Heredity in Protozoa 507 



ferences in size and fission-rate are well known in various ciliates. Strains 

 of Tetrahymena in pure cultures have shown minor differences in bio- 

 chemical activities (Chapter VIII), and in the extent to which they can 

 become acclimatized to salt solutions (42). Differences in pathogenicity, 

 noted among strains of parasitic species, may be paralleled by morpho- 

 logical differences. Strains of Entamoeba histolytica with relatively low 

 pathogenicity may show a small average size (16). In Plasmodium vivax, 

 relatively low and high degrees of pathogenicity may be correlated with 

 slow and rapid reproduction. That these racial characteristics are in- 

 herited is indicated by their persistence in cultures or in infected animals. 



Tendency toward genetic uniformity 



Although non-hereditary differences, induced perhaps by environ- 

 mental factors, may be expected within a race, reproduction by fission or 

 budding should insure exact duplication of genes from generation to 

 generation, barring mutations or mitotic accidents. Therefore, genetic 

 constancy of the race would be expected in the absence of sexual phe- 

 nomena. In general, this expectation has been realized. Such was the case 

 in the early work of Jennings (24) on Paramecium. Although separation 

 of wild populations into several races was usually possible, selection for 

 size within the race was no longer effective. Similar findings of Ackert 

 (1) and Jollos (33) also indicated that the race is relatively constant. 

 However, certain apparent exceptions have been reported. 



Apparently spontaneous changes 



Selection within the race, continued for many generations, has 

 produced distinct stocks of Difflugia corona differing in number and 

 length of spines, diameter and height of the shell, and diameter of the 

 mouth (27). Comparable effects have been observed in Centropyxis 

 aculeata (70) and Arcella dentata (21). These results remain unexplained. 

 Although it is possible that gene mutations were involved, undetected 

 environmental differences might have been perpetuated under the ex- 

 perimental conditions of continued selection. In the latter case, the dif- 

 ferent types probably could be considered results of acclimatization 

 rather than mutations. 



Comparable changes within the race have been reported in a few 

 ciliates. By opposite selection through more than 150 generations. Middle- 

 ton (48) established two strains of Stylonychia pustulata differing in rate 

 of fission. During selection there was a gradual increase in the average 

 difference, indicating that the effects of selection were cumulative. Since 

 these differences persisted after conjugation and also through fission for 

 several months after selection was discontinued, Middleton suggested that 

 the selection of small variations may be "an effective evolutionary pro- 

 cedure." Similar changes, involving size, division-rate, and resistance to 



