HEREDITY AND EVOLUTION IN 



PROTOZOA 



By Prof. R. W. Hegner, Ph.D., 



Department of Medical Zoology 



School of Hygiene and Public Health, Johns Hopkins University, Baltimore, 



Md., U.S.A. 



Ever since the general acceptance of the theory of evolution, 

 biologists have been attempting to discover the method by 

 which evolution has taken place. Certain experimental studies 

 of the Protozoa that have been carried on in the laboratories 

 of the Johns Hopkins University in Baltimore, Maryland, 

 U.S.A., throw some light on this subject, and some of the 

 results and conclusions of these investigations will be discussed 

 in the following paragraphs. 



Many investigators have shown that selection is ineffective 

 as a means of changing the genotypic constitution of plants 

 and animals when practised on self-fertilised plants, on plants 

 reproducing by tubers or cuttings, and on animals reproducing 

 by parthenogenesis, budding, and fission. Among the animals 

 in which this has been found to be true is the ciliate protozoon 

 Paramcecium (Jennings, 1908; Ackert, 1916). 



The data presented below, however, all indicate that by 

 selection it is possible to isolate heritably diverse strains 

 within a family of protozoons derived by fission from a single 

 specimen without the intervention of conjugation. The in- 

 vestigations described are — (1) heritable diversities in fission 

 rate in Stylonychia pustulata, by Middleton (191 5) ; (2) heritable 

 diversities in shell size, spine number and spine length, and 

 number of teeth in Difflugia corona, by Jennings (191 6) ; 

 (3) heritable diversities in spine number, shell size and form, 

 and mouth size and form in Centropyxis aculeata, by Root 

 (191 8) ; and (4) heritable diversities in shell size, spine number, 

 shape of mouth, and size and number of nuclei in four species 

 of Arcella, by Hegner (191 8, 1919a, 1919b, 1920). 



Stylonychia pustulata, which was used by Middleton in his 

 experiments, is a ciliate infusorian that divides, at a tem- 

 perature of 70 ° F., about twice per day. It was found that 



608 



