igoS.] JENNINGS— HEREDITY IN PROTOZOA. 515 



(b) The collection includes various growth stages of each of the 

 lines represented. The youngest stages of each line are little more 

 than half the lengths of the adults; all intermediate stages may be 

 present, and the adults themselves shorten again as they approach 

 fission. A very v^ide range of variation in length may be brought 

 about by these growth stages, all within the limits of a single pure 

 line or race. Of course when many different lines are present, an 

 immense number of combinations are thus produced. 



(c) The collection includes individuals of the various races that 

 have lived under slight or considerable differences in environment, 

 particularly in the matter of nutrition. Those that have been able 

 to get more food will be much larger and will multiply more fre- 

 quently (thus giving more young) than those that get less. Even 

 slight environmental differences make decided differences in dimen- 

 sions. While the environment shows its effects most strongly on 

 comparison of different cultures, even within the same culture, and 

 when all the individuals are of one race and of approximately the 

 same age, there are marked differences due to this cause. This is 

 shown, for example, in Table XLI. (appendix) ; here variations in 

 length from 140 to 200 microns must be considered environmental 

 effects. A few drops of water form a varied microcosm to the 

 infusoria. When diverse pure lines, diverse growth stages, and 

 diverse environmental conditions are found in a culture (as is usually 

 the case), of course, the number of different sizes and forms due to 

 the varied combinations of all these factors are very great. The 

 same sizes may, of course, be produced in different ways ; two diverse 

 lines in different stages of growth or in different environments, or 

 in some combination of the two, may produce forms outwardly iden- 

 tical. The actual variety, as defined by the physiological conditions, 

 is therefore much greater than the measurements show, for the latter 

 throw together heterogeneous combinations. 



Combinations of all the three factors inducing diversity might 

 give us in a single collection individuals varying in length from 50 

 microns to 332 microns. While these are the extremes given by our 

 data, presumably the actual extremes would be still more divergent. 



(d) In different collections the observed mean lengths depend 

 upon the three different sets of factors just mentioned. The inclu- 



