igos.] JENNINGS— HEREDITY IN PROTOZOA. 507 



are weak, sickly ones, and the physiological difference persists at least 

 for some generations (a matter for further study). 



On February 5 about half of each lot was killed and measured. 

 This gave 57 specimens from the larger lot, 19 from the smaller. 

 The mean dimensions were, for the larger lot, 169.754 X 46.877 

 microns; for the smaller lot, 169.895 X 43-579 microns. 



Thus the two were practically identical ; one could not expect a 

 closer approximation in two identical lots kept separate for seventeen 

 days. The slight difference in breadth is only what we might expect 

 when we consider the extreme sensitiveness of that dimension to 

 faint environmental differences. The most striking differences that 

 we can find as a result of physiological isolation for 250 generations 

 have equalized themselves in a short time, when we got both sets to 

 multiplying freely under the same conditions. 



It seems hardly worth while to continue this series, since the two 

 sets have now become equalized. However, they were continued 

 for some time, and samples of 100 each were measured on February 

 15 and February 27. In these two measurements we find certain 

 differences between the two sets, but these are in opposite directions 

 in the two cases. The means are as follows : 



February 15. February 27. 



Large D 180.240X46.880 175.360X47-100 



Small D 173.240X49-760 193-680X52-320 



Evidently slight environmental differences between the two cultures 

 had crept in. It is clear that the two sets show no constant differ- 

 ences, such, for example, as we find between the two lines, g and i, 

 in Table XXIII., page 488. 



Another set of experiments dealt with the two differentiated 

 lines, g and i. The line g consists of individuals that are constantly 

 larger than those of the line i, when the two are under the same 

 conditions (see Table XXIII. , p. 488). The experiments consisted 

 in an attempt to separate these races still farther by propagating 

 continually from the largest specimens of g and from the smallest 

 specimens of /. Thus, if selection is effective, g must become larger, 

 i smaller. The IcngtJi was the dimension mainly attended to in these 

 selections. 



On November 23, 1907, the mean size for g was 129.333 X 34-933 



