xgoS] JENNINGS— HEREDITY IN PROTOZOA. 513 



ferent stages of growth are brought together in Table X., page 428. 



4. Environmental conditions were found to play a very large 

 part in determining dimensions, variations and correlation in Para- 

 mecium. Conditions of nutrition were found to be particularly 

 effective. By changes in nutrition the mean length of a given culture 

 could be changed in a week from 146 microns to 191 microns; the 

 breadth from 31 to 54 microns; in twenty- four hours the coefficient 

 of variability for length was thus changed from 7.003 to 12.767, for 

 breadth from 12.473 to 28.879; the coefficient of correlation from 

 .3906 to .8463. Changes of the most varied sort could be produced 

 and reversed with the greatest ease in short periods ; many examples 

 of this are summarized in Table XVIIL, page 460. Within a given 

 culture at a given time many of the differences between individuals 

 are due to slight environmental differences in different regions. 

 The breadth is more sensitive to environmental changes than the 

 length ; to such an extent is this true that it is difficult to use the 

 breadth dimensions for accurate study of any other factors. A sum- 

 mary on the effects of the environment on dimensions, proportions, 

 variation and correlation is found on pages 476 to 484. 



5. After the study of growth and environmental action, an inves- 

 tigation was made of the internal factors in dimensions and variation ; 

 of the inheritance of size. Are all the observed differences between 

 the individuals of a culture mere matters of growth and environ- 

 ment? Or may we find different races or lines that retain their 

 relative sizes even in the same stage of growth and in the same 

 environment ? 



A thorough experimental study showed that a given " wild " 

 culture usually contains many different lines or races, which maintain 

 their relative sizes throughout all sorts of changing conditions. 

 Eight of these differing pure lines were isolated and propagated; 

 these varied in mean length from a little less than 100 to a little 

 more than 200 microns (see Tables XXIII. and XXV.). Other 

 lines could unquestionably be distinguished by sufficiently accurate 

 experimentation. 



These different lines fall usually into two main groups, one group 

 having a mean length greater than 170 microns, the other having a 

 mean length below 140 microns. These two groups correspond to 



