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THE AMERICAN NATURALIST [Vol. XL 



recent number of Science^ calls attention to the danger of accepting 

 this hypothesis without more conclusive proof, and I cannot do 

 better than to close this communication with his remarks. He 

 says: "While the mutation theory may be a good hypothesis to 

 consider in respect to these peculiarly unstable groups of birds, 

 it must be noted that the method of their origin and the results, 

 as now known, are very unlike the methods and results of muta- 

 tion in plants, as made known by de Vries. The facts and condi- 

 tions are not to any great extent parallel. Instead of the resultant 

 'mutants' remaining constant and breeding true, as in the case of 

 primroses, they are in this case unstable and are believed to inter- 

 breed freely with each other and the parent stock." 



I am indebted to the following gentlemen for assistance in the 

 preparation of this paper: Dr. Henry A. Pilsbry, Academy of 

 Natural Sciences, Philadelphia, Pa., for the loan of a type speci- 

 men of Lymnaa shurtleffi; Mr. L. E. Daniels, La Porte, Indiana, 

 for specimens of L. palustris from Minnesota; Dr. W. A. Nason, 

 Algonquin, Illinois, for specimens of L. palustris from Michigan; 

 and Mr. Frank M. Woodruff, Chicago Academy of Sciences, for 

 making the excellent photographs which illustrate this paper. 

 Chicago Academy of Sciences 



■"The Probable Origin of Certain Birds." Science, n. s., vol. 22, p. 431, 

 1905. 



