Weismann Theory

WEISMANN THEORY (Weismann, 1834-1914), weismannism, an extremely developed form of evolutionary teaching proposed by Professor of Freiburg University Weismann, originating from Darwin's theory of natural selection but containing many original ideas, especially in the field of the doctrine of heredity. Often under W. t. is meant only this part of his teaching - the doctrine of germ plasm. The starting point of W.'s reasoning was the contrast between single-celled organisms-'simplest'-and multicellular organisms. The body of the former, in W.'s opinion, is essentially immortal, since it passes directly during reproduction (division) into the bodies of daughter individuals. They have no natural death. On the contrary, the lifespan of multicellular organisms is limited. They age and die. Only their sex cells do not age, but multiply indefinitely. In these sex cells lies the immortal part of the body. When the egg divides and gives rise to new cells of the organism, the immortal plasm of it passes in unchanged and complete form only to those cells which will give rise to the sex cells of the new organism. Thus there is outlined a continuous path, an initial path (Keimbahn) of this germ plasm (Keimplasma) from one generation to another. Germ plasm, in contrast to bodily or somatic (Somatoplasma), alone possesses the ability to restore an entire organism with all its properties. Any change in somatic plasm, whether it is produced under the influence of external causes or functional exercise of organs-is not inherited. W. was the first to decisively speak out against the inheritance of acquired characteristics, which was recognized by Darwin and on which especially rely the teachings known under the name of 'neo-Lamarckism.' By cutting off the tails of mice for 22 generations and not noticing in the offspring of these mice any decrease in tail length, W. proved the non-inheritability of mechanical injuries. Similarly, he presented many weighty proofs in favor of the non-inheritability of functional changes (the appearance of functional peculiarities in asexual individuals of social insects). In W.'s opinion, only a change in germ plasm can cause the appearance of a new hereditary characteristic. Finding that the egg in virgin reproduction (parthenogenesis) secretes only one directing body, and in fertilization-two, he explained the difference by the fact that in the latter case half of the germ plasm of the egg goes into one of the two directing bodies, in order to be replaced during fertilization by half of the germ plasm of the spermatozoon. During fertilization, there thus occurs amphimixis, the mixing of germ plasm of two individuals. But since during maturation the germ plasm is reduced and each sex cell receives from each parent only half of the hereditary properties ('reduction hypothesis'), amphimixis is the source of an extremely large number of combinations of hereditary rudiments received by children from parents, combinations which become the main cause of variability of forms. Natural selection, preserving from these forms the most adapted, and from the combinations of hereditary rudiments-the most useful, becomes not only the main, but the only factor directing evolution. W.'s teaching, as placing natural selection in the forefront (in this respect W. goes further than Darwin), can therefore be attributed to the group of evolutionary theories united under the name 'neo-Darwinism.' W.'s reduction theory soon found a very interesting illustration in the cytological phenomena of sex cell maturation, well studied with the help of the microscope, and his teaching on amphimixis as the source of combinations coincided remarkably well with those explanations which Mendel gave to the laws of inheritance discovered by him. Less successful was the purely speculative detailed elaboration of W.'s teaching on germ plasm, which he represented as consisting of separate, subordinate to each other, structural hereditary units (biofores, determinants, ids and idants). However, even here, identifying idants with chromosomes and ids-with parts of chromosomes, W. became one of the founders of the teaching on chromosomes as carriers of hereditary properties-carriers which include within themselves separate hereditary material rudiments of properties of the organism. However one may relate to W.'s theory, it must be recognized that it exerted enormous influence on the development of questions of heredity and that many modern teachings have grown to a considerable part on its soil.