I908.J JENNINGS— HEREDITY IN PROTOZOA. 511 



line; the variation of the parents from the type of the pure line has 

 no effect. The difference in the figures above is either purely statis- 

 tical in character or means a faint variation in the culture fluid. 



(c) Summary on Selection zvithin Pure Lines. 



Thus, we come uniformly to the result in all our experiments, 

 that selection has no effect within a pure line ; the size is determined 

 by the Hne to which the animals belong, and individual variations 

 among the parents have no effect on the progeny. 



But for our results with different lines, it might be maintained 

 that the reason why we get no constant differences between the 

 progeny of different individuals of the same line is because the 

 effects of environment are so much greater than the eft'ects of selec- 

 tion that the latter are covered up and obscured. But as soon as 

 we are dealing with lines that are really different (though by but a 

 small amount) we have no such difficulty ; the dift'erent lines retain 

 their relative sizes in spite of environmental action. This is clearly 

 shown in Tables XXIII. and XXV., pages 488 and 494. 



The significance of these results will be dealt with in the next 

 section. 



VL SUMMARY AND DISCUSSION. 

 I. Resume of the Investigations. 



The present paper is an experimental study of the factors involved 

 in variation and inheritance of size in the infusorian Paramecium^ 

 in the period when reproduction is taking place by fission, without 

 conjugation. 



I. The first question proposed is whether the differences in size 

 among different individuals of a culture are inherited. The pre- 

 liminary study showed that in a typical culture there were two 

 permanently dift'erentiated groups of large and small individuals, 

 respectively, corresponding to what had been described as the two 

 species, Paramecium caudatum and Paramecium aurelia. But when 

 a culture was produced from a single individual of either of these 

 groups, forming thus a " pure line," it was found that though the 

 different individuals of the single pure line differed much in size. 



