162 Life and Death, Heredity and Evolution 



We must now revert for a moment to the relation of this 

 production of diverse stocks through mating to the problem 

 of rejuvenescence. Among the characters in which mat- 

 ing induces hereditary diversities is the rate of fission. 

 This has been illustrated on page 158. 



Now, as we have seen, if we take all the lines that have 

 conjugated and average them, we find that their average 

 fission rate is somewhat less than the average rate of those 

 that have not conjugated. Thus in a very extensive ex- 

 periment in which 69 lines derived from conjugants were 

 compared for a period of three weeks with 145 lines derived 

 from non-con jugants, the daily fission rate of the con- 

 jugants averaged 1.097, that of the non-con jugants 1.144 

 (Jennings, 1913, p. 349). For the period of 21 days the 

 average number of fissions in each line was for the 

 descendants of the conjugants 23.041; for the descendants 

 of the non-conjugants, 24.034. When we study the separate 

 lines we find that those descended from the non-conjugants 

 all show nearly the same number of fissions ; the slowest line 

 had 18 fissions, the fastest 28, in 21 days. But the lines 

 descended from the conjugants differ greatly among them- 

 selves ; the slowest line has for the twenty-one days only 

 9 fissions, while the most rapid one has 31. That is, 

 mating has caused much hereditary diversity in the fission 

 rate of the descendants of the conjugants, and those at one 

 extreme of the variation have a higher fission rate than 

 those descended from the non-conjugants. Out of the de- 

 scendants of 56 ex-conjugants, in this experiment, 9 pro- 

 duced families with a higher fission rate than any of the 

 families descended from the 130 non-conjugants. On the 

 other hand, 9 of the families produced from the ex- 

 conjugants had a lower fission rate than the low family (with 

 18 fissions) produced from the non-conjugants. 



