can but inappreciably modify that moving equilibrium of functions which all these antecedent sets of forces have established. Though there may result a considerable perturbation of certain functions—a considerable divergence from their ordinary rhythms—yet the general centre of equilibrium cannot be sensibly changed. On the removal of the perturbing cause the previous balance will be quickly restored: the effect of the new forces being almost obliterated by the enormous aggregate of forces which the previous balance expresses.
§ 71. As thus understood, the phenomena of adaptation fall into harmony with first principles. The inference that organic types are fixed, because the deviations from them which can be produced within assignable periods are relatively small, and because, when a force producing deviation ceases, there is a return to something like the original state; proves tobe an invalid inference. Without assuming fixity of species, we find good reasons for anticipating that kind and degree of stability which is observed. We find grounds for concluding,a priori, that an adaptive change of structure will soon reach a point beyond which further adaptation will be slow; for concluding that when the modifying cause has been but a short time in action, the modification generated will be evanescent; for concluding that a modifying cause acting even for many generations, will do but little towards permanently altering the organic equilibrium of a race; and for concluding that on the cessations of such cause, its effects will become unapparent in the course of a few generations.
INDIVIDUALITY.
§ 72. What is an individual? is a question which many readers will think it easy to answer. Yet it is a question that has led to much controversy among Zoologists and Botanists, and no quite satisfactory reply to it seems possible. As applied to a man, or to any one of the higher animals, which are all sharply-defined and independent, the word individual has a clear meaning: though even here, when we turn from average cases to exceptional cases—as a calf with two heads and two pairs of fore-limbs—we find ourselves in doubt whether to predicate one individuality or two. But when we extend our range of observation to the organic world at large, we find that difficulties allied to this exceptional one meets us everywhere under every variety of form.
Each uniaxial plant may perhaps fairly be regarded as a distinct individual; though there are botanists who do not make even this admission. What, however, are we to say of a multiaxial plant? It is, indeed, usual to speak of a tree with its many branches and shoots as singular; but strong reasons may be urged for considering it as plural. Every one of its axes has a more or less independent life, and when cut off and planted may grow into the likeness of its parent; or, by grafting and budding, parts of this tree may be developed upon another tree, and there manifest their specific peculiarities. Shall we regard all the growing axes thus resulting from slips and grafts and buds, as parts of oneindividual or as distinct individuals? If a strawberry-plant sends out runners carrying buds at their ends, which strike root and grow into independent plants that separate from the original one by decay of the runners, must we not say that they possess separate individualities; and yet if we do this, are we not at a loss to say when their separate individualities were established, unless we admit that each bud was from the beginning an individual? Commenting on such perplexities Schleiden says—"Much has been written and disputed concerning the conception of the individual, without, however, elucidating the subject, principally owing to the misconception that still exists as to the origin of the conception. Now the individual is no conception, but the mere subjective comprehension of an actual object, presented to us under some given specific conception, and on this latter it alone depends whether the object is or is not an individual. Under the specific conception of the solar system, ours is an individual: in relation to the specific conception of a planetary body, it is an aggregate of many individuals." ... "I think, however, that looking at the indubitable facts already mentioned, and the relations treated of in the course of these considerations, it will appear most advantageous and most useful, in a scientific point of view, to consider the vegetable cell as the general type of the plant (simple plant of the first order). Under this conception,Protococcusand other plants consisting of only one cell, and the spore and pollen-granule, will appear as individuals. Such individuals may, however, again, with a partial renunciation of their individual independence, combine under definite laws into definite forms (somewhat as the individual animals do in the globe of theVolvox globator[25]). These again appear empirically as individual beings, under a conception of a species (simple plants of the second order) derived from the form ofthe normal connexion of the elementary individuals. But we cannot stop here, since Nature herself combines these individuals, under a definite form, into larger associations, whence we draw the third conception of the plant, from a connexion, as it were, of the second power (compound plants—plants of the third order). The simple plant proceeding from the combination of the elementary individuals is then termed a bud (gemma), in the composition of plants of the third order."
The animal kingdom presents still greater difficulties. When, from sundry points on the body of a common polype, there bud out young polypes which, after acquiring mouths and tentacles and closing up the communications between their stomachs and the stomach of the parent, finally separate from the parent; we may with propriety regard them as distinct individuals. But when in the allied compoundHydrozoa, we find that these young polypes continue permanently connected with the parent; and when by this continuous budding-out there is presently produced a tree-like aggregation, having a common alimentary canal into which the digestive cavity of each polype opens; it is no longer so clear that these little sacs, furnished with mouths and tentacles, are severally to be regarded as distinct individuals. We cannot deny a certain individuality to the polypedom. And on discovering that some of the buds, instead of unfolding in the same manner as the rest, are transformed into capsules in which eggs are developed—on discovering that certain of the incipient polypes thus become wholly dependent on the aggregate for their nutrition, and discharge functions which have nothing to do with their own maintenance, we have still clearer proof that the individualities of the members are partially merged in the individuality of the group. Other organisms belonging to the same order, display still more decidedly this transition from simple individualities to a complex individuality. In theDiphyesthereis a special modification of one or more members of the polypedom into a swimming apparatus which, by its rhythmical contractions, propels itself through the water, drawing the polypedom after it. And in the more differentiatedPhysaliavarious organs result from the metamorphosis of parts which are the homologues of individual polypes. In this last instance, the individuality of the aggregate is so predominant that the individualities of its members are practically lost. This combination of individualities in such way as to produce a composite individual, meets us in other forms among the ascidians. While in some of these, as in theClavelinaand in theBotryllidæ, the animals associated are but little subordinated to the community they form, in others they are so combined as to form a compound individual. The pelagic ascidianDoliolumis an example. "Here we find a large individual which swims by contractions of circular muscular bands, carries a train of smaller individuals attached to a long dorsal process of the test. These are arranged in three rows: those constituting the lateral row have wide mouths and no sexual organs or organs of locomotion—they subserve the nutrition of the colony, a truth which is illustrated by the fact that as soon as they are properly developed the large individual (the mother) loses her alimentary canal;" while from the median row are eventually derived the sexual zoids.
On the hypothesis of Evolution, perplexities of this nature are just such as we might anticipate. If Life in general commenced with minute and simple forms, like those out of which all organisms, however complex, now originate; and if the transitions from these primordial units to organisms made up of groups of such units, and to higher organisms made up of groups of such groups took place by degrees; it is clear that individualities of the first and simplest order would merge gradually in those of a larger and more complex order, and these again in others of an order having stillgreater bulk and organization. Hence it would be impossible to say where the lower individualities ceased and the higher individualities commenced.
§ 73. To meet these difficulties, it has been proposed that the whole product of a single fertilized germ shall be regarded as a single individual; whether such whole product be organized into one mass, or whether it be organized into many masses that are partially or completely separate. It is urged that whether the development of the fertilized germ be continuous or discontinuous (§ 50) is a matter of secondary importance; that the totality of living tissue to which the fertilized germ gives rise in any one case, is the equivalent of the totality to which it gives rise in any other case; and that we must recognize this equivalence, whether such totality of living tissue takes a concrete or a discrete arrangement. In pursuance of this view, a zoological individual is constituted either by any such single animal as a mammal or bird, which may properly claim the title of azoon, or by any such group of animals as the numerousMedusæthat have been developed from the same egg, which are to be severally distinguished aszooids.
Admitting it to be very desirable that there should be words for expressing these relations and this equivalence, it may be objected that to apply the word individual to a number of separate living bodies, is inconvenient: conflicting so much, as it does, with the ordinary conception which this word suggests. It seems a questionable use of language to say that the countless masses ofAnacharis Alsinastrum(nowEloidea canadensis) which, within these few years, have grown up in our rivers, canals, and ponds, are all parts of one individual: and yet as this plant does not seed in England, these countless masses, having arisen by discontinuous development, must be so regarded if we accept the above definition.
It may be contended, too, that while it does violence to our established way of thinking, this mode of interpretingthe facts is not without its difficulties. Something seems to be gained by restricting the application of the title individual, to organisms which, being in all respects fully developed, possess the power of producing their kind after the ordinary sexual method, and denying this title to those incomplete organisms which have not this power. But the definition does not really establish this distinction for us. On the one hand, we have cases in which, as in the working bee, the whole of the germ-product is aggregated into a single organism; and yet, though an individual according to the definition, this organism has no power of reproducing its kind. On the other hand, we have cases like that of the perfectAphis, where the organism is but an infinitesimal part of the germ product, and yet has that completeness required for sexual reproduction. Further, it might be urged with some show of reason, that if the conception of individuality involves the conception of completeness, then, an organism which possesses an independent power of reproducing itself, being more complete than an organism in which this power is dependent on the aid of another organism, is more individual.
§ 74. There is, indeed, as already implied, no definition of individuality that is unobjectionable. All we can do is to make the best practicable compromise.
As applied either to an animate or an inanimate object, the word individual ordinarily connotes union among the parts of the object and separateness from other objects. This fundamental element in the conception of individuality, we cannot with propriety ignore in the biological application of the word. That which we call an individual plant or animal must, therefore, be some concrete whole and not a discrete whole. If, however, we say that each concrete living whole is to be regarded as an individual, we are still met by the question—What constitutes a concrete living whole? A young organism arising by internal or externalgemmation from a parent organism, passes gradually from a state in which it is an indistinguishable part of the parent organism to a state in which it is a separate organism of like structure with the parent. At what stage does it become an individual? And if its individuality be conceded only when it completely separates from the parent, must we deny individuality to all organisms thus produced which permanently retain their connexions with their parents? Or again, what must we say of theHectocotylus, which is an arm of the Cuttle-fish that undergoes a special development and then, detaching itself, lives independently for a considerable period? And what must we say of the larval nemertine worm the pilidium of which with its nervous system is left to move about awhile after the developing worm has dropped out of it?
To answer such questions we must revert to the definition of life. The distinction between individual in its biological sense, and individual in its more general sense, must consist in the manifestation of Life, properly so called. Life we have seen to be, "the definite combination of heterogeneous change, both simultaneous and successive, in correspondence with external co-existences and sequences." Hence, a biological individual is any concrete whole having a structure which enables it, when placed in appropriate conditions, to continuously adjust its internal relations to external relations, so as to maintain the equilibrium of its functions. In pursuance of this conception, we must consider as individuals all those wholly or partially independent organized masses which arise by multicentral and multiaxial development that is either continuous or discontinuous (§ 50). We must accord the title to each separate aphis, each polype of a polypedom, each bud or shoot of a lowering plant, whether it detaches itself as a bulbil or remains attached as a branch.
By thus interpreting the facts we do not, indeed, avoid all anomalies. While, among flowering plants, the power of independent growth and development is usually possessedonly by shoots or axes; yet, in some cases, as in that of the Begonia-leaf awhile since mentioned, the appendage of an axis, or even a small fragment of such appendage, is capable of initiating and carrying on the functions of life; and in other cases, as shown by M. Naudin in theDrosera intermedia, young plants are occasionally developed from the surfaces of leaves. Nor among forms like the compoundHydrozoa, does the definition enable us to decide where the line is to be drawn between the individuality of the group and the individualities of the members: merging into each other, as these do, in different degrees. But, as before said, such difficulties must necessarily present themselves if organic forms have arisen by insensible gradations. We must be content with a course which commits us to the smallest number of incongruities; and this course is, to consider as an individual any organized mass which is capable of independently carrying on that continuous adjustment of inner to outer relations which constitutes Life.
CELL-LIFE AND CELL-MULTIPLICATION.
§ 74a. The progress of science is simultaneously towards simplification and towards complication. Analysis simplifies its conceptions by resolving phenomena into their factors, and by then showing how each simple mode of action may be traced under multitudinous forms; while, at the same time, synthesis shows how each factor, by cooperation with various other factors in countless modes and degrees, produces different results innumerable in their amounts and varieties. Of course this truth holds alike of processes and of products. Observation and the grouping into classes make it clear that through multitudinous things superficially unlike there run the same cardinal traits of structure; while, along with these major unities, examination discloses innumerable minor diversities.
A concomitant truth, or the same truth under another aspect, is that Nature everywhere presents us with complexities within complexities, which go on revealing themselves as we investigate smaller and smaller objects. In a preceding chapter (§§ 54a,54b) it was pointed out that each primitive organism, in common with each of the units out of which the higher and larger organisms are built, was found a generation ago to consist of nucleus, protoplasm, and cell-wall. This general conception of a cell remained for a time the outcome of inquiry; but with the advance of microscopy itbecame manifest that within these minute structures processes and products of an astonishing nature are to be seen. These we have now to contemplate.
In the passages just referred to it was said that the external layer or cell-wall is a non-essential, inanimate part produced by the animate contents. Itself a product of protoplasmic action, it takes no part in protoplasmic changes, and may therefore here be ignored.
§ 74b. One of the complexities within complexities was disclosed when it was found that the protoplasm itself has a complicated structure. Different observers have described it as constituted by a network or reticulum, a sponge-work, a foam-work. Of these the first may be rejected; since it implies a structure lying in one plane. If we accept the second we have to conceive the threads of protoplasm, corresponding to the fibres of the sponge, as leaving interstices filled either with liquid or solid. They cannot be filled with a continuous solid, since all motion of the protoplasm would be negatived; and that their content is not liquid seems shown by the fact that its parts move about under the form of granules or microsomes. But the conception of moving granules implies the conception of immersion in a liquid or semi-liquid substance in which they move—not a sponge-work of threads but a foam-work, consisting everywhere of septa interposed among the granules. This is the hypothesis which sundry microscopists espouse, and which seems mechanically the most feasible: the only one which consists with the "streaming" of protoplasm. Ordinarily the name protoplasm is applied to the aggregate mass—the semi-liquid, hyaline substance and the granules or microsomes it contains.
What these granules or microsomes are—whether, as some have contended, they are the essential living elements of the protoplasm, or whether, as is otherwise held, they are nutritive particles, is at present undecided. But the fact, allegedby sundry observers, that the microsomes often form rows, held together by intervening substance, seems to imply that these minute bodies are not inert. Leaving aside unsettled questions, however, one fact of significance is manifest—an immense multiplication of surfaces over which inter-action may take place. Anyone who drops into dilute sulphuric acid a small nail and then drops a pinch of iron filings, will be shown, by the rapid disappearance of the last and the long continuance of the first, how greatly the increasing of surfaces by multiplication of fragments facilitates change. The effect of subdivision in producing a large area in a small space, is shown in the lungs, where the air-cells on the sides of which the blood-vessels ramify, are less than1⁄100th of an inch in diameter, while they number 700,000,000. In the composition of every tissue we see the same principle. The living part, or protoplasm, is divided into innumerable protoplasts, among which are distributed the materials and agencies producing changes. And now we find this principle carried still deeper in the structure of the protoplasm itself. Each microscopic portion of it is minutely divided in such ways that its threads or septa have multitudinous contacts with those included portions of matter which take part in its activities.
Concerning the protoplasm contained in each cell, named by some cytoplasm, it remains to say that it always includes a small body called the centrosome, which appears to have a directive function. Usually the centrosome lies outside the nucleus, but is alleged to be sometimes within it. During what is called the "resting stage," or what might more properly be called the growing stage (for clearly the occasional divisions imply that in the intervals between them there has been increase) the centrosome remains quiescent, save in the respect that it exercises some coercive influence on the protoplasm around. This results in the radially-arranged lines constituting an "aster." What is the nature of the coercion exercised by the centrosome—a body hardly distinguishablein size from the microsomes or granules of protoplasm around—is not known. It can scarcely be a repelling force; since, in a substance of liquid or semi-liquid kind, this could not produce approximately straight lines. That it is an attractive force seems more probable; and the nature of the attraction would be comprehensible did the centrosome augment in bulk with rapidity. For if integration were in progress, the drawing in of materials might well produce converging lines. But this seems scarcely a tenable interpretation; since, during the so-called "resting stage," this star-like structure exists—exists, that is, while no active growth of the centrosome is going on.
Respecting this small body we have further to note that, like the cell as a whole, it multiplies by fission, and that the bisection of it terminates the resting or growing stage and initiates those complicated processes by which two cells are produced out of one: the first step following the fission being the movement of the halves, with their respective completed asters, to the opposite sides of the nucleus.
§ 74c. With the hypothesis, now general, that the nucleus or kernel of a cell is its essential part, there has not unnaturally grown up the dogma that it is always present; but there is reason to think that the evidence is somewhat strained to justify this dogma.
In the first place, beyond the cases in which the nucleus, though ordinarily invisible, is said to have been rendered visible by a re-agent, there are cases, as in the already-namedArcherina, where no re-agent makes one visible. In the second place, there is the admitted fact that some nuclei are diffused; as inTrachelocercaand some other Infusoria. In them the numerous scattered granules are supposed to constitute a nucleus: an interpretation obviously biassed by the desire to save the generalization. In the third place, the nucleus is frequently multiple in cells of low types; as in some families of Algæ and predominantly among Fungi.Once more, the so-called nucleus is occasionally a branching structure scarcely to be called a "kernel."
The facts as thus grouped suggest that the nucleus has arisen in conformity with the law of evolution—that the primitive protoplast, though not homogeneous in the full sense, was homogeneous in the sense of being a uniformly granular protoplasm; and that the protoplasts with diffused nuclei, together with those which are multi-nucleate, and those which have nuclei of a branching form, represent stages in that process by which the relatively homogeneous protoplast passed into the relatively heterogeneous one now almost universal.
Concerning the structure and composition of the developed nucleus, the primary fact to be named is that, like the surrounding granular cytoplasm, it is formed of two distinct elements. It has a groundwork or matrix not differing much from that of the cytoplasm, and at some periods continuous with it; and immersed in this it has a special matter named chromatin, distinguished from its matrix by becoming dyed more or less deeply when exposed to fit re-agents. During the "resting stage," or period of growth and activity which comes between periods of division, the chromatin is dispersed throughout the ground-substance, either in discrete portions or in such way as to form an irregular network or sponge-work, various in appearance. When the time for fission is approaching this dispersed chromatin begins to gather itself together: reaching its eventual concentration through several stages. By its concentration are produced the chromosomes, constant in number in each species of plant or animal. It is alleged that the substance of the chromosomes is not continuous, but consists of separate elements or granules, which have been named chromomeres; and it is also alleged that, whether in the dispersed or integrated form, each chromosome retains its individuality—that the chromomeres composing it, now spreading out into a network and now uniting into a worm-like body, form a group which neverloses its identity. Be this as it may, however, the essential fact is that during the growth-period the chromatin substance is widely distributed, and concentration of it is one of the chief steps towards a division of the nucleus and presently of the cell.
During this process of mitosis or karyokinesis, the dispersed chromatin having passed through the coil-stage, reaches presently the star-stage, in which the chromosomes are arranged symmetrically about the equatorial plane of the nucleus. Meanwhile in each of them there has been a preparation for splitting longitudinally in such way that the halves when separated contain (or are assumed to contain) equal numbers of the granules or chromomeres, which some think are the ultimate morphological units of the chromosomes. A simultaneous change has occurred: there has been in course of formation a structure known as theamphiaster. The two centrosomes which, as before said, place themselves on opposite sides of the nucleus, become the terminal poles of a spindle-shaped arrangement of fibres, arising mainly from the groundwork of the nucleus, now continuous with the groundwork of the cytoplasm. A conception of this structure may be formed by supposing that the radiating fibres of the respective asters, meeting one another and uniting in the intermediate space, thereafter exercise a tractive force; since it is clear that, while the central fibres of the bundle will form straight lines, the outer ones, pulling against one another not in straight lines, will form curved lines, becoming more pronounced in their curvatures as the distance from the axis increases. That a tractive force is at work seems inferable from the results. For the separated halves of the split chromosomes, which now form clusters on the two sides of the equatorial plane, gradually part company, and are apparently drawn as clusters towards the opposing centrosomes. As this change progresses the original nucleus loses its individuality. The new chromosomes, halves of the previous chromosomes, concentrate to foundtwo new nuclei; and, by something like a reversal of the stages above described, the chromatin becomes dispersed throughout the substance of each new nucleus. While this is going on the cell itself, undergoing constriction round its equator, divides into two.
Many parts of this complex process are still imperfectly understood, and various opinions concerning them are current. But the essential facts are that this peculiar substance, the chromatin, at other times existing dispersed, is, when division is approaching, gathered together and dealt with in such manner as apparently to insure equal quantities being bequeathed by the mother-cell to the two daughter-cells.
§ 74d. What is the physiological interpretation of these structures and changes? What function does the nucleus discharge; and, more especially, what is the function discharged by the chromatin? There have been to these questions sundry speculative answers.
The theory espoused by some, that the nucleus is the regulative organ of the cell, is met by difficulties. One of them is that, as pointed out in the chapter on "Structure," the nucleus, though morphologically central, is not central geometrically considered; and that its position, often near to some parts of the periphery and remote from others, almost of itself negatives the conclusion that its function is directive in the ordinary sense of the word. It could not well control the cytoplasm in the same ways in all directions and at different distances. A further difficulty is that the cytoplasm when deprived of its nucleus can perform for some time various of its actions, though it eventually dies without reproducing itself.
For the hypothesis that the nucleus is a vehicle for transmitting hereditary characters, the evidence seems strong. When it was shown that the head of a spermatozoon is simply a detached nucleus, and that its fusion with thenucleus of an ovum is the essential process initiating the development of a new organism, the legitimate inference appeared to be that these two nuclei convey respectively the paternal and maternal traits which are mingled in the offspring. And when there came to be discerned the karyokinesis by which the chromatin is, during cell-fission, exactly halved between the nuclei of the daughter-cells, the conclusion was drawn that the chromatin is more especially the agent of inheritance. But though, taken by themselves, the phenomena of fertilization seem to warrant this inference, the inference does not seem congruous with the phenomena of ordinary cell-multiplication—phenomena which have nothing to do with fertilization and the transmission of hereditary characters. No explanation is yielded of the fact that ordinary cell-multiplication exhibits an elaborate process for exact halving of the chromatin. Why should this substance be so carefully portioned out among the cells of tissues which are not even remotely concerned with propagation of the species? If it be said that the end achieved is the conveyance of paternal and maternal qualities in equal degrees to every tissue; then the reply is that they do not seem to be conveyed in equal degrees. In the offspring there is not a uniform diffusion of the two sets of traits throughout all parts, but an irregular mixture of traits of the one with traits of the other.
In presence of these two suggested hypotheses and these respective difficulties, may we not suspect that the action of the chromatin is one which in a way fulfils both functions? Let us consider what action may do this.
§ 74e. The chemical composition of chromatin is highly complex, and its complexity, apart from other traits, implies relative instability. This is further implied by the special natures of its components. Various analyses have shown that it consists of an organic acid (which has been called nucleic acid) rich in phosphorus, combined with analbuminous substance: probably a combination of various proteids. And the evidence, as summarised by Wilson, seems to show that where the proportion of phosphorized acid is high the activity of the substance is great, as in the heads of spermatozoa; while, conversely, where the quantity of phosphorus is relatively small, the substance approximates in character to the cytoplasm. Now (like sulphur, present in the albuminoid base), phosphorus is an element which, besides having several allotropic forms, has a great affinity for oxygen; and an organic compound into which it enters, beyond the instability otherwise caused, has a special instability caused by its presence. The tendency to undergo change will therefore be great when the proportion of the phosphorized component is great. Hence the statement that "the chemical differences between chromatin and cytoplasm, striking and constant as they are, are differences of degree only;" and the conclusion that the activity of the chromatin is specially associated with the phosphorus.[26]
What, now, are the implications? Molecular agitation results fromdecompositionof each phosphorized molecule: shocks are continually propagated around. From the chromatin, units of which are thus ever falling into stabler states, there are ever being diffused waves of molecular motion, setting up molecular changes in the cytoplasm. The chromatin stands towards the other contents of the cell in the same relation that a nerve-element stands to any element ofan organism which it excites: an interpretation congruous with the fact that the chromatin is as near to as, and indeed nearer than, a nerve-ending to any minute structure stimulated by it.
Several confirmatory facts may be named. During the intervals between cell-fissions, when growth and the usual cell-activities are being carried on, the chromatin is dispersed throughout the nucleus into an irregular network: thus greatly increasing the surface of contact between its substance and the substances in which it is imbedded. As has been remarked, this wide distribution furthers metabolism—a metabolism which in this case has, as we infer, the function of generating, not special matters but special motions. Moreover, just as the wave of disturbance a nerve carries produces an effect which is determined, not by anything which is peculiar in itself, but by the peculiar nature of the organ to which it is carried—muscular, glandular or other; so here, the waves diffused from the chromatin do not determine the kinds of changes in the cytoplasm, but simply excite it: its particular activities, whether of movement, absorption, or structural excretion, being determined by its constitution. And then, further, we observe a parallelism between the metabolic changes in the two cases; for, on the one hand, "diminished staining capacity of the chromatin [implying a decreased amount of phosphorus, which gives the staining capacity] occurs during a period of intense constructive activity in the cytoplasm;" and, on the other hand, in high organisms having nervous systems, the intensity of nervous action is measured by the excretion of phosphates—by the using up of the phosphorus contained in nerve-cells.
For thus interpreting the respective functions of chromatin and cytoplasm, yet a further reason may be given. One of the earliest general steps in the evolution of theMetazoa, is the differentiation of parts which act from parts which make them act. TheHydrozoashow us this. In the hydroid stage there are no specialized contractile organs:these are but incipient: individual ectoderm cells have muscular processes. Nor is there any "special aggregation of nerve-cells." If any stimulating units exist they are scattered. But in theMedusa-stage nerve-matter is collected into a ring round the edge of the umbrella. That is to say, in the undeveloped form such motor action as occurs is not effected by a specialized part which excites another part; but in the developed form a differentiation of the two has taken place. All higher types exhibit this differentiation. Be it muscle or gland or other operating organ, the cause of its activity lies not in itself but in a nervous agent, local or central, with which it is connected. Hence, then, there is congruity between the above interpretation and certain general truths displayed by animal organization at large. We may infer that in a way parallel to that just indicated, cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes: leaving behind the substance excited, now distinguished as cytoplasm.
§ 74f. Some further general aspects of the phenomena appear to be in harmony with this interpretation. Let us glance at them.
In Chapters III and IIIAof the First Part, reasons were given for concluding that in the animal organism nitrogenous substances play the part of decomposing agents to the carbo-hydrates—that the molecular disturbance set up by the collapse of a proteid molecule destroys the equilibrium of sundry adjacent carbo-hydrate molecules, and causes that evolution of energy which accompanies their fall into molecules of simpler compounds. Here, if the foregoing argument is valid, we may conclude that this highly complex phosphorized compound which chromatin contains, plays thesame part to the adjacent nitrogenous compounds as these play to the carbo-hydrates. If so, we see arising a stage earlier that "general physiological method" illustrated in§ 23f. It was there pointed out that in animal organisms the various structures are so arranged that evolution of a small amount of energy in one, sets up evolution of a larger amount of energy in another; and often this multiplied energy undergoes a second multiplication of like kind. If this view is tenable, we may now suspect that this method displayed in the structures of theMetazoawas initiated in the structures of theProtozoa, and consequently characterizes those homologues of them which compose theMetazoa.
When contemplated from the suggested point of view, karyokinesis appears to be not wholly incomprehensible. For if the chromatin yields the energy which initiates changes throughout the rest of the cell, we may see why there eventually arises a process for exact halving of the chromatin in a mother-cell between two daughter-cells. To make clear the reason, let us suppose the portioning out of the chromatin leaves one of the two with a sensibly smaller amount than the other. What must result? Its source of activity being relatively less, its rate of growth and its energy of action will be less. If a protozoon, the weaker progeny arising by division of it will originate an inferior stirp, unable to compete successfully with that arising from the sister-cell endowed with a larger portion of chromatin. By continual elimination of the varieties which produce unequal halving, necessarily at a disadvantage if a moiety of their members tend continually to disappear, there will be established a variety in which the halving is exact: the character of this variety being such that all its members aid the permanent multiplication of the species. If, again, the case is that of a metazoon, there will be the same eventual result. An animal or plant in which the chromatin is unequally divided among the cells, must have tissues of uncertain formation. Assume that an organ has, by survival of thefittest, been adjusted in the proportions and qualities of its parts to a given function. If the multiplying protoplasts, instead of taking equal portions of chromatin, have some of them smaller portions, the parts of the organ formed of these, developing less rapidly and having inferior energies, will throw the organ out of adjustment, and the individual will suffer in the struggle for life. That is to say, irregular division of the chromatin will introduce a deranging factor and natural selection will weed out individuals in which it occurs. Of course no interpretation is thus yielded of the special process known as karyokinesis. Probably other modes of equal division might have arisen. Here the argument implies merely that the tendency of evolution is to establishsomemode. In verification of the view that equal division arises from the cause named, it is pointed out to me that amitosis, which is a negation of mitosis or karyokinesis, occurs in transitory tissues or diseased tissues or where degeneracy is going on.
But how does all this consist with the conclusion that the chromatin conveys hereditary traits—that it is the vehicle in which the constitutional structure, primarily of the species and secondarily of recent ancestors and parents, is represented? To this question there seems to be no definite answer. We may say only that this second function is not necessarily in conflict with the first. While the unstable units of chromatin, ever undergoing changes, diffuse energy around, they may also be units which, under the conditions furnished by fertilization, gravitate towards the organization of the species. Possibly it may be that the complex combination of proteids, common to chromatin and cytoplasm, is that part in which the constitutional characters inhere; while the phosphorized component, falling from its unstable union and decomposing, evolves the energy which, ordinarily the cause of changes, now excites the more active changes following fertilization. This suggestion harmonizes with the fact that the fertilizing substance which in animalsconstitutes the head of the spermatozoon, and in plants that of the spermatozoid or antherozoid, is distinguished from the other agents concerned by having the highest proportion of the phosphorized element; and it also harmonizes with the fact that the extremely active changes set up by fertilization are accompanied by decrease of this phosphorized element. Speculation aside, however, we may say that the two functions of the chromatin do not exclude one another, but that the general activity which originates from it may be but a lower phase of that special activity caused by fertilization.[27]
§ 74g. Here we come unawares to the remaining topic embraced under the title Cell-Life and Cell-Multiplication. We pass naturally from asexualmultiplicationof cells to sexualmultiplication—from cell-reproduction to cell-generation. The phenomena are so numerous and so varied that a large part of them must be passed over. Conjugation among theProtophytaandProtozoa, beginning with cases in which there is a mingling of the contents of two cells in no visible respect different from one another, and developing into a great variety of processes in which they differ, must be left aside, and attention limited to the terminal process of fertilization as displayed in higher types of organisms.
Before fertilization there occurs in the ovum an incidental process of a strange kind—"strange" because it is a collateral change taking no part in subsequent changes. I refer to the production and extrusion of the "polar bodies." It is recognized that the formation of each is analogous to cell-formation in general; though process and product are both dwarfed. Apart from any ascribed meaning, the fact itself is clear. There is an abortive cell-formation. Abortiveness is seen firstly in the diminutive size of the separated body or cell, and secondly in the deficient number of its chromosomes: a corresponding deficiency being displayed in the group of chromosomes remaining in the egg—remaining, that is (on the hypothesis here to be suggested), in the sister-cell, supposing the polar body to be an aborted cell. It is currently assumed that the end to be achieved by thus extruding part of the chromosomes, is to reduce the remainder to half the number characterizing the species; so that when, to this group in the germ-cell, the sperm-cell brings a similarly-reduced group, union of the two shall bring the chromosomes to the normal number. I venture to suggest another interpretation. In doing this, however, I must forestall a conclusion contained in the next chapter; namely, the conclusion that gamogenesis begins when agamogenesis is being arrested by unfavourable conditions, and that the failing agamogenesis initiates the gamogenesis. Of numerous illustrations to be presently given I will, to make clear the conception, name only one—the formation of fructifyingorgans in plants at times when, and in places where, shoots are falling off in vigour and leaves in size. Here the successive foliar organs, decreasingly fitted alike in quality and dimensions for carrying on their normal lives, show us an approaching cessation of asexual multiplication, ending in the aborted individuals we call stamens; and the fact that sudden increase of nutrition while gamogenesis is being thus initiated, causes resumption of agamogenesis, shows that the gamogenesis is consequent upon the failing agamogenesis. See then the parallel. On going back from multicellular organisms to unicellular organisms (or those homologues of them which form the reproductive agents in multicellular organisms), we find the same law hold. The polar bodies are aborted cells, indicating that asexual multiplication can no longer go on, and that the conditions leading to sexual multiplication have arisen. If this be so, decrease in the chromatin becomes an initial cause of the change instead of an accompanying incident; and we need no longer assume that a quantity of precious matter is lost, not by passive incapacity, but by active expulsion. Another anomaly disappears. If from the germ-cell there takes place this extrusion of superfluous chromatin, the implication would seem to be that a parallel extrusion takes place from the sperm-cell. But this is not true. In the sperm-cell there occurs just that failure in the production of chromatin which, according to the hypothesis above sketched out, is to be expected; for, in the process of cell-multiplication, the cells which become spermatozoa areleftwith half the number of chromosomes possessed by preceding cells: there is actually that impoverishment and declining vigour here suggested as the antecedent of fertilization. It needs only to imagine the ovum and the polar body to be alike in size, to see the parallelism; and to see that obscuration of it arises from the accumulation of cytoplasm in the ovum.
A test fact remains. Sometimes the first polar body extruded undergoes fission while the second is being formed.This can have nothing to do with reducing the number of chromosomes in the ovum. Unquestionably, however, this change is included with the preceding changes in one transaction, effected by one influence. If, then, it is irrelevant to the decrease of chromosomes, so must the preceding changes be irrelevant: the hypothesis lapses. Contrariwise this fact supports the view suggested above. That extrusion of a polar body is a process of cell-fission is congruous with the fact that another fission occurs after extrusion. And that this occurs irregularly shows that the vital activities, seen in cell-growth and cell-multiplication, now succeed in producing further fission of the dwarfed cell and now fail: the energies causing asexual multiplication are exhausted and there arises the state which initiates sexual multiplication.
Maturation of the ovum having been completed, entrance of the spermatozoon, sometimes through the limiting membrane and sometimes through a micropyle or opening in it, takes place. This instantly initiates a series of complicated changes: not many seconds passing before there begins the formation of an aster around one end of the spermatozoon-head. The growth of this aster, apparently by linear rangings of the granules composing the reticulum of the germ-cell, progresses rapidly; while the whole structure hence arising moves inward. Soon there takes place the fusion of this sperm-nucleus with the germ-nucleus to form the cleavage-nucleus, which, after a pause, begins to divide and subdivide in the same manner as cells at large: so presently forming a cluster of cells out of which arise the layers originating the embyro. The details of this process do not concern us. It suffices to indicate thus briefly its general nature.
And now ending thus the account of genesis under its histological aspect, we pass to the account of genesis under its wider and more significant aspects.