506 JENNINGS— HEREDITY IN PROTOZOA. [ApriU4, 



single individual to show more and more variation as the strain 

 became older; it would break into several or many strains, which 

 would, however, remain intermingled. 



Therefore, the best method of procedure will be to take an old 

 strain, which, derived from a single individual, has for a long time 

 been multiplying freely without conjugation. From this the largest 

 and the smallest individuals should be separated and allowed to 

 propagate under identical conditions. If hereditary variations in 

 size have occurred, we should in this way reach the same result as 

 by actual selection and isolation through many generations. Physio- 

 logical isolation has been as complete as would be experimental 

 isolation. 



A race fulfilling these conditions we have in the pure line derived 

 from the individual D, on which most of the work described in the 

 first parts of this paper was done. On January 19, 1908, large 

 cultures of D had been multiplying without conjugation since April 

 12, 1907, a period of about nine months. During this time about 250 

 generations must have been produced ; these had remained physio- 

 logically isolated. The superfluous individuals had been removed by 

 periodic " catastrophic " destruction ; the greater part of the culture 

 was thrown out, and a remnant saved, without selection, for a new 

 culture. 



On January 19, 1908, I took from the large stock culture of D 

 (i) the ten largest individuals that I could find; (2) the ten smallest 

 individuals I could find. They were separated in two watch-glasses 

 and kept under identical conditions. The difiference between the two 

 sets was very marked ; the smaller lot were certainly not more than 

 two-thirds the length of the larger, and they were very slender, while 

 the large ones were both long and broad. It was clear that both sets 

 were adults. 



It was found that the smaller lot multiplied much less rapidly 

 than the large lot, and some of the small ones died. By January 

 30 there were but twenty of the small lot, while a very large number 

 had arisen from the large lot. On this date the culture fluid was 

 changed and but fifty of the larger lot retained. The small lot con- 

 tinued to multiply very slowly. It is clear that the small specimens 



