237 THE AMERICAN NATURALIST [VOL. XLV 



by the rare crossing of unlike individuals. Great credit 

 is due to Johannsen 2 for demonstrating that in such 

 self-fertilized plants, types also exist which are not 

 readily distinguishable by simple inspection, but whose 

 occurrence may be completely demonstrated by the 

 refined methods of the mathematician. Not only has 

 Johannsen 's work been so extensive as to justify the 

 conclusions arrived at by him, but various other investi- 

 gators, working with different classes of research ma- 

 terial, have shown that the conditions found by Johann- 

 sen in beans and barley are duplicated in many other 

 species and varieties. Perhaps the strongest support in 

 this direction has come from the work of East 3 with 

 potatoes and that of Jennings 4 with various microscopic 

 organisms, especially with paramecium. 



The fact that Drab a verna, and many other wild species 

 in which the existence of numerous elementary species 

 has been demonstrated, as well as wheat, oats, barley and 

 beans, are all predominantly self-fertilizing, and that 

 potatoes and paramecium have an asexual reproduction, 

 has led some to the erroneous notion that the discrete- 

 ness, uniformity and permanence of the types which have 

 been discovered among these and other similar organisms, 

 are in some way dependent upon the absence of crossing. 



It must be admitted that conclusions drawn from self- 

 fertilized and asexual material do not necessarily apply 

 to plants and animals whose successful existence is de- 

 pendent upon repeated crossing. Nevertheless, the con- 

 ception of pure and permanent genotypes in cross-bred 

 material has become familiar simultaneously, owing 

 to the work done in Mendelian heredity; for homozygous 



2 Johannsen, W., "Ueber Erbliehkeit in Populationen und in reinen 

 Linien," 68 pp., Jena, 1903. 



3 East, E. M., l ' The transmission of variations in the potato in asexual 

 reproduction," Conn. Exp. Sta. Eeport 1909-1910, pp. 119-160, 5 pis. 



* Jennings, H. S., "Heredity, variation and evolution in Protozoa 

 II. Heredity and variation of size and form in Paramecium, with studies 

 of growth, environmental action and selection," Proc. Amer. PhiL Soc., 

 47: 393-546, 1908. 



