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
