364 GENETICS 



the great number of injurious mutations there may occur 

 a few that are advantageous. It is these that would form the 

 basis for evolutionary progress. 



On the other hand it may turn out that gene mutations of 

 the sort thus far observed are not the material of progres- 

 sive evolution. It may be that we have not yet recognized 

 the actual steps in progressive evolution, and that when 

 these come clearly into view, it will be found that they are 

 not the result of action of destructive agents, nor connected 

 with injuries to the genetic system, but rather bear a re- 

 semblance to the changes of growth. These questions must 

 be reserved for the future. 



NOTES AND REFERENCES ON CHAPTER XrV 



1. Page 340. Many of the references given at the end of Chapter 

 XIII deal with mutations as well as with other genetic variations. 

 See particularly those cited in Notes i and 4 of Chapter XIII. 



2. Page 341. On the frequency of mutations, see H. J. Muller 

 (1928), The Measurement of Gene Mutation Rate in Drosophila, 

 Its High Variability and Its Dependence on Temperature. Genetics, 

 vol. 13, pp. 279-357. 



3. Page 342. On the characteristics resulting from mutation in 

 Drosophila, see Note i to Chapter I. 



4. Page 346. On somatic mutations in Drosophila, see J. T. Pat- 

 terson (1929), The Production of Mutations in Somatic Cells of 

 Drosophila melanogaster by Means of X-rays. Journal of Experimental 

 Zoology, vol. 53, pp. 327-372. 



5. Page 347. H. J. Muller (1927), Artificial Transmutation of the 

 Gene. Science, vol. 66, pp. 84-87. See also the papers by Muller listed 

 in Note 4, Chapter XIII. 



6. Page 348. H. J. Muller and L. M. Mott-Smith (1930), Evi- 

 dence that Natural Radioactivity is Inadequate to Explain the Fre- 

 quency of "Natural" Mutations. Proc. of the National Academy of 

 Sciences, vol. 16, pp. 277-285. 



7. Page 348. R. Goldschmidt (1929), Experimentelle Mutation 

 und das Problem der sogenannten Parallelinduktion. V ersuche an 

 Drosophila. Biologisches Zentralblatt, Bd. 49, pp. 437-448. — V. Jollos 

 (1930), Studien zum Evolutionsproblem. I. Vber die experimentelle 



