THE MUTATION THEORY 341 



condition, we do not know very much about the causes of chromosomal 

 mutations. 



The causes of gene mutations in Drosophila have remained, until 

 recently, a complete mystery. Muller had been able very slightly to 

 increase the rate of mutation by raising the temperature of fly cultures, 

 but he was not completely satisfied with these results. In 1926, how- 

 ever, he startled the scientific world by the announcement that he had 

 been able, by exposing adult animals to fairly heavy doses of X-rays, 

 vastly to accelerate the rate of mutations. Where some gene muta- 

 tion occurred under normal conditions in only about 1 in 300 gametes, 

 the X-rays raised this ratio to 1 in 2 gametes, an increase of 150 per 

 cent. All of the old familiar mutations that had been occurring in 

 untreated cultures were induced in much larger numbers by X-rays, but 

 only a very few new mutations were produced. It seems that the X- 

 ray acts merely as a catalyst, hastening a process that would go on 

 slowly without such an agent. Numerous experimenters have fol- 

 lowed Muller's lead in the use of X-rays as an agent for accelerating 

 the rate of mutations. Muller has proposed a general theory that the 

 ordinary, so-called "spontaneous," mutation of genes may be due to the 

 presence of minute amounts of radio-active substances accumulated 

 in the tissues of plants and animals. This theory seems somewhat 

 far-fetched at the present writing. We are still in almost complete 

 ignorance as to the causes of gene mutations and probably shall con- 

 tinue to be in ignorance until we gain some more accurate information 

 as to the exact physical and chemical nature of genes and how they 

 act in producing character differences. 



MUTATION AND EVOLUTION 

 T. H. MORGAN 1 



What bearing has the appearance of these new types of Drosophila 

 on the theory of evolution may be asked. The objection has been 

 raised in fact that in the breeding work with Drosophila we are dealing 

 with artificial and unnatural conditions. It has been more than im- 

 plied that the resuijts obtained from the breeding pen, the seed pan, the 

 flower pot and the milk bottle [used as breeding-container for Droso- 

 phila] do not apply to evolution in the "open," nature "at large" or 

 to "wild" types. To be consistent, this same objection should be 

 extended to the use of the spectroscope in the study of the evolution 



■ From .4 Critique of the Theory of Evolution. Princeton University Press, 1 916. 



