70 Regulation of Size in Unicellular Organisms 



of the immediate parent was often as strong a deter- 

 miner of body size. No experiments upon selection 

 for size were carried out by Root. 



Summary. In the shelled rhizopods there is, ob- 

 viously, a somewhat complicated control of the size of 

 offspring. On the one hand each individual tends to 

 resemble its immediate parent; and on the other hand 

 all individuals on the average tend to assume a size 

 which is inherited through many vicissitudes and over 

 years of time. The first sort of factor has been termed 

 by Jollos ('13) a persisting modification (Dauermodi- 

 fikation) ; the second is obviously inheritance in every 

 possible understanding of the term. The first factor can 

 certainly be distinguished from the second; whether 

 it too is an inheritance is a matter of definition. 



Persisting modifications in size (without selection) 

 lasted only one generation in Centropyxis, but much 

 longer in DifBugia and Arcella. When large and small 

 individuals were repeatedly selected, then the progeny 

 gradually reverted to the mean for the clone, yet per- 

 sisted in Diffiugia for at least eight generations, and 

 in Arcella for a hundred or more generations. But 

 finally (in Arcella) the mean size was reattained after 

 six months (figure 30). It may easily be imagined 

 that in the production of a shell, during the fission of 

 one of these species, a large influence is furnished by 

 the old shell or by the old volume of protoplasm. This 

 picture is certainly true to some extent; it is a way 

 of accounting both for the parental resemblance and 

 for the requirement of four or five generations for re- 

 version after cutting in Arcella. But it may be too 

 simple to account for the six months' persistence of 

 selected effects in Arcella. Jollos ('24) has shown that 

 other sorts of persisting modifications are transmitted 

 through many shell-less generations in this genus, and 

 also in shell-less infusoria ('21). 



