BIPARENTAL INHERITANCE IN PARAMECIUM 421 



On the contrary, the fact that the number of pairs that sur- 

 vive (or of those that die) is as a rule greater than would be 

 expected as a result of a random distribution of deaths indicates 

 that the two members of pairs are inore alike than individuals 

 taken at random; that there is thus in the pairing something that 

 tends to make the two members ha^^e a similar fate. 



This second conclusion could not be drawn with any great 

 degree of confidence from my own experiments, summarized in 

 the preceding section. In most of these the number of pairs 

 was indeed greater than would be expected, but the excess was 

 usually not so great but that it would occur once out of seven or 

 eight cases purely as a matter of chance. It is Miss Cull's experi- 

 ment that thus far gives the only incontrovertible evidence for 

 something causing a like fate in the members of pairs. 



Miss Cull's experiment is of course much better adapted for 

 testing this particular matter than are my own, or those of Cal- 

 kins. In my experiments the animals were cultivated on slides, 

 for determining the rate of reproduction, so that it was possible 

 to keep only a small number of representatives of each line. If 

 these few died, the line was extinct. In Miss Cull's experiment, 

 however, all the progeny of a given line were retained, in small 

 mass cultures. They were thus practically assured against acci- 

 dental extinction; if any died out, it indicated some intrinsic 

 weakness. In Miss Cull's experiment, as we have seen, the num- 

 ber of complete pairs that died, and the number that survived, 

 was enormously greater than would be expected ; so that the result 

 cannot be attributed to chance. Particularly in the latter part 

 of the experiment does this appaer, so that the pairs tend to be- 

 come grouped into two classes, in one of which both members 

 die out, while in the other both members live. The chances 

 are 21,000 to 1 against the result actually reached, if there is not 

 something in the pairing tending to give the two members a similar 

 fate; in other words the chances are 21,000 to 1 that the latter is 

 the case. 



In Part II of this paper further evidence will be given on this 

 point. 



