SPENCER AS BIOLOGIST: INDUCTIONS OF BIOLOGY
Growth—Development—Structure and Function—Waste and Repair—Adaptation—Cell-Life—Genesis—Nutrition and Reproduction—The Germ-Cells
Growth.—Perhaps the widest and most familiar induction of Biology, is that organisms grow. But there is growth in crystals, in terrestrial deposits, in celestial bodies; in fact, growth, as being an integration of matter, is the primary trait of evolution; it is universal, in the sense that all aggregates display it in some way at some period. "The essential community of nature between organic growth and inorganic growth is, however, most clearly seen on observing that they both result in the same way. The segregation of different kinds of detritus from each other, as well as from the water carrying them, and their aggregation into distinct strata, is but an instance of a universal tendency towards the union of like units and the parting of unlike units (First Principles, § 163). The deposit of a crystal from a solution is a differentiation of the previously mixed molecules; and an integration of one class of molecules into a solid body, and the other class into a liquid solvent. Is not the growth of an organism an essentially similar process? Around a plant there exist certain elements like the elements which form its substance; and its increase in size is effected by continually integrating these surrounding like elements with itself." And so on.
Passing over the far-fetched statement that the deposit of sediment in distinct strata illustrates the universal tendency towards the union of like units and the parting of unlike units, we must point out that Spencer begins his discussion of organic growth by describing it in such general terms that its essential characteristic is lost sight of. A minute crystal of alum is dropped into a saturated solution of alum, and it grows rapidly under our eyes out of material the same as its own, but the living creature grows larger at the expense of material different from its own. The grass grows at the expense of air, water, and salts, and the lamb grows at the expense of the grass. Though the living creature cannot, of course, transform one element into another, and must have carbon, hydrogen, oxygen, nitrogen, etc., in its food, it utilises materials chemically very different from its own complex compounds.
Spencer's inductions as to growth were the following:—
(1) The growth of an organism is dependent on the available supply of such environing materials as are of like natures with the matters composing the organism.
(2) Other things being equal, the degree of growth varies according to the surplus of nutrition over expenditure.
(3) In the same organism the surplus of nutrition over expenditure differs at different stages, and growth is unlimited or has a definite limit, according as the surplus does or does not rapidly decrease. There is almost unceasing growth in organisms that expend relatively little energy and definitely limited growth in organisms that expend much energy. [There are many difficulties here, e.g., the apparent absence of a limit of growth in many very energetic fishes.]
(4) Among organisms which are large expenders of force, the size ultimately attained is, other things equal, determined by the initial size. [By initial size Spencer means the bulk of the organism when it begins to feed for itself.] A calf and a lamb commence their physiological transactions on widely different scales; their first increments of growth are similarly contrasted in their amounts; and the two diminishing series of such increments end at similarly-contrasted limits.
[But the further we penetrate into details, the more inevitable seems the conclusion that adult size is an adaptive phenomenon; in other words that growth has been punctuated by natural selection.]
(5) Where the likeness of other circumstances permits a comparison, the possible extent of growth depends on the degree of organization; an inference testified to by the larger forms among the various divisions and sub-divisions of organisms.
In connection with growth and its limit Spencer made a simple but shrewd observation, which seems also to have occurred to Prof. Leuckart and to Dr Alexander James. He pointed out, that in the growth of similarly shaped bodies the increase of volume continually tends to outrun the increase of surface. The volume of living matter must grow more than the surface through which it is kept alive, if the surface remain regular in contour. In spherical and all other regular units the volume increases as the cube of the radius, the surface only as the square of the radius. Thus a cell, for instance, as it grows, must get into physiological difficulties, for the nutritive necessities of the increasing volume are ever less adequately supplied by the less rapidly increasing absorbent surface. There is less and less opportunity for nutrition, respiration, and excretion. A nemesis of growth sets in, for waste gains upon, overtakes, balances, and threatens to exceed repair. Growth may cease at this limit, and a balance be struck; or the form of the unit may be altered and surface gained by flattening out, or very frequently by ramifying processes; or—and this the most frequent solution—the cell may divide, halving its volume, gaining new surface, and restoring the balance. In more general terms, growth expresses the preponderance of constructive processes or anabolism; increase of volume with less rapid increase of nutritive, respiratory, and excretory surface involves a relative predominance of katabolism; the limit of growth occurs when further increase of volume would prejudicially increase the ratio of katabolism to anabolism; at that point the cell restores the balance by dividing. And what is true of the unit applies also in a general way to organs, such as leaves which increase their surface by becoming much divided, and even to organisms which exhibit many adaptations for increasing their nutritive, respiratory, and excretory surfaces.
Development.—Growth is increase in bulk, development is increase in structure, and Spencer's chief induction in regard to development is that we see a change from an incoherent, indefinite homogeneity to a coherent, definite heterogeneity. "The originally like units called cells become unlike in various ways, and in ways more numerous and marked as the development goes on. The several tissues which these several classes of cells form by aggregation, grow little by little distinct from each other; and little by little put on those structural complexities that arise from differentiations among their component units. In the shoot, as in the limb, the external form, originally very simple, and having much in common with simple forms in general, gradually acquires an increasing complexity and an increasing unlikeness to other forms. Meanwhile, the remaining parts of the organism to which the shoot or limb belongs, having been severally assuming structures divergent from one another and from that of this particular shoot or limb, there has arisen a greater heterogeneity in the organism as a whole." Moreover, "whereas the germs of organisms are extremely similar, they gradually diverge widely in modes now regular and now irregular, until in place of a multitude of forms practically alike we finally have a multitude of forms most of which are extremely unlike." In other words, there is in individual development (ontogeny) some condensed recapitulation of the steps in racial evolution (phylogeny). Furthermore, in the progressing differentiation of each organism there is a progressing differentiation of it from its environment; it becomes freer from the environmental grip and more master of its fate. Here again there is an individual progress parallel to that seen in the course of historic evolution.
A general criticism must be made, that Spencer thought of the germ-cell much too simply. It is a microcosm full of intricacy; the nucleus is often exceedingly definite and coherent; the early cells are often from the first defined, with prospective values which do not change. The fertilised ovum has only apparent simplicity; it has a complex individualised organisation—often visible. No one can doubt that development is progressive differentiation, but it is rather a realisation of a complex inheritance of materialised potentialities than a change from an incoherent, indefinite homogeneity to a coherent, definite heterogeneity.
Structure and Function.—To the question, does Life produce Organisation, or does Organisation produce Life? Spencer answered that "structure and function must have advanced pari passu: some difference of function, primarily determined by some difference of relation to the environment, initiating a slight difference of structure, and this again leading to a more pronounced difference of function; and so on through continuous actions and reactions." As structure progresses from the homogeneous, indefinite, and incoherent, so does function, illustrating progressive division of labour. From an evolutionist point of view, Spencer argued that life necessarily comes before organisation; "organic matter in a state of homogeneous aggregation must precede organic matter in a stage of heterogeneous aggregation. But since the passing from a structureless state to a structured state is itself a vital process, it follows that vital activity must have existed while there was yet no structure: structure could not else arise. That function takes precedence of structure, seems also implied in the definition of Life. If Life is shown by inner actions so adjusted as to balance outer actions—if the implied energy is the substance of Life while the adjustment of the actions constitutes its form; then may we not say that the actions to be formed must come before that which forms them—that the continuous change which is the basis of function, must come before the structure which brings function into shape?"
But all such discussions of "structure" and "function" in the abstract tend to verbal quibbling. We cannot have activity without something to act, we cannot have metabolism without stuff. No one can tell what the first thing that lived on the earth was like, what organisation it had, or what it was able to do, but we may be sure that vital organisation and vital activity are only static and kinetic aspects of the same thing. It is quite probable, however, that there is no one thing that can be called protoplasm, for vital function may depend upon the inter-relations or inter-actions of several complex substances, none of which could by itself be called alive; which are, however, held together in that unity which makes an organism what it is. Just as the secret of a firm's success may depend upon a particularly fortunate association of partners, so it may be with vitality.[7]
[7] See J. Arthur Thomson's Progress of Science in the Nineteenth Century, 1903, p. 317, and E. B. Wilson's The Cell in Development and Inheritance, 1900.
Waste and Repair.—Organisms are systems for transforming matter and energy and the law of conservation holds good. "Each portion of mechanical or other energy which an organism exerts implies the transformation of as much organic matter as contained this energy in a latent state," and the waste must be made good by repair. We thus see why plants with an enormous income of energy and little expenditure of energy have no difficulty in sustaining the balance between waste and repair; we understand the relation between small waste, small activity, and low temperature in many of the lower animals; we understand conversely the rapid waste of energetic, hot-blooded animals. The deductive interpretation of waste is easy, but it is different with repair, for here the analogy between the organism and an inanimate engine breaks down. The living creature is a self-stoking, self-repairing, and also—it may be noted in passing—a self-reproducing engine. Spencer did not do more than restate the difficulty when he said that the component units of organisms have the power of moulding fit materials into other units of the same order.
In passing to consider the ability which an organism often has of recompleting itself when one of its parts has been cut off, just as an injured crystal recompletes itself, Spencer was led to the hypothesis that "the form of each species of organism is determined by a peculiarity in the constitution of its units—that these have a special structure in which they tend to arrange themselves; just as have the simpler units of inorganic matter." "This organic polarity (as we might figuratively call this proclivity towards a specific structural arrangement) can be possessed neither by the chemical units nor the morphological units, we must conceive it as possessed by certain intermediate units, which we may term physiological." But if in each organism the physiological units which result from the compounding of highly compound molecules have a more or less distinctive character, the germ-cell is not so very indefinite after all.
Many of the facts of regeneration are very striking. A crab may regrow its complex claw, a starfish arm may regrow an entire body. A snail has been known to regenerate an amputated eye-bearing horn twenty times in succession, a newt can replace a lost lens, a lizard can regrow its tail and part of its leg, a stork can regrow the greater part of its bill. In many cases, the surrender of parts which are afterwards regrown is exceedingly common, as in some worms and Echinoderms, and is a life-saving adaptation. Organically, though not consciously, the brainless starfish has learned that it is better that one member should perish than that the whole life should be lost. This regenerative capacity no doubt implies certain properties in the living matter and in the organism, but we are far from being able to picture how it comes about. What does seem clear is that the distribution and mode of occurrence of the regenerative capacity—in external organs often, but in internal organs very rarely; in most lizard's tails, but not in the chamæleon's; in the stork's bill but not in its toes—are adaptive, being related to the normal risks of life, as Réaumur, Lessona, Darwin, and Weismann have pointed out. According to Lessona's Law, which Weismann has elaborated, regeneration tends to occur in those organisms and in those parts of organisms which are in the ordinary course of nature most liable to injury. To which we must add two saving-clauses—(a) provided that the lost part is of some vital importance, and (b) provided that the wound or breakage is not in itself very likely to be fatal. In Weismann's words, the theory is, that "the power of regeneration possessed by an animal or by a part of an animal is regulated by adaptation to the frequency of loss and to the extent of the damage done by the loss."
Adaptation.—Wherever we look in the world of organisms we find examples of adaptation; we see form suited for different kinds of motion, organs suited for their uses, constitution suited to circumstances in such external features as colouring and in such internal adjustments as the regulation of temperature; we find effective weapons and effective armour, flowers adapted to insect visitors and insect visitors adapted to flowers, one sex adapted in relation to the other, the mother adapted to bearing and rearing offspring, the embryo adapted to its pre-natal life; everywhere there is adaptation in varying degrees of perfection. The adaptation is a fact, in regard to which all naturalists are agreed; difference of opinion arises when we ask how these adaptations have come to be.
In the chapter "Adaptation" Spencer practically restricted his attention to a certain kind of adaptation, namely the direct modifications which result from use or disuse, or from environmental influence. The blacksmith's arm, the dancer's legs, the jockey's crural adductors, illustrate direct results of practice; "à force de forger on devient forgeron." The skin forms protective callosities where it is much pressed or rubbed, as on the schoolboy's hands or the old man's toothless gums. The blood-vessels may respond by enlargement to increased demands made on them; the fingers of the blind become extraordinarily sensitive.
Spencer points to the general truth that extra function is followed by extra growth, but that a limit is soon reached beyond which very little, if any, further modification can be produced. Moreover, the limited increase of size produced in any organ by a limited increase of its function, is not maintained unless the increase of function is permanent. When the modifying influence is removed, the organism rebounds or tends to rebound. A lasting change of importance involves a re-organisation, a new state of equilibrium.
On inductive and deductive grounds, Spencer summed up in four conclusions:—
(1) An adaptive change of structure will soon reach a point beyond which further adaptation will be slow.
(2) When the modifying cause has been but for a short time in action, the modification generated will be evanescent.
(3) A modifying cause acting even for many generations will do little towards permanently altering the organic equilibrium of a race.
(4) On the cessation of such cause, its effects will become unapparent in the course of a few generations.
But two cautions must be emphasised (a) that Spencer, in this discussion, dealt only with those direct adjustments which are referable to the action of use or disuse, or of surrounding influences; and (b) that we have no security in regarding these as being as such transmissible.
By adaptations biologists usually mean permanent adjustments, and there are two theories of the origin of these: (a) by the action of natural selection on inborn variations, or (b) by the inheritance of the directly acquired bodily modifications.
Cell-Life.—In this chapter, interpolated in the revised edition, Spencer summed up the main results of the study of the structural units or cells which build up a body. "Nature everywhere presents us with complexities within complexities, which go on revealing themselves as we investigate smaller and smaller objects." Thus protoplasm itself has a complicated structure; the nucleus of the cell is a little world in itself; and the cell-firm has other partners, such as the centrosome. When a cell divides, the readily stainable bodies or chromosomes, present in definite number within the nucleus, are divided, usually by a most intricate process, in such a manner that equal amounts are bequeathed by the mother-cell to each of the two daughter-cells. Spencer favoured the view that the chromatin, which "consists of an organic acid (nucleic acid) rich in phosphorus, combined with an albuminous substance, probably a combination of various proteids" may be peculiarly unstable and active.
"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 of an organism which it excites." "We may infer that 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."
But the suggestion that chromosomes may be stimulating, change-exciting elements, does not, Spencer goes on to say, conflict with the conclusion that the chromosomes are the vehicles conveying hereditary traits. "While the unstable units of chromatin, ever undergoing changes, diffuse energy around, they may also be units which, under the conditions furnished by fertilisation, gravitate towards the organisation of the species. Possibly it may be that the complex combination of proteids, common to chromatin and cytoplasm, is that part in which constitutional characters inhere; while the phosphorised component, falling from its unstable union and decomposing, evolves the energy which, ordinarily the cause of changes, now excites the more active changes following fertilisation."
From this speculation Spencer passes to a brief consideration of what occurs before and during the fertilisation of the ovum. Before fertilisation is accomplished the nucleus of the ovum normally divides twice in rapid succession, and gives off two abortive cells—known as polar bodies—which come to nothing. The usual result of this "maturation," as it is called, is that the number of chromosomes in the ovum is reduced to a half of the normal number characteristic of the cells of the species to which it belongs. In the history of the male element or spermatozoon, there is an analogous reduction, so that when spermatozoon and ovum unite in fertilisation the normal number is restored. It is now recognised that the maturation-divisions are useful in obviating the doubling of the number of chromosomes which fertilisation would otherwise involve, and it has also been suggested that this continually recurrent elimination of chromosomes may be one of the causes of variation.
Spencer suggested another interpretation. He pointed out the general fact that sexual reproduction (gamogenesis) commonly occurs when asexual reproduction (agamogenesis) is arrested by unfavourable conditions, that failing asexual reproduction initiates sexual reproduction. Now as egg-cells and sperm-cells are the outcome of often long series of cell divisions (asexual multiplication), may not the polar bodies, which are aborted cells, indicate that asexual multiplication can no longer go on, and that the conditions leading to sexual multiplication have set in? "As the cells which become spermatozoa are left with half the number of chromosomes possessed by preceding cells, there is actually that impoverishment and declining vigour here suggested as the antecedent of fertilisation." In short, the germ-cells, separately considered, are cells in which the power of further asexual multiplication is exhausted, as it is known to become exhausted in Infusorians and such body-cells as nerve-cells; there arises a state which initiates a sexual union or amphimixis of the two kinds of germ-cells, and the decrease in the chromatin is an initial cause of that state.
We quote this speculation as a good instance of Spencer's continual endeavour to rationalise puzzling and exceptional facts by showing that there is a general principle underlying them. But the objections to his hypothesis are numerous. Mature ova or spermatozoa will not normally divide if left to themselves, but that is because they are specialised to secure amphimixis, not because their powers are in any way declining or impoverished. A parthenogenetic ovum gives off one polar body—though without reduction in the number of chromosomes—and then proceeds by asexual multiplication or ordinary cell division to build up a body. The spore of a fern or a moss has only half the number of chromosomes that the cells of its producer have, yet it proceeds by asexual multiplication or ordinary cell-division to build up the gametophyte or sexual generation.
Genesis.—Spencer attempted a classification of the various modes of reproduction that occur among organisms—asexual reproduction (agamogenesis) by fission and budding, sexual reproduction (gamogenesis) by specialised germ-cells usually involving fertilisation or amphimixis, and all the complications involved in "alternation of generations" (metagenesis), the development of eggs without fertilisation (parthenogenesis), and so on. But what gives particular importance to the chapter on genesis is not the discussion of the modes of reproduction, but the general conclusion that nutrition and reproduction are antithetic processes—a very fruitful idea in biology.
Where there is alternation of generation, sexual and asexual, we find that asexual reproduction continues as long as the forces which result in growth are greatly in excess of the antagonistic forces. Conversely the recurrence of sexual reproduction occurs when the conditions are no longer so favourable to growth. Similarly, where there is no alternation, "new individuals are usually not formed while the preceding individuals are still rapidly growing—that is, while the forces producing growth exceed the opposing forces to a great extent; but the formation of new individuals begins when nutrition is nearly equalled by expenditure."
In illustration Spencer points to facts like the following: "Uniaxial plants begin to produce their lateral, flowering axes, only after the main axis has developed the great mass of its leaves, and is showing its diminished nutrition by smaller leaves, or shorter internodes, or both"; "root-pruning" and "ringing," which diminish the nutritive supply, promote the formation of flower-shoots; high nutrition in plants prevents or arrests flowering.
Similarly, the aphides or green-flies, hatched from eggs in the spring, multiply by parthenogenesis throughout the summer; with extraordinary rapidity one generation follows on another; but when the weather becomes cold and plants no longer afford abundant sap, males reappear and sexual reproduction sets in. It has been shown that in the artificial summer of a green-house, parthenogenesis may continue for four years. In a large number of cases of ordinary reproduction, e.g. in birds, the connexion between cessation of growth and commencement of reproduction is very distinct.
It is not difficult to see the advantages in the postponement of sexual reproduction until the rate of growth begins to decline. "For so long as the rate of growth continues rapid, there is proof that the organism gets food with facility—that expenditure does not seriously check assimilation; and that the size reached is as yet not disadvantageous: or rather, indeed, that it is advantageous. But when the rate of growth is much decreased by the increase of expenditure—when the excess of assimilative power is diminishing so fast as to indicate its approaching disappearance—it becomes needful, for the maintenance of the species, that this excess shall be turned to the production of new individuals; since, did growth continue until there was a complete balancing of assimilation and expenditure, the production of new individuals would be either impossible or fatal to the parent. And it is clear that 'natural selection' will continually tend to determine the period at which gamogenesis commences, in such a way as most favours the maintenance of the race."
That natural selection punctuates the life to advantage does not imply that it works directly towards such a remote goal as species-maintaining; it means that the arrangements which do secure this end most effectively are those which tend to establish themselves. Those that do not secure this end are eliminated.
Nutrition and Reproduction.—Spencer's doctrine of the antithesis between Nutrition and Reproduction is of great importance in biology, and we must dwell on it a little longer.
The life of organisms is rhythmic. Plants have their long period of vegetative growth, and then suddenly burst into flower. Animals in their young stages grow rapidly, and as the growth ceases reproduction normally begins; or again, just as perennial plants are strictly vegetative through a great part of the year or for many successive years, but have their periodic recurrence of flowers and fruit, so it is with many animals which after remaining virtually asexual for prolonged periods, exhibit periodic returns of a reproductive or sexual tide. Foliage and fruiting, periods of nutrition and crises of reproduction, hunger and love, must be interpreted as life-tides, punctuated by the seasons and other circumstances through the agency of Natural Selection, but none the less as expressions of the fundamental organic rhythm between rest and work, upbuilding and expenditure, repair and waste, which on the protoplasmic plane are known as anabolism and katabolism.[8]
[8] P. Geddes and J. Arthur Thomson, The Evolution of Sex, revised edition, 1901, p. 238.
Anabolism and katabolism are the two sides of protoplasmic life, and the major rhythms of the respective preponderance of these give the antitheses of growth and multiplication, asexual and sexual reproduction. The contrasts of metabolism represent the swings of the organic see-saw; the periodic contrasts correspond to alternate weightings or lightenings of the two sides.
Spencer's induction that "an approach towards equilibrium between the forces which cause growth and the forces which oppose growth, is the chief condition to the recurrence of sexual reproduction," is an approximate answer to the question—When does sexual reproduction recur? But there remains, he says, the more difficult question—Why does sexual reproduction recur? Why cannot multiplication be carried on in all cases, as it is in many cases, by asexual reproduction?
As yet, he says, biology is not advanced enough to give a reply, but a certain hypothetical answer may be suggested. "Seeing, on the one hand, that gamogenesis recurs only in individuals which are approaching a state of organic equilibrium; and seeing, on the other hand, that the sperm-cells and germ-cells thrown off by such individuals are cells in which developmental changes have ended in quiescence, but in which, after their union, there arises a process of active cell-formation; we may suspect that the approach towards a state of general equilibrium in such gamogenetic individuals is accompanied by an approach towards molecular equilibrium in them; and that the need for this union of sperm-cell and germ-cell is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ—a result probably effected by mixing the slightly different physiological units of slightly different individuals."
Now, while Spencer was probably right in saying that fertilisation promotes change, we cannot think that he succeeded in finding what he was seeking, namely a primary physiological reason why sexual reproduction should occur. It may be pointed out that it is only in a limited sense that sperm-cells or egg-cells can be spoken of as in a state of "quiescence," and that it is only in a limited sense that the organism which has finished growing and is beginning to be sexual can be spoken of as in a state of general or molecular equilibrium. An egg-cell is quiescent, as a seed lying in the ground is quiescent, awaiting its stimulus of warmth and moisture; a sperm-cell is quiescent, as a spore floating in the air is quiescent, awaiting its appropriate soil. The egg-cells and sperm-cells cannot be very quiescent since they do so much when they unite. Moreover, we have simply to recall the facts of natural parthenogenesis on the one hand or of artificial parthenogenesis on the other, to see that the quiescence of the egg is a secondary restriction adapted to secure amphimixis. Moreover, the familiar external and internal changes which occur in the bodies of organisms when they are approaching sexual maturity suggest the very opposite of general or molecular equilibrium.
It may be pointed out that although asexual multiplication persists in many organisms both large and small, and is sometimes the only method of multiplication, yet it is apt to be a somewhat expensive process and would be difficult to arrange for in highly differentiated animals. On the other hand, asexual multiplication succeeds admirably in many cases; it does not imply degeneration; it is not inconsistent with the occurrence of variations; and it is conceivable that it might have been arranged for even in the highest animals. What other reason can there be why the circuitous process of sexual reproduction has been preferred? It may be said that the arrangement by which multiplication is secured through special germ-cells, more or less apart from the cells which build up the body, may be justified as an arrangement which prevents or tends to prevent the transmission of bodily modifications, many of which are detrimental. But as this cuts both ways, preventing or tending to prevent the transmission of useful modifications, there must be some other reason why the circuitous process of sexual reproduction has been preferred. We believe the answer to be that sexual reproduction is an adaptive process securing the benefits of amphimixis, for in amphimixis and in the changes preparatory to it, there is an important source of variation. In one of his essays Weismann wrote as follows:—
"Sexual reproduction is well known to consist in the fusion of two contrasted reproductive cells, or perhaps even in the fusion of their nuclei alone. These reproductive cells contain the germinal material or germ-plasm, and this again, in its specific molecular structure, is the bearer of the hereditary tendencies of the organisms from which the reproductive cells originate. Thus in sexual reproduction two hereditary tendencies are in a sense intermingled. In this mingling, I see the cause of the hereditary individual characteristics; and in the production of these characters, the task of sexual reproduction. It has to supply the material for the individual differences from which selection produces new species."
When we inquire into the reasons for the occurrence of a process such as sexual reproduction, there are four different questions which may be put: (1) We may inquire into the historical evolution of the process, so far as that can be legitimately imagined or inferred from still persistent grades. (2) We may try to discover what factors may have operated in the course of evolution in raising the process from one step of differentiation to another. (3) We may also try to show how the process is justified by its advantages either self-regarding or species-maintaining. (4) We may inquire into the physiological sequences in the internal economy of the individual organism which lead up to the process in question. There is no doubt always an immediate necessity for the occurrence of an organic process, but we are in many cases quite unable at present to do more than describe the series of events without understanding their causal nexus. The reason for this is apparent, since the organism is much more than a detached inanimate engine; it is a system which has summed up in it the long results of time, the history of ages. Its rhythms and periodicities and crises puzzle us because they originated under conditions which obtained untold millennia ago. Thus some processes in higher animals may have had originally a reference to tides from the reach of which their present possessors are far withdrawn.
We have entered on this digression partly for clearness sake, and partly to explain why Spencer had, as we think, very limited success in his answer to the question: Why does sexual reproduction occur? The curious reader may be referred to the discussion of these problems in The Evolution of Sex, Contemporary Science Series, Revised Edition, 1901.
The Germ-Cells.—But we cannot leave the interesting chapter on genesis without referring to another of Spencer's conclusions, which does not seem to us to be quite consistent with facts.
"The marvellous phenomena initiated by the meeting of sperm-cell and germ-cell, or rather of their nuclei, naturally suggest the conception of some quite special and peculiar properties possessed by these cells. It seems obvious that this mysterious power which they display of originating a new and complex organism, distinguishes them in the broadest way from portions of organic substance in general. Nevertheless, the more we study the evidence the more are we led towards the conclusion that these cells are not fundamentally different from other cells." The evidence he gives is: (1) that small fragments of tissue in many plants and inferior animals may develop into entire organisms; (2) that the reproductive organs producing eggs and sperms are organs of low organisation, with no specialities of structure "which might be looked for, did sperm-cells and germ-cells need endowing with properties unlike those of all other organic agents." "Thus, there is no warrant for the assumption that sperm-cells and germ-cells possess powers fundamentally unlike those of other cells."
To this it must be answered: (1) though sperm-cells and egg-cells, being living units, cannot be "fundamentally unlike" other living units, such as ordinary body-cells, yet they may be very unlike them; (2) that the germ-cells are very unlike ordinary body-cells is shown by the fact that they can do what no single body-cell can do, build up a whole organism; (3) so specific are germ-cells that in certain cases and in favourable conditions a small fraction of an egg, bereft of its own nucleus, may, if fertilised, develop into an entire and normal larva; (4) it is quite consistent with the idea of evolution that in lower organisms the contrast between body-cells and germ-cells should be less pronounced than in higher forms. But the fundamental answer is found when we inquire into the history of the germ-cells. In many cases, and the list is being added to, the future reproductive cells are segregated off at an early stage in embryonic development. Even before differentiation sets in, the future reproductive cells may be set apart from the body-forming cells. The latter develop in manifold variety into skin and nerve, muscle and blood, gut and gland; they differentiate, and may lose almost all protoplasmic likeness to the mother ovum. But the reproductive cells are set apart; they take no share in the differentiation, but remain virtually unchanged, continuing unaltered the protoplasmic tradition of the original fertilised ovum. After a while their division-products will be liberated as functional reproductive cells or germ-cells, handing on the tradition intact to the next generation.
An early isolation of the reproductive cells has been observed in the harlequin fly (Chironomus) and in some other insects, in the aberrant worm-type Sagitta, in leeches, in thread-worms, in some Polyzoa, in some small Crustaceans known as Cladocera, in the water-flea Moina, in some Arachnoids (Phalangidæ), in the bony fish Micrometrus aggregatus, and in other cases. In the development of the threadworm of the horse according to Boveri, the very first cleavage of the ovum establishes a distinction between somatic and reproductive cells. One of the first two cells is the ancestor of all the cells of the body; the other is the ancestor of all the germ-cells. "Moreover, from the outset the progenitor of the germ-cells differs from the somatic cells not only in the greater size and richness of the chromatin of its nucleus, but also in its mode of mitosis (division), for in all those blastomeres (segmentation-cells) destined to produce somatic cells a portion of the chromatin is cast out into the cytoplasm, where it degenerates, and only in the germ-cells is the sum-total of the chromatin retained" (E. B. Wilson, The Cell in Development and Inheritance, 1896, p. 111).
In the majority of cases, we admit, the reproductive cells are not to be seen in early segregation, and the continuous lineage from the fertilised ovum cannot be traced. In the majority of cases, the germ-cells are seen as such after considerable differentiation has gone on, and although they are linear descendants of the ovum, their special lineage cannot be traced. But it seems legitimate to argue from the clear cases to the obscure cases, and to say that the germ-cells are those cells which retain the complete complement of heritable qualities. Adopting the conception of the germ-plasm as the material within the nucleus which bears all the properties transmitted in inheritance, we may still say, in Weismann's words, "In every development a portion of this specific germ-plasm, which the parental ovum contains, is unused in the upbuilding of the offspring's body, and is reserved unchanged to form the germ-cells of the next generation.... The germ-cells no longer appear as products of the body, at least not in their more essential part—the specific germ-plasm; they appear rather as something opposed to the sum-total of body-cells; and the germ-cells of successive generations are related to one another like generations of Protozoa." In terms of this conception, which fits many facts, we may say that in plants and lower animals the distinction between germ-plasm and somato-plasm has not been much accentuated, and that in some organisms the body-cells retain enough undifferentiated germ-plasm to enable them in small or large companies to regrow an entire organism.
It may be said that Spencer must also have regarded the germ-cells as containing the whole complement of hereditary qualities. It must be so. The point is that he rejected the theory which gives a rational account of how the germ-cells have this content and their power of developing into an organism, like from like. The sentence in which he points out that the reproductive organs have "none of the specialities of structure which might be looked for, did the sperm-cells and germ-cells need endowing with properties unlike those of all other organic agents," shows how far he deliberately stood from the conception we have outlined.
Here we may note that the "Inductions" regarding Heredity are discussed in our eleventh chapter, and those regarding Variation in our twelfth chapter. We have not dealt with the suggestive concrete sections which deal with structural and functional evolution, partly because they are too concrete to be dealt with briefly, and partly because they are saturated with the hypothesis of the transmission of acquired characters. Spencer's most important conclusion in regard to the Laws of Multiplication is referred to under the heading Population.