THE NATURE OF THE GENE 383 



genes produced in it. Similar methods have been elaborated for de- 

 tecting mutations in the other chromosomes, but the CIB and attached- 

 X methods are the most commonly used. 



Since Muller's success in inducing mutations by X-rays, many 

 attempts have been made to achieve the same result by other 

 methods. Other types of high frequency radiation (gamma rays, a 

 and jS rays, etc.) have been shown to have the same effect, but short 

 wireless waves (3 to 6 metres) and visible hght are ineffective. The 

 upper limit of effective radiation is the ultra-violet region of the 

 spectrum. 



In his first paper on the subject, MuUer raised the possibiHty that 

 spontaneous mutations may be produced by the natural radiation on 

 the earth's surface. Very sHght increases of mutation rate have in fact 

 been found in regions with a high natural radiation intensity. But 

 calculations, based on the measurements of the effects of radiation 

 discussed in the next section, have shown that the natural radiation is 

 much too small (perhaps five hundred times) ^ to account for the ob- 

 served spontaneous mutation rates. It is possible that some pecuhar 

 result is produced by cosmic ray "showers," but none has yet been 

 demonstrated. 



We have seen that the mutation rate is increased by raising the 

 temperature. This is similar to the effect of temperature on a normal 

 chemical process. It has also been suggested that just sub-lethal tem- 

 peratures might have an effect of a different order of magnitude. 

 Goldschmidt- and Jollos claim to have demonstrated this by submitting 

 larvae to temperatures of about 37° C. for periods of twelve to twenty- 

 four hours. Jollos states that if the procedure is repeated in successive 

 generations, a given locus may mutate successively to a more and more 

 extreme allelomorph. The experiments were unfortunately not per- 

 formed in a way which allows of quantitative results. Other workers, 

 using the standard Clb method, have found only a rather sUght increase 

 in mutation rate at sublethal temperatures (which may, however, be 

 rather greater than can be accounted for by the temperature coefficient) 

 and no sign of progressive mutations. If the results of Goldschmidt and 

 Jollos can be confirmed by future research, they may have great theo- 

 retical importance, but at present opinion about them must be held in 

 suspense. 



1 Miiller and Mott-Sinith 1930. 



2 Goldschmidt 1929, Jollos 1934, 1935, cf. Timofeeff-Ressovsky 1937. 



