Thomas [1935] sees further proof for his 

 idea that many important changes in fossil flora 

 are definitely related to periods of mountain 

 formation. For example, in the western part 

 of North America at the time of the Upper Cre- 

 taceous period mountains rose to a height of 

 20, 000 feet. But since this was a warm period, 

 luxuriant in flora, the rise of the mountains did 

 not destroy it, and some species of plants sur- 

 vived and underwent mutations. Shortly after 

 this 83 genera and numerous species are en- 

 countered in the fossil flora of the United States 

 hitherto unknown in Cretaceous flora. Accord- 

 ing to Thomas [1935], this indicates that the 

 formation of new species is related to the growth 

 of plants in high altitudes where the intensity of 

 cosmic rays is very high. To the arguments of 

 several authors who point out that great varia- 

 tions in temperature play an important role in 

 the mountains Thomas [1935] replies that in 

 certain lowland areas where variations are also 

 great, such a multiplicity of forms is not en- 

 countered. 



Muller,^ Timofeeff-Ressovsky,9 Efroimson,9 

 et al are opposed to attaching significance to 

 natural radiation as the cause of spontaneous 

 mutability. They base their arguments on the 

 following considerations. It is known that the 

 frequency of mutations is proportional to the 

 dose of roentgen units (r). A single unit (Ir) 

 produces 2. 1 x 10^ ions. A dose of 3420 r, 

 which produces 7. 2 x 10^2 ions induces 15 [%] 

 sex -linked lethal gene mutations in Drosophila 

 melanogaster. During its life span, which is 

 about 14 days, the fly is affected by natural 

 radiation equivalent to 3. 6 x lO" ions, since in 

 the immediate environment of the fly natural 

 radiation amounts to 30 ions/sq [cubic?] cm/ 

 second. Since the natural frequency of muta- 

 tions is 0. 1%, or 1/150 of the 15% which is 

 induced by a dose of 3420 r, while the ratio of 

 ionization is 1:200, 000 (i. e. , 3. 6 x lO'^: 7. 2 x 

 10l2), the disproportion is 1/150 : 1/200,000, 

 which equals 1/1333. In other words, natural 

 radiation has to be 1333 times as great in order 

 to explain the spontaneous frequency of sex- 

 linked mutations. 



Delbruck and Timofeeff-Ressovsky (1936) 

 think that the more interesting facts presented 

 by Thomas can be explained by the modern con- 

 cepts of evolution. The high altitudes of moun- 

 tainous areas produce specific conditions for the 

 organisms and a great variety of local types, 

 which are conducive to isolation and selection 

 of specially adapted varieties. Each of the free 

 living populations usually has a large number 

 of mutations that can serve as a basis for 

 evolution. 



The same issue of the magazine Nature 



'These references are cited in article by Delbnick 

 and Timofeeff-Ressovsky (1936). 



[p. 359] that contained the article of the above 

 authors also contained H. Thomas' answer. He 

 pointed out that there is a difference between the 

 ionization induced by cosmic rays and that in- 

 duced by the gamma rays of radium, and he 

 further indicated that it would be improper to 

 conclude that with equal mean intensities their 

 actions would be identical. 



In 1933, Friesen [1936] had the idea of sub- 

 jecting Drosophila flies to the effect of cosmic 

 rays in the stratosphere, but it was not until 

 1935 that he succeeded in raising the flies in a 

 "stratostat" to an elevation of 15, 900 meters. 

 For 2 hours the flies were bombarded by cos- 

 mic rays whose intensity was 100 times greater 

 than their intensity on the surface of the earth. 

 Friesen employed the usual method to deter- 

 mine lethal mutations in the X chromosome. At 

 the same time an experimental control was set 

 up. In the 2724 fly chromosomes elevated into 

 the stratosphere 8 mutations were obtained, 

 whereas 10 mutations were found in the 2445 

 control chromosomes. These negative results 

 with cosmic rays of high intensity should make 

 us careful about accepting the theory that re- 

 gards cosmic rays as an important factor in 

 organic evolution. Stubbe, in 1932-1933, set 

 up an interesting experiment for the purpose of 

 investigating the influence of cosmic ultraradia- 

 tion on the mutability of genes in the well-known 

 strain of Antirrhinum majus . He also took as 

 a point of departure the works of Vavilov who 

 pointed out that high mountainous areas consti- 

 tute the so-called "genocenters, " which produce 

 the greatest varieties of species. A high moun- 

 tain station on the Jungfrau was selected as the 

 setting for these experiments. Eight -week -old 

 plants were transported to the station and left 

 there for one or two growing seasons. However, 

 the mutability of the genes of the plants culti- 

 vated at the elevation of 3300 meters did not 

 differ in any respect from those in the valleys. 



These experimental data of Friesen [1936] 

 and Stubbe [1937] indicate that cosmic ultra - 

 radiation plays only the most insignificant part, 

 if any, in the occurrence of spontaneous 

 mutations. 



Since natural radiation and cosmic rays can 

 account only in trifling degree for the frequency 

 of spontaneous mutations, they cannot be con- 

 sidered the sole cause of such mutations. 



REFERENCES 



1. AFANAS'EVA, A. S. 1936a. Persistence of 



action of X rays on wheat. Byull. Moskov . 

 Obshchestva Ispytatelei Prirody 45: 433- 

 440. (In Russian) 



2. . 1936b. The stability of the 

 effect of X rays on the seeds of spring 



113 



