Weismann, AUGUST, biologist, was born in 1834 at Frankfort-on-the-Main, in the lyceum of which city his father was professor of philology. He was educated at the gymnasium till his eighteenth year, studied medicine at Göttingen, and in 1860 became physician to the Archduke Stephen of Austria. This appointment secured time for zoological pursuits, the first outcome of which was a treatise on the Development of the Diptera. Impaired sight compelled abandonment of microscopical work for some years, and Weismann turned to the study of the problem of variability of organisms, on which the doctrine of descent with modification is based. The results of this, drawn mainly from observations on caterpillars and other insects exhibiting metamorphosis, appeared in a series of papers issued between 1868 and 1876, of which an English translation by R. Meldola, with prefatory note by Darwin, was published in 1882 under the title of Studies in the Theory of Descent. But it is round the answer which Weismann, after many years of research, has given in his Essays upon Heredity and Kindred Biological Problems (Eng. trans. vol. i. 1889; 2d ed. 1891; vol. ii. 1892) to the question 'How is it that a single cell of the body can contain within itself all the hereditary tendencies of the whole organism?' that interest and controversy have gathered. In all theories of Heredity (q.v.) biologists have assumed that characters acquired by the individual are transmitted to offspring. This Weismann denies, and, while biologists have concerned themselves with speculation as to the mode by which such transmissions are effected, he challenges them to prove that they are effected at all. The onus probandi is thus thrown upon his opponents, whose assumptions must give way to experimental evidence, which alone can determine, and that only after protracted record of cases, whether individually-acquired characters are transmitted or not.
Death, he contends, is not a primary attribute of living matter; the Protozoa, or one-celled organisms, being immortal in so far that they do not die naturally. The Protozoan, a microscopic, jelly-like, apparently—not really—structureless mass, with no seeming likeness of parts, multiplies by division. Each half becomes a complete individual, and grows in like manner as the whole to which it belonged, till it also divides, and so on with the multiplication of Protozoa ad infinitum. It cannot be said of either half that one is parent and the other offspring, for both are of the same age, and only, in a limited sense, as the subdivisions into separate individuals are repeated, can we speak of succession of generations. In these processes there is nothing analogous to death. 'There are,' Weismann says, 'no grounds for the assumption that the two halves of an amoeba are differently constituted internally, so that, after a time, one of them will die while the other continues to live. Observations show that when division is almost complete the protoplasm of both parts begins to circulate, and for some time passes backwards and forwards between the two halves. A complete mingling of the whole substance of the animal, and a resulting identity in the constitution of each half, is thus brought about before the final separation' (Essays, p. 26, 1st ed.). Consequently, there is unlimited persistence of the individual; potential, although not absolute, immortality so long as life lasts on the earth.
While the one-celled organisms are thus immortal, only the reproductive cells of the Metazoa, or many-celled, are immortal. How has this come about? Weismann accounts for it by the failure of certain Protozoa to divide equally, whereby unlikeness of parts and differences of position of parts resulted. 'The first multicellular organisms were probably clusters of similar cells, but these units soon lost the original homogeneity. As the result of mere relative position there arose division of labour, some of the cells were especially fitted to provide for the nutrition of the colony, while others undertook the work of reproduction' (Ib. p. 27). Clearly, those on the outside, being exposed to the direct and constant action of their surroundings, would be the media of nutrition, and the builders-up of the cell-commonwealth. So the result of this cell-clustering would be that the cells fell into two classes, body cells and germ-cells. While the body cells were solely concerned with the nutrition of the organism, losing in this specialisation of function the power of reproduction, that power became concentrated in the germ-cells, or, speaking more precisely, in the germ-plasm, which is located in the nucleus of the germ-cell. It is these germ-cells which are the immortal part of the Metazoa. 'It is necessary to distinguish between the mortal and immortal part of the individual—the body (soma) in its narrow sense—and the germ-cells. Death affects only the former; the germ-cells are potentially immortal, in so far as they are able, under favourable circumstances, to develop into a new individual' (Ib. p. 122). With increasing subdivision of function there has been increasing modification of the organism, but the twofold classification of the somatic or body cells and the germ-cells has remained. The death of the body cells is involved in the ultimate failure to repair waste, because a worn-out tissue cannot for ever renew itself, and because cell-division has its limits. In brief, death is the penalty paid for complexity of structure. As it is impossible for the germ-cell to be, as it were, an extract of the whole body, and for all the cells of the body to despatch particles to the germ-cells whence these derive their power of heredity (the fundamental idea of Darwin's theory of Pangenesis), the germ-cells, so far as their essential and characteristic substance is concerned, are not derived from the body of the individual, but directly from the parent germ-cell. Heredity, Weismann contends, is secured by the transference from one generation to another of a substance with a definite chemical and molecular constitution—in other words, by the 'continuity of the germ-plasm.' This germ-plasm (which, Weismann's critics argue, runs perilously near a metaphysical concept) is assumed to possess a highly complex but extremely stable structure, so stable 'that it absorbs nourishment and grows enormously without the least change in its complex molecular structure' (Ib. p. 271). Of this germ-plasm it is further assumed that a small portion contained in the parent egg-cell is not used up in the construction of the body of the offspring, but is reserved unchanged for the formation of the germ-cells of the following generations. 'One might represent the germ-plasm by the metaphor of a long creeping root-stock from which plants arise at intervals, these latter representing the individuals of successive generations' (Ib. p. 266).
Only variations of the germ-plasm itself are inherited, and it is upon these variations that natural selection operates. Variations are due not to the influence of external condition nor to use or disuse of organs, but to sexual conjugation. This process combines two groups of hereditary tendencies derived from the mingled germ-plasms of the male and female parents, resulting in those individual differences which form the material from which new species are produced by the action of natural selection. Those differences multiply in geometrical ratio, so that 'in the tenth generation a single germ contains 1024 different germ-plasms with their inherent hereditary tendencies, and, as continued sexual reproduction can never lead to the reappearance of exactly the same combinations, new ones must always arise' (Ib. p. 276).
So the sum of the matter is that natural selection is the dominant factor, that use and disuse of parts and the action of the environment count for nothing, or, at the most, for but a little. Here and there Weismann makes concessions as to the modifying influences of body cells on the germ-cells (Ib. p. 170), and as to the ultimate origin of hereditary individual differences in the direct action of surroundings (Ib. p. 279), which are a partial surrender of his main contention as to the isolation of the germ-plasm. The vulgar notions concerning the transmission of mutilations and developments of non-vital parts are altogether without evidence, as are the beliefs in coincidence between maternal shocks and impressions and 'birth-marks,' and other malformations in the offspring; but the case is altered when we deal with subtle processes initiating changes in vital parts. It is not easy to reconcile the theory of an insulated 'germ-plasm' with the ceaseless manufacture, secretion, and expulsion of germ-cells, the materials of which are derived from the materials nourishing the entire organism; nor with the subtle influence of the nervous system on the reproductive organs. The chief arguments against Weismann's theory are summarised in the article on Heredity (q.v.), but perhaps the most serious difficulty is in the reconciliation of psychological evolution with the continuity of the germ-plasm. For the researches of Spencer, Balfour, and others have demonstrated that the nervous system had its origin in modifications of the primitive skin due to the direct action of the environment.
Be this as it may, the wide-reaching sociological significance of the doctrine of Heredity—which may be regarded as the physical correlate of Determinism—gives an importance to the labours of Weismann that cannot be overrated, and makes urgent the record of extended observations on the lines already laid down by Galton.
For bibliography of subject, see list of books at end of article HEREDITY, also the articles DARWINIAN THEORY, VARIATION, and cognate articles in this work; Weismann's Essays (1892) and The Germ Plasm (1893); G. J. Romanes, An Examination of Weismannism (1893); Herbert Spencer, A Rejoinder to Professor Weismann (1894); and numerous articles in Nature.