164 On the Genetics of the Ciliate Protozoa 



B. The Sexual Period. 



90. Turning our attention now to the sexual phase of ciliate life 

 we must consider the causes and effects' of conjugation. We may begin 

 with certain isreliminaries to the process — considering first " assortative 

 mating." This has been proved biometrically by Pearl (1907) to occur 

 in Paramecium caudatum. He found that conjugants are smaller^ less 

 variable, and more alike than non-conjugants (as regards size and form). 

 The likene.ss between a pair of conjugants is "not due to any local 

 environmental factor" but to " homogamy " — individuals tending to 

 pair with their likes, not at random. These observations have been 

 confirmed by Jennings (1911 a) in both P. caudatum and P. aurelia. 

 Watters (1912) reports similar conditions in Blepharisma. 



91. That assortative mating occurs in ciliates generally has not 

 been proved, nor has it been tlemonstratetl that conjugants are always 

 smaller than non-conjugants^ or that they are usually of the same size. 

 Marked differences in the sizes of a conjugating pair have often been 

 noted — for example, by Mulsow (1913) in Stentor. He noticed that 

 conjugants are smaller than non-conjugants, but found that only about 

 half the pairs'* were composed of similar sized individuals. In other 

 cases one individual was smaller than its partner — the difference being 

 sometimes very considerable. Doflein (1907) says that " in Paramecium 

 putrinum almost half the pairs are composed of distinctly different 

 individuals." Indeed, he was so impressed with the dissimilarities 

 observable between two conjugating individuals in many .sj)ecies that 

 he enunciated a " working hypothesis " to account for them. The 

 frequent suggestion that the differences between the members of a 

 conjugating pair are sexual in nature — the larger being female, the 

 smaller male — is manifestly due to a misunderstanding of the nature 

 of a conjugant (§ 14). For my own part, I do not consider that 



have been able to show that " the rhythms in the division rate of Paramecium are the 

 phj-siologieal expression of profound nnclear changes," which "involve the formation of a 

 complete new nuclear apparatus."..." This nuclear reorganization is evidently a normal 

 substitute for typical conjugation."] 



' I use these elusive terms in their colloquial sense, without prejudice to any conception 

 of causality. 



- Previously observed by Maupas, Gruber and others. 



^ Maupas (1889) enumenites 10 species in which the conjugants are smaller than the 

 non-conjugants, and 7 in which they are of the same size. 



^ About 3000 pairs were studied— but not biometrically. 



