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THE AMERICAN NATURALIST [Vol. LI 



unit of genetic segregation ; it is of unknown nature, but probably 

 consists of a genetically indivisible portion of a chromosome (a 

 locus) in a particular state. 



The presenee-and-absence scheme of factor notation properly 

 employs only the first of these meanings; the Morgan-Castle 

 scheme, on the other hand, may use either. 



Howard B. Frost 



EivEESiDE, Calif. 



BIBLIOGRAPHY 



Castle, William E. 1913. Simplification of Mendelian formulae. Am. Nat., 

 47: 170-182. 



East. Edward M. 1912. The Mendelian Notation as a Description of 



Physiological Facts. Am. Nat., 46: 633-655. 

 Emerson, Eollins A. 1913. Simplified Mendelian Formulae. Am. Nat., 



47: 307-311. 



Gates, E. Euggles. 1915. The Mutation Factor in Evolution, with Par- 

 ticular Eeference to CEnothera. 14 + 353 p. London, MaemiUan. 

 Goodspeed, Thomas II., and Clausen, E. E. 1917. Mendelian Factor Dif- 



Morgan, Thomas H. 1913. Factors and Unit Characters in Mendelian He- 

 redity. Am. Nat., 47: 5-16. 

 1915. The Eole of the Environment in the Eealization of a Sex-linked 



Morgan, Thomas H., Sturtevant, A^H., Muller, H. J., and Bridges, Calvin B. 

 ]!)15. The .Mechanism of Mendelian Heredity. 13 + 262 p. New 

 York, Henry Holt & Co. 



THE SELECTION PROBLEM 

 I'xLEss history fails to repeat itself, geneticists, whose atten- 

 tion is focused upon variation, should sooner or later overem- 

 phasize its importance as a factor in shaping the organic complex. 

 There is, indeed, reason to believe that already a tendency for 

 some among them to do so is becoming apparent. Dr. Pearl's^ 

 recent paper under the title above affords an example in point. 



In that communication its author fails to discriminate sharply 

 between two distinct phases of his subject. Whether selection 

 may affect the course of evolution is a matter entirely apart from 

 the possibility that it alters the germ plasm. Racial history 

 may possibly be modified, if the genetic composition of a mixed 

 population may be affected by selection based upon somatic dif- 

 1 Pearl, Raymond, 1917, "The Selection Problem," The American Nat- 



