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Weismannism

weismann, selection, germ-plasm, variation, germ, changes and external

WEISMANNISM. The essence of the doc trines which were taught by August Weismal111. and which differ from the more moderate views of Darwin, Romanes, and others, is the 'all sufficiency' of natural selection; the sweeping denial that use-inheritance (q.v.) operates at all the view that variation is mainly due to sexual reproduction, or `amphimixisC and the elaborate and complicated relations of the ultimate ele ments or determinants (biophores) of the germ plasm. He thus states his theory as to the na ture of the nuclear substance: "The germ-plasm, or hereditary suhstanee of the Aletazoa and phyta, therefore, consists of a larger or smaller number of idants, which in turn are composed of ids; each id has a definite and special architec ture, as it is composed of determinants, each of which plays a perfectly definite part in develop ment" (The Germ-Plasm, p. 453). He admits that the primary cause of variation is always the effect of external infinences, but when these changes of conditions only atfeet the body in general their effects are limited to the simple life of the individual, and are not transmitted by heredity; lmt when they occur in the germ-plasm they are transmitted to the next generation, and valise corresponding hereditary variations in the body. The opponents of this view hold that what ever external changes affect the lasly in general must necessarily affect the reproductive cells and the chromosomes of the nucleus of such cells. Indeed, the Weismannians treat the germ-plasm as if it were a parasite, getting shelter and food from the body containing it.

In maintaining the 'all-sufficiency' of natural selection Weismann argues that external influ ences act only as stimuli by which latent powers in the germ are called forth. 'Hence the changes in external conditions which appear to deter mine character's are not in any sense a true cause cflicicas, but merely a necessary condition for the appearance of that which is inherent in the organism at sonic stage of development. Ilis critics assert that in taking such extreme ground as this lie overlooks the thoroughgoing effects of past geological changes, and their persistent effects as seen in the different lines of develop ment of series of extinct animals. Weismannism, they say, also overlooks the fact that in the be ginning natural selection no materials to operate with, the earliest types having been originated by the action of the primary factors of organic evolution. Accounting for the dis

appearance of a typical organ, Weismann says it is "always clue to variations of the primary con stituents of the germ," whereas Darwin and Lamarck attribute the loss of or reduet ion in the number of digits or other parts to simple disuse.

Another phase of Weismannism is the prin ciple of intra-selection. Taking the hint from Roux's principle of selection, or the struggle for existence between the parts of the organism, Weismann has greatly extended it. claiming that the selective process must take place not only in cells and tissues, but also in the smallest con ceivable particles of the germ-plasm, which lie calls `biophores.' This process lie calls infra selection. Ilere as elsewhere his opponents argue that \\•eismann minimizes, or sets aside even in the ease of plants. the effects of the action of the primary factors, such as gravity. light. heat, moisture, and ehemical stimuli, and asserts that all the various adaptations of the parts of plants "must likewise be referred to the process of intra selection." In the same manner Weismann. in the words of a friendly critic, driven haek from acquired characters as a cause of phylogenetic variation, came to regard the mingling of germ characters in amphimixis as the source of all variation, though he does allow that in the low est organisms variation is due to the direct in fluence of changes in the environment.

Although Weismann struck a death blow at the ancient preformation theory, his speculations have led him and his followers to a modern phase of preformation. Tie denies that epigenetic de velopment exists, and claims that individual development (ontogenesis) "can be explained . only by evolution, and not by epigenesis." In his view the germ "is an exceedingly complicated living being, a microcosm in the truest sense, in which every independently variable part that ever appear throughout the whole life is repre seated by a living particle" (Hertwig). It should be said that Weismann from time to time modified his views.

For bibliography, see under WEISMANN; tor criticisms of Weismannism, consult Roinanes, .1 a Examina I ion of Weismannism ( Chicago, 1893) ; 0. Hertwig, The Biological Problem. of To-day (New York, 1894).