VARIATION AND HEREDITY 225 



Table 9. — Relation between the number of days cultivated in peptone 

 medium and the number of days cultivated in salt-sugar medium needed to 

 change from type 1 to type 5 in Chlamydomonas debaryana (Moewus). 



acters. To the last-mentioned category belongs perhaps a strain of 

 Tetrahymena pyriformis in which, according to Furgason (1940), a 

 pure-line bacteria-free culture derived from a single individual was 

 found to be composed of individuals differing in shape and size which 

 became more marked in older cultures. 



The first comprehensive study dealing with the variation in 

 size and its inheritance in asexual reproduction of Protozoa was 

 conducted by Jennings (1909). From a "wild" lot of Paramec- 

 ium caudatum, eight races or biotypes with the relative mean 

 lengths of 206, 200, 194, 176, 142, 125, 100, and 45/x were isolated. 

 It was found that within each clone derived from a single parent, 

 the size of individuals varies greatly (which is attributable to 

 growth, amount of food, and other environmental conditions), any 

 one of which may give rise to progeny of the same mean size. Thus 

 selection within the pure race has no effect on the size, and the differ- 

 ences brought about merely by environment are not inherited. Jen- 

 nings (1916) examined the inheritance of the size and number of 

 spines, size of shell, diameter of mouth, and size and number of 

 teeth of the testacean Difflugia corona, and showed that "a popula- 

 tion consists of many hereditarily diverse stocks, and a single stock, 

 derived from a single progenitor, gradually differentiates into such 

 hereditarily diverse stocks, so that by selection marked results are 

 produced." Root (1918) with Centropyxis aculeata, Hegner (1919) 

 with Arcella dentata, and Reynolds (1924) with A. polypora, ob- 

 tained similar results. Jennings (1937) studied the inheritance of 

 teeth in Difflugia corona in normal fission and by altering through 

 operation, and found that operated mouth or teeth were restored to 



