the number of polyprotoplast cells. Examina- 

 tion of both control groups did not reveal any 

 changes in the nuclei. Consequently, the experi 

 ments indicate that the changes induced by X 

 rays are stable and do not disappear or lessen 

 even when irradiated seeds are stored for 3 

 years. 



NATURAL RADIATION AND COSMIC RAYS 



Muller (1928) in his first papers dealing with 

 the increase in frequency of mutations induced 

 by X rays suggested that spontaneously arising 

 mutations were possibly due to natural short- 

 wave irradiation. The suggestion was all the 

 more probable since none of the experimentally 

 obtained mutations differed in any way from 

 natural mutations. 



Olson and Lewis (1928) in their article, 

 "Natural Reactivity and the Origin of Species, " 

 wrote that in nature all living creatures are 

 constantly bombarded by gamma rays of low 

 intensity which are due to a wide distribution of 

 radioactive substances. On the basis of the 

 material obtained by Goodspeed, the above 

 authors indicated that as a result of natural 

 radiation two variants per thousand should occur 

 in tobacco, i. e. , the percentage that was 

 actually found to exist. They also set out to 

 discover whether or not in areas of increased 

 radioactivity there was not also an increased 

 percentage of mutations in the fauna and flora. 

 Similar ideas were expressed by Haldane (1929*)- 

 Dixon (1929) thought it was possible to conclude 

 from the work of Goodspeed and Olson [1928] 

 that cosmic rays (which have the same properties 

 as X rays) were a factor in the occurrence of 

 changes by direct action on the germ plasm. 

 After these theoretical assumptions, a series 

 of experiments was begun for the purpose of 

 proving the influence of radioactivity and cos- 

 mic rays on the mutability of genes under natural 

 conditions. 



Babcock and Collins (1929a and b) selected 

 for their investigations one of the genetics lab- 

 oratories in Berkeley and also a small tunnel in 

 San Francisco. The two locations were sepa- 

 rated from each other by a distance of 15 miles. 

 Electroscopic examination revealed the amount 

 of ionizing radiation in the tunnel was twice as 

 great as in the laboratory in Berkeley. With this 

 exception, the conditions of the two experiments 

 were for all practical purposes identical. In 

 Berkeley 3481 experiments were performed; in 

 the tunnel, 2500. 



Average percentage of lethals 



in the tunnel 0.526 + 0.074 



Average percentage of lethals 



in the laboratory 0.251 ±0.044 



Difference 0.275 + 0.086 



From this it is possible to conclude with 



certainty that the natural ionizing radiation on 

 earth is an important controlling factor in the 

 frequency with which new hereditary character- 

 istics occur in plants and animals. Although 

 this does not prove that radiation is the direct 

 cause of mutations, it does open the way for an 

 experimental solution of the problem. 



Whether natural radiation on earth induces 

 mutations or only affects their frequency is not 

 important; what is obvious is that it plays a 

 significant role in evolution. Radioactive sub- 

 stances are distributed in the crust of the earth 

 as well as on the surface. The greater part of 

 the deposits of radioactive substances which 

 are located near the surface of the earth, acts 

 as a source of ionizing radiation that affects the 

 mutability of organisms in its vicinity. 



Engelstad and Moxnes (1934) performed 

 experiments with the action of cosmic rays on 

 white mice. Silver mines to the west of Oslo 

 were a suitable place for the work since 350 

 meters of ore provided protection against cos- 

 mic radiation. A building accessible to cosmic 

 rays served as a control station. Experiments 

 with the animals lasted more than 10 months; 

 438 specimens were used representing five 

 generations, four of which were born during the 

 course of the experiment. The experimental 

 animals did not differ from the control animals 

 either in behavior or in skeletal structure. The 

 only difference was that the animals living in 

 the mine were heavier. 



Thomas ([1935 and] 1936) wrote two interest- 

 ing articles in connection with this question. 

 He points out that there can be no doubt about 

 the occurrence of mutations; it is sufficient to 

 point out the new species Spartina townsendii 

 which arose and spread in our time^ It is diffi - 

 cult to imagine that such changes can be spon- 

 taneous; we must seek some external cause of 

 their appearance in the environment. In a 

 survey of experiments on obtaining mutations 

 by artificial means, Timofeeff -Ressovsky [1934] 

 showed that the only effective method for induc- 

 ing mutations is bombardment by short-wave 

 radiation or high-speed electrons. At the same 

 time, numerous experiments on the effect of X 

 rays on the structure of the nucleus indicate that 

 irradiation can also cause cytological changes 

 which can be associated with specific differ- 

 ences. However, since the time of Muller' s 

 discovery that the frequency of mutations can 

 be raised by the action of X rays, there has 

 been little hope of proving that species arise in 

 nature due to natural short-wave radiation or 

 radioactive particles. 



The consensus of investigators in this subject 

 is that "the mutation process capable of being 

 induced by natural radiation is out of the ques- 

 tion since sufficient amounts of energy required 

 for artificial induction of mutations are not 

 available. " Thomas [1936] considers this to be 



HI 



