338 GENETICS 



four pairs of chromosomes, one pair straight, two pairs 

 V-shaped, one pair small and nearly spherical. Type B, in 

 another species, is like type A except that the small chromo- 

 somes have united with some of the larger ones, leaving but 

 3 pairs. Type C, found in a third species, is like type A 

 except that the chromosomes of one of the V-shaped pairs 

 have broken at the point of the V into two straight chromo- 

 somes; so there are in this type 5 pairs. In type F, both the 

 V pairs of type A have broken into two straight chromo- 

 somes, giving six pairs instead of four. The various other 

 types result from similar rather simple changes. There are 

 many other cases of related species having chromosome 

 groups that thus differ by slight alterations. 



NOTES AND REFERENCES ON CHAPTER XIII 



1. Page 316. Genetic variations resulting from changes and irregu- 

 larities in the genetic system have been very fully described and figured 

 in C. C. Hurst's The Mechanism of Creative Evolution (1932. 365 

 pages). See also T. H. Morgan (1928)^ The Theory of the Gene. 343 

 pages; and (1932), The Scientific Basis of Evolution. 286 pages. 



2. Page 319. The work on Datura is by A. F. Blakeslee and his 

 associates. See A. F. Blakeslee and J. Belling (1928), Chromosomal 

 Alutations in the Jimson Weed, Datura stramonium. Journal of 

 Heredity, vol. 15, pp. 195—206; and the references there given. For 

 details as to Oenothera, see R. R. Gates (1928), The Cytology of 

 Oenothera. Bibliographia Genetica, vol. 4, pp. 401-492. 



3. Page 325. See the lists of the chromosome numbers of different 

 species of animals and plants in Wilson's The Cell in Development 

 and Heredity, pp. 855-865. 



4. Page 326. Much of the knowledge of the effects of radiation 

 in organisms, and particularly in Drosophila, is due to the v^^ork of 

 H. J. Muller and his associates. The account in the text is based 

 largely on the following: 



H. J. Muller and A. Dippel (1928), Chromosome Breakage by 

 X-rays and the Production of Eggs from Genetically Male Tissue in 

 Drosophila. British Journ. of Experimental Biology, vol. 3, pp. 85- 

 122; H. J. Muller (1930), Types of Visible Variations Induced by 

 X-rays in Drosophila. Journ. of Genetics, vol. 22, pp. 299-334; 



