VARIATION AND HEREDITY 227 



ability of the pure line. It is considered that some differences in 

 dimensions among strains are apparently due to environment (host), 

 but others cannot be considered as due to this cause, since they per- 

 sist when several strains showing such differences are inoculated 

 into the same host. The two strains of T. cruzi isolated from human 

 hosts and maintained for 28 and 41 months by Hauschka (1949), 

 showed well defined and constant strain-specific levels of virulence, 

 different degrees of affinity for certain host tissues, unequal suscepti- 

 bility to the quinoline-derivative Bayer 7602, and a difference in re- 

 sponse to environmental temperature. The five strains of Tricho- 

 monas gallinae studied by Stabler (1948) were found to possess a 

 marked variation in virulence to its hosts. 



According to Kidder and his associates, the six strains (H, E, T, 

 T-P, W, GHH) of Tetrahymena pyriformis and the two strains (V, 

 PP) of T. vorax differ in biochemical reactions. They found the ap- 

 pearance of a biochemical variation between a parent strain (T) and 

 a daughter strain (T-P) during a few years of separation and a 

 greater difference in the reactions between the two species than that 

 between the strains of each species. These strains show further dif- 

 ferences in antigenic relationships. Five strains of pyriformis con- 

 tain qualitatively identical antigens, but differ quantitatively with 

 respect to amount, concentration or distribution of antigenic ma- 

 terials. The sixth strain (T) contains all the antigens of the other five 

 strains and additional antigens. The two strains of vorax are said to 

 be nearly identical antigenically. The antigenic differences between 

 the two species were marked, since there is no cross-reaction within 

 the standard testing time. In these cases, thus, some aspects of the 

 physiological difference among different strains are understood. 



Jollos (1921) subjected Paramecium caudatum to various environ- 

 mental influences such as temperature and chemicals, and found that 

 the animals develop tolerance which is inherited through many gen- 

 erations even after removal to the original environment. For exam- 

 ple, one of the clones which tolerated only 1.1% of standard solution 

 of arsenic acid, was cultivated in gradually increasing concentrations 

 for four months, at the end of which the tolerance for this chemical 

 was raised to 5%. After being removed to water without arsenic 

 acid, the tolerance changed as follows: 22 days, 5%; 46 days, 4.5%; 

 151 days, 4%; 166 days, 3%; 183 days, 2.5%; 198 days, 1.25% and 

 255 days, 1%. As the organisms reproduced about once a day, the 

 acquired increased tolerance to arsenic was inherited for about 250 

 generations. 



There are also known inherited changes in form and structure 



