BIPARENTAL INHERITANCE IN PARAMECIUM 449 



excess of five pairs over the most probable number. At the end of 

 four days the excess- was four pairs, so that in the succeeding six- 

 teen days the excess has increased by but 1 pair; a result that 

 might readily be produced by chance. 



Thus in the split pairs it is clear that there is an excess in the 

 number of pairs included among those that die immediately after 

 separation, without fission; and tliis is what might be expected 

 from the violence sometimes necessary in separating them. But 

 in the remainder of the experiment there is little evidence of a 

 tendency for the two members of the pairs to have a common fate. 

 There is a very slight excess in the number of cases where both 

 members of the split' pairs died. If this is not due to chance, it 

 may be the result of the assortative mating which we know to 

 occur. 



But when we compare the spht pairs with the pairs that had 

 completed conjugation, as in Miss Cull's experiment, and in our 

 experiment 16, we find a very great difference in this respect. 

 In the conjugants, the tendency for the fate of the members of 

 pairs to be alike becomes greater and greater as time passes, until 

 finally we get such extreme results as are found in the third period 

 of ]\Iiss CulFs experiment (page 413), or in the later periods of 

 our experiment 16 (table 44, page 437). 'If in experiment 16 

 we examine for the conjugants the deaths in the first four days 

 of the experiment, we find but five, including no pairs whate^'er. 

 It is clear, therefore, that deaths due to coiximon injury of the two 

 members played no part in the case of the conjugants, yet in the 

 experiment as a whole the tendency for the two members to have 

 the same fate is much greater than in t-he split pairs, where such 

 injury certainly plays a large part. 



Summary. The difference between the split pairs and the pairs 

 that have completed conjugation is then in this respect very 

 great, showing that in the conjugants something has occurred 

 to make the two members of the pairs more alike than they were 

 before conjugating. It is clear therefore that hy conjugatioii the 

 'progeny of the two members of pairs are rnade alike in vitality, so 

 that they tend to have a similar fate, both surviving or both dying 

 out. 



