MATERIALS OF HEREDITY 35 



plex conditions than do the X-chromosomes, because they 

 are always in pairs, while in some individuals (males), X 

 is single. This makes detection of the effects of substituting 

 one chromosome for another more difficult. But the same 

 type of experimentation illustrated above for the X-chro- 

 mosomes demonstrates that the other chromosomes also 

 influence the characteristics of organisms. These matters 

 are taken up in detail in later chapters. 



It is clear therefore that the chromosomes are indeed 

 "materials of heredity." That is, they influence the char- 

 acteristics of the individuals that contain them, for these 

 characteristics are altered when one chromosome is substi- 

 tuted for another. We turn next to a study of these ma- 

 terials. 



NOTES AND REFERENCES ON CHAPTER I 



I. Page 23. The relations between the materials of heredity and the 

 characteristics of the organism have been far more thoroughly inves- 

 tigated in the fruit-fly, Drosophila melanogaster, than in any other 

 organism, so that much of the detailed knowledge of these matters is 

 based upon this animal. Most of the fundamental work on Drosophila 

 has been done by T. H. Morgan and his associates, C. B. Bridges and 

 A. H. Sturtevant, though in recent years many investigators in all 

 parts of the world have been at work on the genetics of this organism, 

 adding greatly to our knowledge. A detailed account of the genetics 

 of Drosophila will be found in the following: 



T. H. Morgan, C. B. Bridges, and A. H. Sturtevant (1925), The 

 Genetics of Drosophila. Bibliographia Genetica, vol. 2, pp. 1-262. 



The detailed investigations of these authors are published for the 

 most part in the Publications of the Carnegie Institution of Washing- 

 ton. Among the more important are the following: 



T. H. Morgan and C. B. Bridges ( 1916), Sex-linked Inheritance in 

 Drosophila. Carn. Inst. Publ. No. 237. 87 pages ; T. H. Morgan, C. B. 

 Bridges and A. H. Sturtevant (1919), Contributions to the Genetics 

 of Drosophila melanogaster. Carn. Inst. Publ. No. 278. 388 pages; 

 C. B. Bridges and T. H. Morgan (1923), The Third Chromosome 

 Group of Mutant Characters of Drosophila melanogaster. Carn. Inst. 



