The Germ-plasm : a theory of heredity / by August Weismann ; translated by W. Newton Parker and Harriet Ronnfeldt.

  • Weismann, August, 1834-1914.
Date:
1893
Catalogue details

Licence: Public Domain Mark

Credit: The Germ-plasm : a theory of heredity / by August Weismann ; translated by W. Newton Parker and Harriet Ronnfeldt. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

    486/510 (page 462)
    exhibited by human beings. It has long been known that hcBmophilia occurs in men only, but is transmitted by women. If we assume that the visible sexual differences, as well as those existing in the system which we are unable actually to recognise, are due to the presence of double determinants, the peculiar limitation of this uncommon disease to one sex is explained. The disease, like a secondary sexual character, is only trans- mitted to the sex in which it first appeared, for this half of the double determinants of the ' mesoblast germ' has alone been modified by the disease. The sexual polyjnorphism exhibited by certain butterflies may also be explained by assuming the presence of double deter- minants of several local varieties of the same species which interbreed with one another. The polymorphism of bees and other animals which fonn communities, requires, however, the assumption of triple or quadruple determinants. In these animals the female half of the double determinant again becomes doubled, and this may also be the case as regards the male half (Termites). Lastly, the assumption of double determinants in the idio- plasm accounts for temporary dimorphism, such as seasonal dimorphism. The occurrence oi dimorphism is in all cases attributable to the pi-esence of two kinds of detenninants ; but the causes which determine which of the two is to become active, are extremely varied, and cannot in many cases be accurately indicated. The determining influences, however, are always external ones —such as fertilisation, nutrition, and the effect of light in cases of dichogeny in plants. It is self-evident from the theory of heredity here propounded, that only those characters are transmissible which have been controlled—i.e.., produced—by determinants of the germ, and that consequently only those variations are hereditary which result from the modification of several or many determinants in the germ-plasm, and not those which have arisen subse- quently in consequence of some influence exerted upon the cells of the body. In other words, it follows from this theory that somatogenic or acquired characters cannot be transmitted. This, however, does not imply that external influences are incapable of producing hereditary variations ; on the contrary, they always give rise to such variations when they are capable