296 HEREDITY IN THE PROTOZOA 
of raising the temperature of cultures containing 
certain flagellates. In the course of seven years 
he succeeded in producing a strain that could live 
at a temperature of 70° C., although at first the 
entire culture would be killed at a temperature 
of 61° C. 
Recently, Jollos (’13—'14) caused Paramecium 
caudatum to acquire a resistance to compounds of 
arsenic. At first the individuals were killed when 
1.1 parts of a standard solution were added to 100 
parts of water. Gradually they became accustomed 
to live in 5 parts to 100. For several months they 
retained their acquired resistance when returned to 
their original environment, but they lost it at last. 
The loss was slow in fission lines, but sometimes 
sudden after a conjugation period. Jollos calls this 
sort of an induced change, ‘‘ Dauer-modification,” 
or long-persisting modification. He believes that it 
has nothing to do with a nuclear change in the 
hereditary constitution; 7.e., it is not a mutation 
at all but is probably a change in the plasma or in 
the macronucleus. The latter is absorbed during 
conjugation, also during parthenogenesis, hence the 
sudden disappearance of the Dauer-modification at 
this time. On the other hand, certain kinds of 
these modifications may even survive a conjugation 
period. For instance, after the changes induced by 
calcium treatment or by high temperature treat- 
ment the effects may survive for a long time if only 
fission takes place; they may survive several 
parthenogenetic periods, or even a conjugation 
