Читать книгу Parallel Paths: A Study in Biology, Ethics, and Art - T. W. Rolleston - Страница 8

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DE MINIMIS

Immense have been the preparations for me,

Faithful and friendly the arms that have help’d me.

******

“Before I was born out of my mother generations guided me,

My embryo has never been torpid, nothing could overlay it.”

Walt Whitman.

There are two functions of organic life which are often confused together, but which it is well to keep distinct in our thought. These are Growth and Development. The mark of growth is that an organism, by assimilation from the outside world, becomes larger than it was. But in development it becomes different from what it was. The history of an embryo in the womb presents a succession of phenomena which, when one comes to realize them, almost stagger thought; for, while remaining the same thing all through, it is continually becoming a different class of thing—first two cells, then one cell, then a fish, a quadruped, ultimately a human being. This is Development. Once born, it is laid hold of by the principle of Growth which lasts until maturity. Now in the groups called Species, as well as in individuals, we observe exactly the same distinction. The members of a species multiply and increase their numbers. This is Growth. But under certain conditions, which we have now to investigate, they vary in type and ultimately give rise to new species differing widely from that from which they sprang. This we call Development or, in the more popular term for the process when applied to species, Evolution.

The investigation of this process in all its details has been the master-impulse of biology ever since the fact of the process was established by the researches of Darwin.

In Darwin’s time the study of evolution was mainly an affair of what is called Natural History But it has now been realized that fully to comprehend the processes involved—so far as they can ever be comprehended—it is necessary to find out of what kind of material living beings are composed, and how their fundamental processes take place. “The ultimate problems of sex, fertilization, inheritance, and development,” says Wilson, have been now “shown to be cell-problems.”26 Before going further, therefore, we must give some account of the leading facts connected with the structure and vital action of the cell.

Since the publication of the Origin of Species, probably the most important contribution to biological theory is to be found in the researches of Dr. A. Weismann, and particularly in his large work, The Evolution Theory, of which a masterly English translation has recently appeared.27 Weismann, on one side, represents an heroic attempt to bring back to the strictly mechanical principles of Darwinism the tide of biological speculation, which has been flowing more and more in the direction of recognizing an essential and not a merely fortuitous connexion between the goal of the evolution of natural forms and the means taken by nature to attain it. On another side he has brought the physiology of the cell into true relation with the natural history of the organism and of the species, and has become the author, or at least the first great expounder and systematizer, of a theory of heredity—the now famous Germ-Plasm theory—much of which seems a solid, permanent, and deeply important contribution to knowledge. But this theory seems to lead straight to a non-mechanical or psychic conception of the driving-force of evolution, and Weismann has therefore supplied the other part which, in the view of the present writer and of many others better qualified to judge, seems to be of the nature of a baseless and improbable hypothesis, devised to find a means of avoiding recourse to any non-mechanical conception of the ultimate nature of evolutionary processes.

As we shall be much concerned with Weismann’s views, let us place at the head of our study of them a couple of passages in which his general attitude towards the phenomena of vital processes is expressed.

“In our time,” he writes, “the great riddle has been solved—the riddle of the origin of what is best suited to its purpose without the co-operation of purposive forces.”28 “We must certainly assume,” he declares, “that the mechanical theory of life is correct.”29

A longer passage shows us what he understands by ‘mechanical’:—

“The living machine differs essentially from other machines in the fact that it constructs itself; it arises by development from a cell, by going through numerous stages of development, but none of these stages is a dead thing, each in itself is a living organism whose chief function is to give rise to the next stage. Thus each stage of the development may be compared to a machine whose function consists in producing a similar but more complex machine. Each stage is thus composed, just like the complete organism, of a number of such ‘constellations’ of elementary substances and elementary forces, whose number in the beginnings is relatively small, but increases rapidly with each new stage.”30

It would have been simpler, but it would not have suited Weismann’s conception of nature, to say that the “living machine” differs essentially from other machines in not being a machine at all, or anything in the least like one. No machine constructs itself. No machine can do anything but repeat a certain series of movements, each series exactly similar to the last. What Weismann has described is not a machine, just because it is a living organism. It is surely as true in biology as it is in mechanics that in any purely physical chain of sequences you cannot by any possibility get more out at the end than you put in at the beginning, unless you take it in upon the way.

“Development,” writes Weismann, “is an expression of life.”31 But “life,” again, is merely “a chemico-physical phenomenon.”32 To say that development is an expression of a chemico-physical phenomenon does not seem a very illuminating or helpful generalization. The fact is that the statement that life is a chemico-physical phenomenon does not take us further towards an understanding of the subject than when we say, what is equally true, that chemical and physical phenomena are a manifestation of life. Life is everywhere. We use it as a convenient term for the energies associated with ‘living’ protoplasm, because we observe that when it is present protoplasmic structures act and react (as in the phenomena of nutrition, for instance) in certain chemico-physical ways, while, if it be absent, the same protoplasm acts in other ways, also chemico-physical, but quite different from the former, and analogous to the ways of minerals and of gases into which dead protoplasm finally resolves itself. The chemico-physical actions and reactions appear in a living plant or animal to be under the direction of a force devoted to the preservation of that particular organism. The smallest atom of organic life includes not only a chemical compound but a chemist. In the mineral world we may say broadly that there is no individuality of parts.33 With protoplasmic structure, therefore, a stage is reached in the evolution of life which we may rightfully call ‘life’ par excellence, but there has been no breach of continuity, and it is highly probable, as Weismann himself suggests, that far below the limits of microscopic observation the transformation of ‘dead’ into ‘living’ matter is continually going forward. When, therefore, we speak of the action of living protoplasm the distinction is rather between this action and that of a piece of mechanism than between protoplasm and minerals or gases.

The phenomena of cell-growth, reproduction, and heredity are those which lie at the basis of all organized protoplasmic life, and in all the forms of that life, vegetable as well as animal, they are extraordinarily similar; there is, in fact, nothing which all the species of living things have so much in common. One of the most wonderful and fascinating chapters in the whole range of science is that which contains the account of these processes, and it is only within the last few years that it has been possible to write it. Weismann, in a certain section of his Evolution Theory, has brought the facts together in a manner which, for its lucidity and mastery of the subject-matter, deserves to be called a classic example of scientific exposition.34 To understand the basis of the higher manifestations of life, these processes, as we have said, must first be understood, and an account of them, based on Weismann, and accepting his germ-plasm theory so far as it seems to accord with established facts, will be given, of course only in the broadest outlines.35 At the same time it will be attempted, here and there, to throw some light on the rationale of the processes described.

All animal and vegetable structure arises from cellular tissue, and in fact is either cellular tissue or, as in the case of bones, scales, etc., the mineral deposit formed by the action of cells. The simplest living forms are composed of single cells, and the most complex and huge of them were each once nothing more than a single cell, possessed of the powers of development and growth. In multicellular organisms, this single originating cell is usually formed by the fusion of two imperfect cells by what is indifferently called conjugation, sexual reproduction, or ‘amphimixis.’ All cells, whether they are the product of conjugation or not, grow, when they do grow, fundamentally in the same way, and this way must now be described.

The contents of the typical cell are broadly differentiated into (1) a more or less hardened envelope containing (2) a substance called cytoplasm (Gk. κύτος, a cell), and (3) a small, rounded, dark-coloured body called the nucleus. Until recently nothing more than this was known of the structure of the cell, and nothing at all of the functions of the nucleus. Now, keener microscopic research and better instruments have thrown a flood of light on cell-organization, and the nucleus is revealed as a powerful factor in the vital processes of the cell and the bearer of its hereditary substance36—that which makes it a cell of some particular organism, plant or animal, and of no other. This hereditary substance, divined by the botanist Nägeli, and since observed by Weismann and others, is called ‘chromatin’ (from the fact that it is observed by means of the stain it takes from the addition of an aniline dye), or ‘idioplasm’ (Nägeli’s appellation), which might be rendered the ‘selfhood substance’ of the cell.

Cellular structure begins, as has long been known, by the division of a cell into two, each of the parts then proceeding to grow by the assimilative power of protoplasm and in due time to divide in its turn. A mass of these cells is called ‘cellular tissue.’ The so-called ‘budding’ of a small cell from the side of the parent is, of course, simply a form of division. The process of division and redivision goes on, accompanied by a differentiation in the shape and function of the different cells or groups of cells which are formed, until the structure of the plant or animal is completed. In these operations the nucleus plays the principal part. The division of the cell is essentially the division of the nucleus. A detached portion of a cell which contains nothing of the nucleus can reproduce itself no more; it perishes.

Fig. I.

This illustration, which (by permission of The Macmillan Co.) I take from Wilson’s work, The Cell, is one of remarkable interest, for in it the microscope has caught, in a piece of actual tissue from the skin of the salamander, Amblystoma, three nuclei in different stages of mitotic division. Most of the nuclei, which are seen as large, roundish objects in their respective cells, show the chromatin in its ‘resting’ condition interspersed through the nucleus. The nucleus under a shows the chromatin gathered into chromosomes. At b the centrosomes with their astral figures (which can barely be detected) have been formed, the chromosomes have carried out their longitudinal division, and are being attracted half towards one centrosome and half towards the other. A little above this the process has been carried further, and the sides of the cell are beginning to contract, preparatory to forming two new ones. In Fig. 2 will be found a clear representation of the astral figures.

To face p. 40.

Fig. 2.

The above illustration from Wilson’s The Cell shows in more or less diagrammatic form the stage of nuclear division in which the chromosomes, as yet undivided, have arranged themselves in the centre of the nucleus. The centrosomes with their astral figures have been formed, and have taken their places near each pole of the nucleus. The next stage is represented at b in Fig. 1.

When a cell is about to divide, an organ of recent discovery, termed the ‘centrosome,’ comes into play. This appears as the core of a sort of rayed or star-like figure, and it takes up its position beside the nucleus. When the cell is resting, the chromatin is dispersed through the nucleus in a mass of broken lines, forming a kind of network. When division begins, this broken-up substance forms itself into a series of small threads, sometimes straight, sometimes looped or curved. These are called ‘chromosomes.’ There are always a definite and invariable number of chromosomes for every species of plant or animal—the cell of a man has so many,37 of a grasshopper so many, of a lily so many. The chromosomes range themselves in a belt across the centre of the nucleus, and the centrosome breaks into two parts, which take up a position one at each end of the nucleus. Regarding the nucleus as a tiny globe, we may say that the chromosomes lie in the equatorial plane, while the two parts of the centrosome move towards the North and South Poles respectively.

The centrosomes, at the two poles of the nucleus, are surrounded each with a halo of ray-like processes (the centrosphere), and on the sides next each other these rays penetrate the nucleus and join, forming a spindle-shaped figure with a centrosphere at each end. This spindle figure appears to be the organ by which the division is accomplished, for each of the chromosomes now splits itself in two longitudinally, as one cleaves a log of wood, and one half passes over to each centrosphere, thus making an exact division of the whole chromatin or hereditary substance. An indentation now appears in the outer wall of the cell and also in the nucleus—it deepens and deepens, and finally two cells appear instead of one, each with a nucleus, a centrosome, and a supply of chromatin, the latter now breaking up into its original condition of diffusion through the nucleus. In multicellular organisms the two new cells, of course, do not separate, but a wall is formed between them. Some plant-cells contain several nuclei; in this case division of the nucleus is not necessarily followed by that of the cell.38

Throughout the processes of cell division it is apparent that the utmost care is taken to ensure an exact partition of the chromatin between the two new cells. This partition has to be qualitative as well as quantitative; for one chromosome may, and no doubt does, differ in function and influence from another, and has various elements within itself. The longitudinal division of each chromosome, in which the elements are arranged like beads on a rosary, ensures that the different elements of the whole hereditary substance shall appear in each new cell in exactly the same relative proportion as in the parent cell; just as if two persons had to divide between them a dozen apples of different varieties, and secured perfect equality, not by taking six apples each, but by dividing every apple in two. This is the fundamental cause of the fixity of species, which means the production of offspring having the same specific characteristics as their parents. How, under these conditions, the mutability of species is brought about must be discussed later. It is first of all necessary to inquire more closely into the composition of the chromatin, and to study the special phenomena of cell-growth in connexion with conjugation, where new and extraordinary features come to light.

A chromosome is not, or is not usually, a simple body. In all but the very lowest organisms it is composed, as we have said, of a number of elements. Each of these elements is styled a ‘determinant,’ and it controls the form, colour, and function of some definite part of the future plant or animal. Weismann believes the determinants to be grouped into complex bodies called ‘ids,’ each id containing all the determinants necessary for a whole being, and each chromosome being composed of a number of ids. These ids are microscopically visible; they form the beads on the rosary already referred to; but their exact composition and potency are largely conjectural at present. How far the subdivision of determinants may go, it is, of course, impossible to ascertain. We cannot say, for instance, whether there is a determinant for every hair of the head, or one for the hirsute covering in general, or one for each of the different sections of the scalp. But the division is very minute. Each of the ids may be a very complex body, as we see by the manner in which, in some families, small physical signs like a patch of hair differing from the colour of the rest, or a tiny pit or mole on the skin of a certain part of the body, may be handed down, in that precise position, for generations. There may be, and, in fact, in the higher plants and animals there must be, a number of determinants for each part of the structure, and the final characteristics of that part must be the resultant of a blend of all these determinants, the more powerful predominating in proportion to their vitality and force. The whole body of the chromosomes may therefore be said to represent one or more complete beings in diagrammatic form, each part of the complete animal or plant being represented by some part of a chromosome, though of course not physically resembling it. And we thus strike on the very curious and startling fact that, as far as we can see, every cell in every organism throughout the world of life contains all the elements of the whole being to which it belongs, and is, potentially, that being.39 All the higher organisms possess two kinds of cells—reproductive cells which have the faculty of fusing together to reproduce their kind, and ‘somatic’ or body cells, which, although they all originate in a reproductive cell, multiply only by division, and have the function of forming the various parts of the bodily structure. Of the nucleus of a germ cell “we cannot say that it differs in any essential or definite way from the nucleus of any other cell.”40 All possess the chromatin or hereditary substance of the organism, though, according to Boveri, the germ cells alone receive all the chromatin of the parent cell, the derived somatic cells having to part with some of it.41 There may be some distinction, though on what it may be based it is at present impossible to say, between cells that are capable of developing into a complete organized creature and those that are not.

Every somatic cell is doomed to perish, but every reproductive cell now upon the globe is united, not metaphorically, not by a chain of successive originations or impulses, but by actual identity of substance, with the first beginnings of protoplasmic life in the abyss of time; and it has before it a potential immortality commensurate with life itself. It is not, as used to be thought, a physiological product of the organism in which it dwells; it is a part of the original reproductive cell from which that organism sprang.

To understand these conceptions we must now study the phenomena of reproduction in the light of recent discoveries.

The lowest form of the reproductive process is, of course, by simple division and redivision. This is characteristic of many of those organisms which consist only of a single cell, and it may co-exist, even in these, with a considerable degree of structural complexity, as in the ‘trumpet animalcule,’ Stentor raselii. But among the lowest of these unicellular organisms a curious process is sometimes observed to take place, in which we may doubtless recognize the origin of sexual reproduction. Two, three, or more Amœbæ42 approach each other, partially coalesce, and remain united for some time. They then separate again. No new creatures are formed by this contact; there are no visible results at all. But that something which is for the advantage of the organisms takes place during this period of union is certain, and in the light of what is known of processes in other organisms we can make a very good guess at what this something is. Each Amœba parts with some of its chromatin to some other and receives an equivalent in exchange. The creature is thus reconstituted. The element of change, which always provides so marked a stimulus to vital processes, has been obtained. The process has actually been observed in a certain Infusorian, Noctiluca. Two Noctilucas coalesce, and then proceed to divide at right angles to the plane of contact. This necessarily has the effect of giving to each of the two new Noctilucas which result from the division half the nucleus and chromatin of one parent and half of the other. There is, however, no actual new birth or multiplication of beings; there are only two Noctilucas as before.

We can now imagine that if a certain class of unicellular organisms are in the habit of approaching each other for the purpose of this interchange of portions of their chromatin, they might occasionally, under the influence of the approaching conjugation, expel those portions of chromatin before another cell was in a position to receive it. What would happen if two cells, each of which had thus got rid of half its chromatin, were to come into contact? Plainly, they would fuse together; they would not separate again; they would become a new organism. Each would have supplied just what the other lacked.

This process, forming the bridge from mere cell division to sexual reproduction, is a hypothetical one; it has not, I believe, been actually observed in unicellular organisms, but it is exactly what we find to be taking place when we reach the stage of sexual reproduction among multicellulars. Multicellular organisms of more or less elaborate structure plainly cannot, without breaking up, fuse together like single cells. How, then, are they, as a species, to gain the advantages of the temporary union and interchange of elements which we have observed in the low unicellular organisms? Only in one way—by producing special cells for this purpose. These cells must represent the whole parent, they must be capable of shedding half their chromatin, and, when they have fused, must be capable of growing into a complete organism like the parent. When these specialized cells have been formed, the others, the somatic cells, will at the same time have been specialized for other functions, and will thus naturally lose the original capacity for interchanging chromatin with other cells, i.e. for conjugation. We see the significance, then, of Weismann’s remark, “germ cells made their appearance along with the multicellular body.”43 They are an instance of that differentiation of structure and function which takes place in all highly organized life. We must note also that the benefits of conjugation which are realized individually by the lowest unicellular forms are only realized as a species by the multicellulars. A species must, then, be regarded as in some sense an organic whole, and not as a mere aggregate of individuals.

In some very curious cases which stand on the borderland between sexual and non-sexual reproduction, the same organism is capable of employing both methods. Thus, among the lower seaweeds (Algæ), the genus Pandorina consists of a colony of sixteen green cells contained in a kind of gelatinous matrix which the cells excrete. Each cell is ordinarily capable of recreating the whole organism by division. But after this process has gone on for some time, the need of conjugation is felt, the colony breaks up and cells begin to fuse with each other, though never with those of the same colony. In Pandorina the two conjugating cells are similar in appearance, but in the genus Volvox we begin to see a difference in the appearance of the two kinds of conjugating cells. What may be called the ‘female’ cells (germ cells) are large and quiescent; the ‘male’ (sperm cells) are smaller and active. The primary meaning of this is that the larger cells have stored up a supply of nutriment for the young organism, and are therefore bulkier and less active, while the others contain only the bare elements of cell-structure and are therefore able, as they are obliged, to be active in order to search out their quiescent mates. A strictly vegetable organism, in this stage, may therefore possess organs of locomotion, and be as free-moving as a fish. A remarkable fact has come to light respecting those organisms (like some Algæ among vegetables and Infusorians among animals), which are capable both of conjugation and of reproduction by division, namely, that the supply of nutriment often determines which method shall be followed. If nutriment is abundant, division is practised; if it becomes scanty, an impulse appears to be given to conjugation. Infusorians, which ordinarily conjugate at pretty regular intervals, can be kept indefinitely from doing so, and confined to division, by the simple process of supplying abundance of nutritive matter in the water in which they live.

“As far as we can see from an a priori point of view,” writes Dr. E. B. Wilson in his great work on cell structure and cell phenomena, “there is no reason why, barring accident, cell-division should not follow cell-division in endless succession in the stream of life. It is possible, indeed probable, that such may be the fact in some of the lower and simpler forms of life where no form of sexual reproduction is known to occur. In the vast majority of living forms, however, the series of cell-divisions tends to run in cycles in each of which the energy of division gradually comes to an end and is only restored by an admixture of living matter derived from another cell. This operation, known as fertilization, or fecundation, is the essence of sexual reproduction, and in it we behold a process by which, on the one hand, the energy of division is restored, and by which, on the other hand, two independent lines of descent are blended into one. Why this dual process should take place we are as yet unable to say.”44

The actual mechanism of sexual reproduction is essentially the same wherever it occurs, whether in a seaweed or a human being. Two cells have to play their part in it, the Germ cell and the Sperm cell, and these, in the higher orders of organized beings, come to be located respectively in distinct classes or sexes of individuals. Reproduction begins by the fusion of a sperm, or male cell with a germ, or female cell.

These cells originally resemble the other cells of the same species, containing the same number of chromosomes. If this number was, say, sixteen, which is believed to be the number in man, then a fusion of two complete cells, if it were possible, would produce a cell with thirty-two chromosomes, and that would mean a different species of animal. What happens is that each of the reproductive cells, male and female, prepares itself for conjugation by getting rid of half its chromosomes. Two divisions of the nucleus take place, not as in the ordinary fashion of cell-division, when the chromosomes split longitudinally, but in such a way that, in each division, four of the sixteen chromosomes are bodily expelled from the nucleus and from the cell, when they either perish or, in some cases, appear to help in forming an envelope of nutritive matter round the germ cell. These divisions are called ‘maturation divisions,’ and until they are accomplished, fecundation is impossible. When a sperm cell after maturation comes into the neighbourhood of a germ cell, it penetrates into its substance, using the long flagellum, or tail-like process, with which it is equipped as an organ of locomotion. The two nuclei come into contact and coalesce, and we have thus a new cell with its sixteen chromosomes complete. This cell is the origin of the new being. It divides in two, and each part divides and redivides, different cells gradually differentiating themselves as muscular tissue, cartilage, blood-corpuscles, nerves, reproductive cells, and so forth, until the whole animal is built up and is ready for birth. One point of cardinal importance must here be noted. The originating cell, as we have seen, has eight of its sixteen chromosomes from one parent and eight from another. When division takes place, these chromosomes, as we have seen, split longitudinally, and the result is that each new cell gets exactly the same mixture of chromatin as that of the originating cell—half from each parent. This principle of division is carried on throughout the whole process of building up the new being—every cell of the latter, down to the minutest details of its structure, containing an exactly equal quantity of hereditary elements from each of its parents.

It will be seen from the above account that the old conception of the germ-cell as a passive body, incapable of a change till ‘fertilized’ by a male or sperm cell, was altogether wrong. Both male and female cells prepare themselves for conjugation long before it takes place, and neither of them can be said to be a more active agent in fertilization than the other. Not ‘fertilization’ but ‘fusion’ is the keyword of the process. The mystical conception, as old as Plato, of the male and female as representing respectively the two halves of a complete being, turns out to be no poetic metaphor. As regards the essential features of reproduction, it is a literal fact.

If we now ask why and by what mysterious law all these exact and elaborate choric movements take place Weismann and his school refer us to “chemotactic forces,” the nature of which is yet unknown. Chemotaxis means simply the effect of the presence of certain substances on vital organisms without specific chemical action. The really essential fact is that these special chemotactic forces are working in living protoplasm. Life is not the product or the slave of any chemotactic forces, but their maker and steersman.

The following passage from a work of the late Prof. Geo. Rolleston may be pertinently quoted here:—

“There exists, as is well known, a tendency to resolve all physiological into physico-chemical phenomena: undoubtedly many have been, and some more may still remain to be, so resolved; but the public may rest assured that in the kingdom of Biology no desire for a rectification of frontiers will ever be called out by any such attempts at, or successes in the way of, encroachment; and that where physics and chemistry can show that physico-chemical agencies are sufficient to account for the phenomena, there their claim upon the territory will be acceded to, as in the cases we have been glancing at [certain animal poisons], and where such claims cannot be established and fail to come up to the quantitative requirements of strict science, as in the cases of continuous and of discontinuous development or self-multiplication of a contagious germ, and in some others, they will be disallowed.”45

This was written in 1870. A generation later the attempt to reduce life to a physico-chemical phenomenon had not made much way, as may be judged by the following passage from Strasburger’s Text Book of Botany:—46

“Vital phenomena are essentially bound up with the living protoplasm. No other substance exhibits a similar series of remarkable and varied phenomena, such as we may compare with the attributes of life. As both physics and chemistry have been restricted to the investigation of lifeless bodies, any attempt to explain vital phenomena solely by chemical and physical laws could only be induced by a false conception of their real significance, and must lead to fruitless results. The physical attributes of air, water, and of the glasses and metals made use of in physical apparatus, can never explain qualities like nutrition, respiration, growth, irritability and reproduction.”

And Wilson concludes his work by the admission that

“the study of the cell has on the whole seemed to widen rather than to narrow the enormous gap that separates even the lowest forms of life from the inorganic world.”47

“The lowest observed forms of life” would have been a more exact way of stating the fact.

Many questions of detail will occur to the reader at this point, which he will find answered in the pages of Weismann or other investigators. Here we must confine ourselves to what has a distinct bearing on the objects of this study. One of the points which may be briefly touched on is the question how it comes that two germ cells, once having passed through their maturation divisions, cannot fuse and form a new being; nor can two sperm cells. Were this possible we might have ‘self-fertilization,’ and virginal conception or parthenogenesis, whenever two germ cells in the ovary of a female animal or in that of a plant happened to come into contact. But since the object of fusion is the union of (more or less) unlike, and not closely related, elements, we find that even when a kind of self-fertilization occurs, as in some plants, the sperm or pollen cells are differentiated visibly, and probably still more invisibly, from the germ cells. But, apart from this, the object of preventing the union of reproductive cells of the same sex is mechanically attained by a very curious device. The cell-organ by which division is carried out is the centrosome. But in the course of the two maturation divisions of the germ cell, that cell loses its centrosome, which seems to be absorbed into the protoplasmic substance of the cell when once its task is accomplished. No fusion of any number of such cells can therefore lead to any further change or growth, for growth is based on cell division, and the centrosome is the organ of division. The sperm cell, on the other hand, does not lose its centrosome; it retains it to form the organ of division for the new cell after conjugation. But, reduced as it is to little more than a bare nucleus without any envelope of nutritive matter, the sperm cell cannot support the intense vital activity called for in the initial stages of the life of a new being, and therefore sperm cells, like the germ cells, though for a different reason, would be incapable of mutual conjugation, even if the element of mutual attraction existed among them.

Another point of interest is the question of the determination of sex. The known facts afford a strong corroboration of the general theory of reproduction outlined above. It has not been ascertained, nor is it, perhaps, ascertainable, whether the sperm cells of the male contain in their chromatin a preponderance of male, while the germ cells provide chiefly the female determinants.48 However this may be, it is certain that determinants which severally control the formation both of male and of female structure are always present in every combination of the sperm and germ cells, those which exhibit the greatest energy and vitality probably prevailing in the determination of the sex of the future being. This accounts at once not only for the cases (rare in the higher animals) of actual hermaphroditism, when the sex is really indistinguishable, but for the universal occurrence in all male animals of rudimentary female organs (such as mammæ) and in all females of rudimentary male organs. Both sets of determinants are always present; the more powerful prevail, but the weaker have a deflecting influence on the total result. When the primary sexual characters of the embryo are determined, they appear to communicate a stimulus which starts into activity the appropriate secondary characters, such as colouring and other modifications not directly sexual. An extraordinary case, which I take from Beddard’s Animal Coloration,49 is that of a chaffinch which was found to have on the left side of its body the plumage of a hen bird and on its right that of a cock. On dissection the meaning of this freak of physiology was revealed. The bird was an hermaphrodite, having the female organs of generation on the left side of its body and the male on the right. Hermaphroditism is not in itself a very uncommon phenomenon in birds (though here it is a monstrosity, not, as in slugs and snails, a natural and useful condition); but the way in which in this instance it governed the distribution of colour is most peculiar; and of course it strongly reinforces Weismann’s conception of distinct determinants for the various details of bodily structure.50

This brings us to the recognition of a competition among determinants which is an important, indeed a cardinal, feature in Weismann’s theory of evolution. He makes, as I am forced to believe, an illegitimate and extravagant use of it, but the principle may really exist and be operative without furnishing the master-word to the riddle of organized being. The master-word, as I shall try to show, is nature’s will to live. But before going fully into this argument, let us fix in our minds the rationale of those processes of elementary organic life which have been described in this chapter. Protoplasmic life may be supposed to have originated, and perhaps to be still originating, in certain molecular combinations of matter. In other words, the combination, when it took place, developed certain peculiar forces through which it was enabled to maintain itself and to grow, by the processes called assimilation and nutrition. These forces, then, were potentially present in nature before the molecules combined to evoke them. They are among the latent powers of life. They waited, ready to be called into action when the required external form should be arrived at in the play of molecular energy. Life first originated, no doubt, in unconnected and inconceivably small units of protoplasm. Between the units thus formed and their combination into the elaborate structure which we now know a cell to be—packed as full of varied energies, it has been said, as an ironclad is of machinery—there is evidently a very wide gap. All we know is that when we have got the cell, we find it in possession of a complex apparatus for subdivision, which, taken together with the faculties of nutrition and growth, enable any one cell to multiply indefinitely by producing replicas of itself. To life and growth, then, has been added the faculty for multiplication. Here we strike on a veritable mystery. Why should any new movement ever take place? Why should a cell ever divide in two? We can only say that it is its property to do so.51 It does so because it is alive. Did this property first arise as one of a multitude of aimless movements—the only one which ensured permanence and multiplicity to the organisms which exhibited it? If so, then Nature, at the time when life began on the earth, behaved in a manner most unlike that in which she behaves at present. If we are to interpret the processes hidden in the remote past by the light of what we see at present, we shall conclude that, at bottom, the will to live made molecular action—and the same force incorporated itself in the combinations which originated protoplasmic life, ordered the structure of the cell, and gave it the need and the power to multiply. Nature is for ever changing, for ever straining after new life, after more life.

Having arrived at the cell with its powers of division, the next step was the power of conjugation between cells with their interchange of vital substance, bringing about, in Weismann’s words, “a wealth and diversity of organic architecture which without it would have been unattainable.” It takes place by means of physical energies, but the process is entirely inexplicable unless we assume that it exists to satisfy a need, a Drang, for life. And this need, although of course it displays itself in physical processes, is not in itself a physical process. At the very beginnings of structural life, if not before it, we are obliged to pass beyond physics in order to comprehend physical phenomena. Whenever we find an aggregate of living units, such as a Pandorina colony, living with a communal life which is other than the sum-total of the lives of the individual units, we are in presence at once of the necessity for a metaphysical conception, to render intelligible the unity in diversity which we perceive.

The response of living protoplasm to the stimuli it receives from the outside world is normally directed to the maintenance of the life and form of the organism. The response of what is called ‘lifeless’ matter is of another nature; not because it is really lifeless, for if it were it would not respond at all, but because it has no organisms to protect and foster. We all know the nature of the action of gravity on Newton’s apple. It was treated as a dead substance, like a stone, and gravity acted upon it as upon all other ponderable matter. But when it had fallen to the earth, had decayed, and one of its pips began to grow, the action of gravity began to be manifested in a quite different and very peculiar fashion. It has been ascertained by a series of ingenious experiments that gravity is the force which obliges the roots of a plant to sink downwards into the earth. This does not, of course, mean that the roots are drawn downwards by attraction of the earth, but that the pull of gravitation gives a certain stimulus to the cells concerned which makes them grow in that direction. Precisely the same stimulus communicated to the cells of the stem has the very opposite effect—these it causes to grow upright into the air and light. Thus the roots are, as it is termed, positively, and the stems negatively, geotropic. The substance of the root cells and of the stem cells is the same, the stimulus is the same, but the effects on growth agree in only one point, that they are respectively what the plant requires them to be. There is no doubt that if a species of plants were placed in such a position that it would serve them for the roots to grow upwards, then upward-growing roots would eventually be evolved; in fact, this is actually the case in the lateral underground roots of certain mangroves which rise to the surface and become modified as breathing organs, and in the aerial roots of various orchids, etc.52 When a change of habitat takes place calling for new developments of structure to meet new conditions, these developments are not, as a matter of actual observation, found to be mechanically ‘selected’ from a mass of random movements and modifications of tissue—they reach their goal, it is true, by a series of gradual approximations, but the goal is in sight from the beginning. In other words, adaptability is a fundamental character of life. Hence the fact that multicellular organisms which cannot, as a whole, fuse with others, adapt themselves to these conditions by the allotment of special cells for that purpose; while, again, the production of multicellular organisms is itself an adaptation to Nature’s need for the higher organization of life.

“The botanist Reinke,” writes Weismann, “has recently called attention once again to the fact that machines cannot be directly made up of primary physico-chemical forces or energies, but that, as Lotze said, forces of a superior order are indispensable, which so dispose the fundamental chemico-physical forces that they must act in the way aimed at by the purpose of the machine.... Organisms also [according to Reinke] are machines which perform a particular and purposeful kind of work, and they are only capable of doing so because the energies which perform the work are forced into definite paths by superior forces; these superior forces are thus ‘the steersmen of the energies.’”53

Weismann admits that there is “undoubtedly a kernel of truth in this view,” but he is content with this perfunctory acknowledgment. His main efforts are devoted to the substitution of fortuitously developed “constellations” of molecular energy for any force which can be deemed to have the slightest tincture of intelligence or purpose. “In our time,” as he writes, “the great riddle has been solved—the riddle of the origin of what is best suited to its purpose without the co-operation of purposive forces.” The nature of the proposed solution can be best described and discussed in another chapter, when we shall be in a position to consider it in relation to the whole history of organic development from its origin in protoplasmic life to the evolution of species in plants and animals.