In Chapter II. we saw that many of the valuable and carefully reasoned results which Weismann had obtained are accompanied by much detailed speculation based upon an extremely limited fact basis. In his later writings especially, the reader who searches for the facts and reasoning which refer to such questions as the non-transmission of acquired characters will find everywhere an abundant commingling with speculations concerning the mechanical process of heredity. I have ventured in the appendix to sketch shortly this speculation of Weismann’s, and to give my own estimation of its value.

Weismann was not the first to speculate or found a mechanical theory of heredity; Darwin himself published a theory of “pangenesis,” and this was, I believe, the only piece of speculation of which Darwin was ever guilty. That he might picture to himself, by means of material particles, the views that he held of heredity, Darwin supposed that during their lifetime every cell of the parent disengages small living particles—gemmules—which find their way to and are stored up in the generative cells, ready to develop in the next generation into cells similar to those from which they came. This theory was actually framed to support those cases where Darwin supposed that acquired characters are transmitted, for he says in one of his replies to Huxley’s letters:[33] “I do not doubt your judgment is perfectly just, and I will try to persuade myself not to publish. The whole affair is much too speculative; yet I think some such view will have to be adopted when I call to mind such facts as the inherited effects of use and disuse,” etc. In the theory of pangenesis the gemmules residing in the body or somatic cells will be subject to such influences as affect the cells, and will naturally transmit any change to which they have been subjected to the offspring that they eventually build up. Darwin’s view may be graphically represented by Fig.2. Since Darwin’s time most biologists have come to doubt whether there is any evidence that acquired characters are transmitted at all, and incline rather to view racial change as altogether due to inborn variations, some of which variations have an advantage over others; Darwin himself, as Huxley pointed out,[34] laid less weight on the influence of acquired habits in his later than in his earlier writings. Modern biologists tend, therefore, to be more Darwinian than Darwin in respect to their thoroughgoing adherence to the action of selective influence; but of necessity they discard his theory of pangenesis, which was a pictorial expression of Darwin’s lingering Lamarckian tendencies. Let me give some idea of what the Neo-Darwinians picture to themselves as the “process of heredity.” It has been known now for many years that every cell in the body, including the sperm cells and ova, are descended from a fertilised ovum. Of these cells of the body all obviously die except those sperm cells and ova which give rise to the next generation, and so on. We have, therefore, a continuing chain of actual organic matter linking every living form with those that are most ancestral and remote, and from these chains all the so-called living individuals that have ever existed have, as it were, been thrown off. Many have emphasised this point, Owen, Haeckel and others; but perhaps Francis Galton must be given much of the credit of clearly stating it as a fact to be remarked, though similar views have more recently been popularised among biologists by the delightful pen of Weismann. The “continuity of the germ plasm,” the title by which this view is generally known, expresses the fact that germ substance continues in an unbroken line from generation to generation; a man is similar to his parents because he develops out of a similar plasm. The continuity of germ plasm (stirp of Galton), like Darwin’s selection, is a fact, not a theory.

Now while Darwin’s fancy regarding pangenesis compels one to believe that the effects of use and disuse, the action of disease and mutilation, must be transmitted, the continuity of the germ plasm in the isolated reproductive cells of the parents renders this extremely doubtful. The anatomical conditions actually found are fully reconcilable with the observed non-transmission of acquired characters.

This fact was shortly and precisely stated by Galton in 1875, and by Weismann in his “Studies in the Theory of Descent,” published in English in 1882. In 1883, in his essay upon Life and Death,[35] Weismann looked upon the germ plasm as the substance of the germinal or reproductive cells, and on p.148 he defines “germ” as follows: “I should propose to include under this term every cell, cytode, or group of cells which, while not possessing the structure of the mature individual of the species, possess the power of developing into it under certain circumstances.” So far Weismann was a fact man, and gave to the facts observed their true and full significance. Since that time, however, he has speculated upon the nature of the germ plasm. To him it consists of ultimate living particles, to which he assigns various and specific purposes, and groups them at will, group within group, like nests of Chinese boxes. The biophores (his conceived units) have the capacity of growth and reproduction—a generous concession indeed. They are groups of chemical molecules, far beyond the highest powers of the microscope, and cannot therefore be investigated by our senses; they are conceptions, not perceptions. These biophores are arranged in groups, called determinants, one for every part of the adult which is capable of variation; groups of determinants are termed “ids,” and groups of ids are termed “idants,” the last being visible in the ovum as a brightly-stained rod of unclear matter. By the multiplication, differentiation, and disintegration of these various groups, the adult body is formed, and Weismann is prepared to explain every step.

Speaking in 1883[36] of Darwin’s theory of pangenesis, Weismann remarks: “We become lost in unfounded hypotheses.” I think this is a true and allowable criticism, but I also think that it applies in far greater measure to the theory of the germ plasm developed by Weismann himself. It may be pointed out that Darwin placed little store upon his theory, and apologised for its speculative nature, not only in the letters already referred to, but in his work upon “The Variations of Plants and Animals under Domestication,” where it appears in a single chapter at the end of a large work of 800 pages. Darwin here says (vol.ii., p.349): “I am aware that my view is merely a provisional hypothesis or speculation; but until a better one be advanced, it will serve to bring together a multitude of facts which are at present left disconnected by any efficient cause.” Weismann’s speculations, equally unfounded on fact, are nevertheless viewed by him as being of sufficient importance to demand the most detailed elaboration. Do not suppose for an instant that I am decrying the use of fancy or provisional hypotheses; the world advances upon the steps, generally the ruined steps, of hypotheses. The value of an hypothesis depends, however, entirely upon whether we can put it to the test of experiment. If it is intangible, then it remains an hypothesis, while the great body of fact workers go on building their sciences, and in the completion of these it has no part. Darwin’s pangenesis has no value as an hypothesis, it seems to me, apart from its being a pictorial way of illustrating how use and disuse might be inherited. This latter is a question which can be solved experimentally; pangenesis was a mental picture present to Darwin’s mind, and he threw it out for what it was worth. In this picture the gemmules were supposed to pass from the cells of the body through the blood into the reproductive cells, and the experiments which Galton, and subsequently Romanes, undertook to disprove pangenesis by showing that certain samples of blood did not contain gemmules, and that therefore pangenesis did not occur, appear to me to show a want of appreciation of the fact that Darwin intended this theory merely as a temporary mental picture, and nothing more.

But Weismann puts forward his views in most sober earnest, elaborates the details of his theory, and remodifies his conceptions so that his story may fit in with the fresh discoveries in embryology and comparative anatomy as these are made. Our criticism is simply this, that the hypothesis, being nothing more than a personal conception of the author’s, is not to be tested by experiment, and that the author could always escape from the clutch of refutation. We are left, moreover, in the same difficulty with which we started, for by giving his living units the functions of growth and reproduction, which must involve heredity itself, Weismann shunts back the question from animal and plant, which we can see and handle, to particles quite beyond our ken. We may also point out that there is no reason to assume that living matter is made up of little parts, or persons, molecules or groups of molecules; experiment, in fact, strongly contradicts this assumption. Let us see how humanity first obtained the idea that matter is made up of little bits, how we obtained the idea of molecule or atom. The idea is an ancient one, and it is easy to see how it occurred to thinkers even in primitive times. Most solids, such as chalk, sand, and rock, and fluids too and gases, are, when broken or divided, of obviously similar parts. If we break a piece of writing chalk across, each fragment of it is chalk—that is, it appears to us to be white and hard, it will write upon the board, etc. If we continue to break it we shall always have smaller particles of chalk, never anything else. Thus men generalised from their commoner experiences, and got the idea that all things are built up of small parts or molecules. Anaxagoras (70th Olympiad), from experiences such as those just described, in all probability arrived at his theory of elements (homoeomeriÆ), viewing all things as built up of elementary things of the same nature, flesh and blood of elements of flesh and blood, etc. Similar views are held to the present day, and are an expression of the fact that you can break most things into similar but smaller parts. Quoting from Tait’s “Properties of Matter,” 1885, p.21, we find: “But the really extraordinary fact, already known in this part of our subject, is the apparently perfect similarity and equality of any two particles of the same kind of gas, probably of each individual species of matter, when it is reduced to a state of vapour. Of such parts, therefore, whether they be further divisible or not, each species of solid or liquid must be looked upon as built up.” It will be noted that the moderns would make a distinction which the ancients did not; a modern will speak of a molecule of a gas or of incandescent iron, but he hesitates before speaking of a molecule of an ordinary solid at ordinary temperature, and would certainly, if an exact thinker, never dream of speaking of a molecule of wood or flesh or bone. If we carefully prepare as perfect a cylinder of wood as we may wish, and divide it into two equal parts, these (unlike the piece of chalk or iron) will not be the same; the graining will show a difference to the naked eye. The microscope will show that the wood is not uniform in its smallest parts; it is what we may term “organised,” the structure being different as we shift the eye along the smallest distance of the thinnest section. The same applies to bone and muscle; in fact, to all parts of the bodies of all animals and plants. The idea, then, of molecule is a conception of an ultimate bit of a substance towards which our experimental breaking has in actual experience partially reduced it, and would never have occurred to the ancients had the commonest objects of nature been, like variegated marble or granite, of obvious heterogeneous structure. When, therefore, we translate this idea to the realm of biology, where bodies are not made up of similar parts, it is obvious that we do this without warrant, in face of our biological experience, and that we are thoughtlessly accepting a physical theory of gases and incandescent solids without appreciating the foundation upon which this theory rests. The fact of the matter is, we are in utter ignorance as to the ultimate constitution of any living matter; but so far as microscopic investigation goes, we learn that it has unlike—not like—parts. Our experience, therefore, rather contradicts the belief that it will be found to consist of ultimate parts, each part having the function of living protoplasm, biophores, or whatever name may be attached to them; and we recall Clifford’s rule that we may believe what goes beyond our experience only when it is inferred from that experience by the assumption that what we do not know is like what we know.[37]

THE END.

FOOTNOTES

[33] “Life and Letters,” first edition, vol.iii., p.44.

[34] “Life and Letters,” vol.ii., p.14.

[35] Essay to be found in vol.i.; English translation, 1889.

[36] Lecture on Heredity, op. cit., p.77.

[37] “Lectures and Essays,” vol.ii., p.210.

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