VARIATION AND HEREDITY 165 



ciliary rows, with three contractile vacuoles, and mode of move- 

 ment, but during conjugation showed the diploid nimiber of chro- 

 mosomes as in the normal form. The tailed form remained true 

 and underwent 20 conjugations during ten months. 



The first comprehensive study dealing with the variation in 

 size with respect to inheritance in the uniparental reproduction 

 of Protozoa was done by Jennings (1909). From a "wild" lot of 

 Paramecium, Jennings isolated eight races with the relative mean 

 lengths of 206, 200, 194, 176, 142, 125, 100, and 45// which were 

 inherited in each race. It was found further that within each 

 clone derived from a single parent the size of different component 

 individuals varies extremely, which is attributable to growth, 

 amoimt of food and other environmental conditions, any one of 

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

 thus showed that selection within the pure race has no effect on 

 the size and that differences brought about merely by environ- 

 ment are not inherited. 



Jennings (1916) also studied the inheritance of size and number 

 of spines, dimensions of tests, diameter of mouth and size and 

 number of teeth of the testacean Difflugia corona, and found that 

 "a population consists of many hereditarily diverse stocks, and a 

 single stock, derived from a single progenitor, gradually differen- 

 tiates 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 (1923) 

 with A. polypora, obtained similar results. Jennings (1937) carried 

 on his study on the inheritance of teeth in Difflugia corona further 

 in normal reproduction and by altering the mouth and teeth of 

 the parent by operation, and observed that operated normal 

 mouth or teeth were restored in three to four generations and that 

 three factors appeared to determine the character and number of 

 teeth: namely, the size of the mouth, the number and arrange- 

 ment of the teeth in the parent, and "something in the constitu- 

 tion of the clone (its genotype) which tends toward the produc- 

 tion of a mouth of a certain size, with teeth of a certain form, 

 arrangement and number." 



In the case of biparental inheritance, two nuclei of two different 

 individuals participate to produce new combinations which 

 would naturally bring about a greater variation among the off- 

 spring. For example, if two individuals from a single clone of a 



