The ordinary species is Paramecium caudatum; superficially, it resembles P. aurelia. The latter differs from the former in smaller size, in rounded instead of attenuated posterior end, and in the possession of two instead of one micronucleus. The last is generally regarded as the most important difference between the two species. In March, 1905, a pair of conjugating Paramecium caudatum was isolated from a culture in an epidemic of conjugations. The ex-conjugates had all of the characteristics of P. aurelia. One died before many generations in culture, the other is still living and is now in the 346th generation. This one retained the characteristics of P. aurelia until about the 45th generation after conjugation, when it lapsed again into the P. caudatum form, with one micronucleus, and other characteristics of P. caudatum. The latter characters are still maintained.The observation indicates one of two things. Either this is an interesting case of mutation of species with lapse into the parent form after...