Durable Modification

DURABLE MODIFICATION, a feature acquired under the influence of external conditions, which persists for several generations, gradually weakening, and ultimately disappearing. The term was introduced by Jollos, who investigated this phenomenon in infusoria. By cultivating infusoria in gradually increasing doses of poisons (calcium arsenite), Jollos succeeded in acclimating them to relatively large doses, exceeding the usual lethal dose. The ability to withstand strong doses of poison is preserved even in a culture raised under normal conditions for many generations of asexual reproduction by division, although it gradually weakens. Thus, a culture initially tolerating 5% poison, after 8 months of being kept under normal conditions, tolerated only 4%, after 9 1/2 months only 2.5%, and after 10 months no more than 1%, i.e., it did not differ from normal cultures. But if asexual reproduction is interrupted by a sexual process, conjugation, the infusoria for the most part lose the ability they developed to withstand strong doses of poisons. In other cases, the acquired feature is preserved even after conjugation. Under the influence of calcium nitrate (Ca(NO₃)₂), the rate of division in infusoria decreases. This property is preserved even under normal conditions for many generations and sometimes does not disappear even after conjugation. However, after a series of divisions or reorganization of the nucleus, this property still disappears. A whole series of experiments of this kind was also conducted on various bacteria (the effect of various conditions on the virulence of Diplococcus pneumoniae—experiments by Toenniessen and others). Among multicellular organisms, phenomena of this kind were observed in Daphnia (crustaceans)—experiments by Woltereck. Scant nutrition changes the size of the "helmet"; this feature is preserved, despite good nutritional conditions, until the third generation. Probably a whole series of experiments on the "inheritance of acquired characteristics" belongs to the same category of phenomena. It is possible to think that temporary, although durable, modifications arise as a result of changes in the plasma of germ cells, or the soma, of protozoa or bacteria, while stable hereditary mutations arise as a result of changes in the nuclear apparatus (Muller's experiments on Drosophila). Lat.: Vladimirsky A., Are Acquired Characteristics Inherited?, M.-L., 1927 (lit.); Shimkevich V., Biological Foundations of Zoology, vol. I-II, M.-L., 1923-25; Jollos V., Untersuchungen über Variabilität und Vererbung bei Infusorien, Arch. f. Protistenkunde, Band XLIII, 1921; Muller H., Artificial transmutation of the gene, Science, v. LXVI, 1927.