Parallel Paths: A Study in Biology, Ethics, and Art. T. W. RollestonЧитать онлайн книгу.
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.
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