144 Life and Death, Heredity and Evolution 



vitality and reproductive power. The single line retained by 

 Calkins was clearly one of the rare extreme variants, with a 

 high fission rate. 



In Calkins' recent work on Uroleptus, 2 it is set forth that 

 in this organism the effect of conjugation is uniformly to in- 

 crease the rate of fission. The fission rate, however, is given 

 in terms of averages of sets of five lines ; such averages were 

 always higher for the sets that had recently conjugated than 

 for those that had not. But it is not possible to discover 

 from them whether the fission rate was higher in all the lines 

 of conjugants or whether as in my own experiments the rate 

 was lower in some of the conjugant lines, higher in the oth- 

 ers. Obviously it was higher in the majority at least of the 

 conjugated lines. 



The fact that some of the lines derived from ex-con jugants 

 may exceed in their fission rate the non-conjugant stocks, 

 whether this is rare, as in Paramecium and apparently in 

 most other infusoria, or common, as in Uroleptus, is unques- 

 tionably a fact of great significance ; this renders it possible 

 to still maintain, as Calkins does, that mating at least some- 

 times produces rejuvenescence, in the sense of increased 

 vigor of reproduction. We shall discuss this matter farther 

 in dealing with the question of the production of inherited 

 variation by conjugation. But if we base our conclusions on 

 the usual or the average effect of conjugation, we are forced 

 to conclude that in most cases it does not increase the rate 

 of multiplication, but rather it decreases this rate. 



It is not generally realized that the work of Maupas 

 and of Richard Hertwig, on whose authority the theories of 

 rejuvenescence by conjugation largely rest, was squarely 

 opposed to the idea that this rejuvenescence manifests itself 

 by an increase of reproductive vigor. It is worth while to 

 2 Calkins, G. N., Proc. Nat. Acad., April, 1919. 



