104 



genes." We may expect evidence of 

 this community in the variations 

 which arise from time to time within 

 the species, whether they be at the 

 time of specific value or not. Such 

 community is not to be inferred from 

 mere similarity in appearance but must 

 rest on a more real homology of 

 germinal cause. This kind of similarity 

 is now apparent between Miis rmiscu- 

 lus and Rattus ?iorvegicJis, which have 

 varied so far from a common type that 

 thev are now inter-sterile and have 

 been placed recently in different gen- 

 era. Yet they have retained a genetic 

 constitution so similar that it contains 

 genes common to both species. 

 Whether this is due to a community 



MULLER 



of descent in the terms of current 

 evolutionary theory or to relationship 

 through some other cause is one of 

 the questions which genetics, aided by 

 the chromosome notation, may be ex- 

 pected at some time to answer. 



BIBLIOGRAPHY 



Castle, W. E. 1920 Genetics and Eugenics. 

 Harvard University Press, Cambridge, 

 Mass. 



Morgan, T. H., Sturtevant, A. H., Muller, 

 H. J., and Bridges, C. B. 1915 The Mecha- 

 nisjn of Mendelian Heredity. Henry Holt 

 and Co., New York. 



Morgan, T. H. 1919 The Physical Basis of 

 Heredity. J. B. Lippincott Co., Philadel- 

 phia and London. 



Wright, S. 1917 "Color Inheritance in Mam- 

 mals." Journal of Heredity, vol. 8, nos. 5-9. 



Variation Due to Change in the Individual Gene 



H. J. MULLER 



Reprinted by author's and publisher's permission 

 from American Nattiralist, vol. 56, 1922, pp. 32- 

 50. 



It is not the practice of scie?itists to expend much effort in making 

 their co?itributio?is readable. If a paper successfidly comvnmicates 

 the experimental data a?id the conclusio7is drawn to the reader, the 

 author is satisfied, and editors find such work quite acceptable. Once 

 in a great while, however, science is blessed with an author both 

 eminently successful at his trade and articulate, so that his contribu- 

 tiofis are a pleasure to read. Such a man is H. J. Muller. Students in 

 Tny general biology course found this article the easiest of the entire 

 collection to read and digest. In Tny opinion, it is the paper that 

 taught them the most. 



We have noted two differ eyit types of papers already; first, the 

 research paper, and secotid, the paper e?nbodying syjithesis of several 

 areas of study. Muller' s co?itributio7i is a third type. It is a thoughtful 

 ajialysis of the progress that had been made in the understanding of 



