Inheritance of Size 57 



under duplicated environmental circumstances, inheri- 

 ted factors for size express themselves in a highly uni- 

 form way. It should be kept in mind, however, that 

 more than one generation of Paramecium may be re- 

 quired for the conservative "nuclear" characters for 

 size to materialize in "somatic" expression. For in- 

 stance, if Paramecia are transferred from 10° to 25°, 

 the average adult body volume is reduced by more than 

 half (figure 36). But obviously more than one binary 

 fission is required to accomplish this reduction of size. 

 The expression of the hereditary character is condi- 

 tioned by the immediate past history of the organism; 

 such conditioning has been demonstrated repeatedly 

 in experiments on inheritance, and Jollos ('21) found 

 that such bodily conditions may prevail through hun- 

 dreds of generations of Paramecium. But the fact that 

 such factors or "persisting modifications" are ulti- 

 mately lost, so that the organisms return to their origi- 

 nal bodily characteristics, shows that inheritance has 

 not been indelibly affected. 



In Paramecium no attempts have been made to 

 measure sizes in successive generations. But in Col- 

 poda (Adolph, '29) it has been found that there is a 

 high correlation between the size of the parent and 

 the sizes of its offspring, when they become adults 

 (figure 25). The correlation is +0.56; the individual 

 resembles its immediate parent more than it resem- 

 bles the average size of the clone. Selection would ob- 

 viously be effective as long as it were continued; but 

 actually in only three or four generations after selec- 

 tion had ceased the selected line reverted to the aver- 

 age. 



Inheritance in uniparental generations is certainly 

 much simpler to trace than in biparental, yet one large 

 complication is attached to micro-organisms in the 

 fact that a very appreciable quantity of a daughter in- 



