ASSORTIVE MATING IN CHROMODORIS ZEBRA 269 



to come together (or 'fit') when the extreme anterior ends are 

 united." This explanation was not based upon personal ob- 

 servation, and later work has shown it to be somewhat too 

 schematic, although entirely correct in principle. 



Jennings ('lib) substantiated Pearl's discovery of assortive 

 mating in Paramecium, and was able to make certain the con- 

 clusion that the observed correlation between the lengths of 

 conjugating individuals is indeed largely due to true homogamy, 

 rather than to any process of mutual equalization of sizes during 

 the progress of conjugation. 'Jennings is also in essential agree- 

 ment with Pearl regarding the mechanical cause of assortive 

 mating. On the basis of direct observation of the mating proc- 

 ess, Jennings showed that in a general way the explanation 

 offered by Pearl, as pre\dously quoted, is quite sound. However, 

 in many cases conjugants of unequal length are able to pair 

 successfully through a series of bending movements and con- 

 tractions which cause their lengths to become more or less equal- 

 ized. This influence is, nevertheless, found inadequate to ac- 

 count for more than a small part of the correlation. It remains 

 true that two individuals conjugate most readily when they are 

 of nearly the same size. This is the automatic outcome of the 

 manner in which the conjugation-postures of Paramecium are 

 brought about, and of the fact that the total length of Para- 

 mecium (and any other dimension which may be used as an index 

 of size) is rather closely correlated with the distance from the 

 anterior end to the mouth. Upon the equality of the latter 

 character in two individuals the possibility of their successful 

 conjugation is in large measure mechanically dependent. 



It does not appear that the pairing of animals, other than Para- 

 mecium, several other infusorians (Jennings, '11 b, p. 85-88), 

 and man, has ever been examined with reference to the possible 

 occurrence of assortive mating. Yet it is generally recognized 

 that this matter of homogamy is one of great importance. In 

 addition to its implications for other phases of evolutionary 

 speculation, assortive mating (where it can be shown to occur) 

 may provide in particular an interesting example of a mechani- 

 cally determined result having the superficial characteristics of a 



