gaining resistance against viruses, antibiotics or toxic substances, various
biochemical deviations, etc.
Mutation Theory
The observed sudden changes in morphology and hereditary features of
the organisms have attracted the attention of investigators. Some of them
have directed their attention to searching and elucidating the reasons for
their appearance. In 1899 the Russian botanist S. I. Korzhinsky has arrived
at the conclusion that these changes arise unevenly, without any transition
between the initial and newly form. The foundations of an orderly theory
has been laid by the Holland botanist Hugo de Vries (1901, 1903). For the
first time he introduced the term mutation to denote sudden and saltatory
hereditary changes.
Even now some of the basic principles of the mutation theory of H. de
Vries are valid. The most important of them are as follows: a) the mutation
arises suddenly, without any transitions; b) the newly forms are completely
constant, i.e. resistant; c) the mutations, unlike the non-hereditary changes
(fluctuations), are qualitative changes; d) mutations are realized in different
directions and can be useful, as well as harmful; e) same mutations can
arise over again.
W. Bateson (1902, 1909) have tried to elucidate the nature of
mutations by relating them to the hereditary factors of Mendel. Later on
Muller (1922) has arrived at the conclusion that mutations are closely
related to the nature of the gene. In 1925 Nadson and Filipov discovered
the mutagenic effect of radium rays on yeasts. Convincing evidence of the
effect of ionizing radiation on heredity and X-ray capability to cause
mutations are given by Muller (1927, 1928 a, b) in Drosophila and by
Stadler (1928 a, b) in barley and maize. There are laid the beginnings of
the experimental, i.e. induction mutagenesis. Also in the thirties, the
mutagenic effect of ultraviolet light is discovered.
Besides by ionizing radiations, mutagenic effect is manifested by some
chemical substances. Intensive research on them begins with the works of Ch.
Auerbach in England and I. A. Rapoport in USSR. The mighty mutagens
found, inducing more than 30—60% mutations, were called supermutagens
(see Rapoport, 1966).
Using the ionizing radiations and chemical mutagenic substances, as
well as their additive effect by combining, increased the power and the
possibilities of experimental mutagenesis. It began its considerable
progress as scientific branch. That not only encompassed theoretical
problems related to arising of mutations, their specificity and frequency, the
nature of gene and the character of chromosomal aberrations, but also
found practical application. There were obtained many new mutant forms in
different kinds of plants, animals and microorganisms, some of them with