Page:The New International Encyclopædia 1st ed. v. 20.djvu/487

* WEISMANN. 411 WEISMANNISM. selection," holding thnt changes in the physical environment only afford the initial stimulus. Although Jiiger first {187(i) proposed the term 'eontinuity of the germ-plasm,' Weismann greatly expanded the idea, and added many new and original suggestions, establishing the pres- ent theory that lieredity (q.v.) has a pliysical basis. lie was also led to deny what is known as use-inhcrilanee (q.v.), or the Lamarckian doetrine that characters acquired during the life- time of an auinuil may he transmitUnl to its pos- terity. Sul}s('(piently lie ]>ro|)Osed a theory of germinal selection. He also called in ques- tion the existence of the influence of a pre- vious sire on the progeny of a subsequent one by the same female, giving the name 'telegony' (q.v.) to the phenomenon. Weismann is an admirat)le investigator, a strong thinker, and the leader of the Neo-Darwinian or Weismannian .school of evolutionists. His theories, which are bold, original, and suggestive, have been criticised as supf rscientific speculations and assumptions, especially that related to the architecture or mechanical arrangement of his hypothetical ele- ments or 'determinants' of the germ-plasm (q.v.), but it will be acknowledged by all that he has been a powerful agent in placing mod- ern biological studies on a higher plane, and rendering the theory of descent of practical value in psychological and philosophical studies. His principal works are: Die Enticickeliing der Diplcren (18C4) ; Shidirn :ur Drscendrn.-lhroric (lS7.';-76; Eng. trans., London, 1SS2) : Bci- iriifie zur Naturgeschichte der Driphnoiden (1870-70) ; Ueher die Zrthl der Richtungfikorper mid iiher Hire liedeutung auf die Terrrbiinfj (1887) ; Essays upon Heredity and Kindred Bio- logical Prohlems (English trans., Oxford, 1889) ; Ampliinii.ris odcr Die Vermischutig der Iiidi- viducn (1801) : Aufsiltze iiber Vercrbuiig (1892) ; The Germ-Plasm, A Theory of Beredity (GJerman, Jena, 1892; English, Xew York. 1893); "The All-Suffieieney of Natural Selection" {Contem- porary Review, February, March, and May, 1893) ; The Effect of Extermil Influences upon Development (the Romanes Lecture, 1894) ; Veue Gedanken zur Vererbungsfrage. Mine Ant- wort an Herbert Spencer (1895) ; 'Xcic Experi- ments on the fteasonal Dimorphism of Lepidop- tera (English trans.. Entomologist, January-Au- giist, 1896); Ueber Germinalselektion (1896); Yortriige ilber Desvendenztheorie (1902). WEISMANNISM. The essence of the doc- trines which were taught by August Weismann. and which differ from the more moderate views of Darwin, Romanes, and other.s, is the 'all- Buflieiency' 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 'amphimixis;' 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 substance of the Metazoa and Meta- 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. 4.53). He nilmits that the primary cause of variation is always Vol. XX.— 27. the effect of external influences, but when lhe.se changes of conditions only aU'ect the body in general their effects are limited to the simple life of the individu;il, and are not transmitted by heredity; but when they occur in the gcrni-|ilasm they ari! transmitted to the next generation, and cause corres|)oiuling hereditary variations in the bod}'. The opponents of tills view hold that what- ever externa! changes affect the body in general must necessarily affect the icproductive cells and the chromosomes of the nucleus of such cells. Indeed, the Weismannians treat the germ-idasm as if it were a parasite, getting shelter and food from the body containing It. In maintaining the 'all-suflieiency' 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 characters are not in any sense a true causa cffieiciis, but merely a necessary eonilitlon f(n- the appearance of that which is inherent in the organism at some stage of development. His critics assert that in taking such extreme ground as this he overlooks the thoroughgoing effects of past geological changes, and their persistent effects as seen in the dlirercnt lines of develop- ment of series of extinct animals. Weismannism, they say, also overlooks the fact that in the be- ginning natural selection h:id no materials to operate with, the earliest types having been originated bj' the action of the primary factors of organic evolution. Accounting for the dis- appearance of a tj'pieal organ, Weismann says it is "always due to variations of the primary con- stituents of the germ," whereas Darwin and Lamarck attribute the loss of or reduction 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 Rotix's principle of .selection, or the struggle for existence between the jiarts 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 he calls 'biophores.' This process he calls intra- selcetion. Here as elsewhere his opponents argue that Weismann minimizes, or sets aside even in the case of plants, the effects of the action of the primary factors, such as gravity, light, heat, moisture, and chemical stimuli, and asserts that all the various adaptations of the parts of plants "must likewise be referred to the process of intra- seleetion." In the same manner W'eismann, in the words of a friendly critic, driven back 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 ^'eismann struck a death blow at the ancient preformation theory, his speculations have led him and his followers to a modern phase of preformation. He 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 r^re-