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TEE AMERICAN NATURALIST [Vol. XLIII 



tremes, then "regression is complete," since the progeny 

 are of the mean size for the race as a whole. 



Work with "pure lines"— where no intercrossing of 

 races or individuals occurs — is possible with but few or- 

 ganisms, and little of it has been done. In the few in- 

 vestigations carried on in this way, the same conditions 

 have been found that we have set forth above for Para- 

 mecium. They were first shown by Johannsen 6 to hold 

 for beans and barley, and later by Elise Hanel for Hydra. 7 

 The fact that there exist diverse races, tending to breed 

 true, has of course been shown for many species, but in 

 most cases it is difficult to maintain pure lines, and thus 

 to absolutely demonstrate the relations above set forth, 

 as has been done for beans, barley, Hydra and Para- 

 mecium. In the Protozoa it has of course been possible 

 to carry the work through an immensely greater number 

 of generations than in many celled organisms. The ex- 

 tremely important work of Barber 8 has shown the same 

 condition of affairs to exist in yeast and bacteria, though 

 with certain additional factors to be mentioned later. 



When we deal with organisms that continually inter- 

 cross, the conditions of course become immensely com- 

 plex. Each individual represents, as it were, many 

 diverse lines, whose appearance and disappearance in the 

 hide-and-seek way characteristic of Mendelian inheri- 

 tance makes interpretation extremely difficult. There is 

 not wanting evidence that the same principles are at work 

 even here; that the results of the study of "pure lines" 

 give the clue by the aid of which the more complex re- 

 sults of selection in biparental inheritance may be un- 

 raveled. If this is the case, then selection would act even 

 here also by isolating diversities that already exist, not 

 by producing diversities. There are of course some not 



"Johannsen, W., < < Erblichkeit in Populationen und in reinen Linien," 

 68 pp., Jena, 1903. 



T Hanel, Elise, "Vererbung bei ungeschlechtlicher Fortpflanzung von 

 Hydra grisea," Jenaische Zeitschr., 43, 1907, pp. 321-372. 



8 Barber, M. A., ' ' On Heredity in Certain Micro-organisms, ' ' The Kansas 

 University Science Bulletin, 4, 1907, 1-48, pi. 1-4. 



