432 H. S. JENNINGS AND K. S. LASHLEY 



• 



probable explanation. The resemblance would in this case be a 

 result of conjugation. 



2. The resemblance might be due to assortative mating. We 

 know that assortative mating does occur in Paramecium, so far 

 as size is concerned, individuals of nearly equal size tending to 

 mate together (Pearl '07, Jennings '11). It appears possible that 

 the similarity^ in size might be accompanied by a similarity in 

 physiological characteristics, resulting in similar rates of fission. 

 This would be especially notable if there were a tendency for mem- 

 bers of the same or related strains to mate together; and this 

 tendency does exist, as I showed in my paper of 1911. Thus it 

 appears not improbable that the correlation is partly due to assor- 

 tative mating. So far as this is the correct explanation, the 

 resemblance would exist before the mating took place; would not 

 be a consequence of conjugation. 



It seems on the whole probable, a priori, that both inheritance 

 and assortative mating play a part in bringing about the greater 

 resemblance in the progeny of the two members of pairs. 



How can we test the vahdity of these explanations? If assor- 

 tative mating plays a part in the matter, then the resemblance in 

 the two strains derived from a pair ought to exist, to a certain 

 degree at least, if the two members of the pairs are separated 

 before conjugation is consummated. Now, this operation was 

 performed, for other purposes, in Experiments 1 and 2 of the 

 preceding paper. In these cases we had the progeny both of 

 pairs and of 'split pairs.' By determining whether there is corre- 

 lation between the two individuals, a and b, of the split pairs (indivi- 

 duals that were uniting for conjugation), we can tell whether assor- 

 tative mating plays any part in the matter. The coefficients are 

 given in the second column of table 43. 



As there appears, there is in fact a positive coefficient of correla- 

 tion, in most cases, between the progeny of animals that were 

 beginning to pair, but did not complete the process. The corre- 

 lation is on the whole of similar value to that found between the 

 two members of pairs that had completed conjugation. If we 

 average the eight coefficients for Experiment 1 in each case, we 

 find that the mean for the conjugants is 0.3176; for the non-con- 



