Races of Paramecium. 499 



the distinctness of the two species, and it has been generally 

 acknowledged since then. This distinction was accepted by 

 Butschli in his great work on the Protozoa ('89) ;^ and by Sche- 

 wiakoff ('96) in his monographic treatment of the holotrichous 

 infusoria. Simpson in 1901 says ''There is no doubt, however, 

 that these two species do exist." In 1906 Calkins brought the 

 matter into question again upon the following grounds: From 

 a ''wild" culture brought into his laboratory in March, 1905, he 

 isolated four pairs of conjugating "caudatum" forms. Upon 

 examining the progeny of one of the exconjugants, after the third 

 division, to determine whether the conjugation had been normal, 

 he found that this exconjugant had "reorganized as an aureUa 

 form with two micro-nuclei." The later behavior of this line he 

 describes as follows: "During the month of May and until three 

 months afterthe culture was started, individuals appearedhere and 

 there with but one micro-nucleus, while the majority of these killed 

 at this time appeared with one of the micro-nuclei larger than the 

 other. By the end of June none of the P. aurelia forms were to be 

 found, and this culture, like the other cultures started at the same 

 time, contained, forms with only one micro-nucleus; Paramecium 

 aureha had become Paramecium caudatum again." (Calkins, 

 '06, p. 5.) He concludes from this that P. aurelia is only a variant 

 of P. caudatum and states further that it is relatively rare in 

 nature. 



The question as to what names we should give these organisms 

 is of course of very subordinate interest; what is of interest is 

 to know the facts as to the permanent differentiations of the va- 

 rious lines or races that exist, whether we call them species or not. 

 I have, therefore, made a careful study of the micro-nuclear 

 relations of the various races of Paramecium, to see whether 

 there exist any permanent differences in these structures. 



The diverse races or "pure lines" of Paramecia studied are 

 mainly those described in the paper of Jennings ('08; see especially 

 pp. 485-500). Each of these races consists of individuals derived 

 from the fission of a single specimen which had been isolated from 



1 It may be noted that Kent in his Manual of the Infusoria ('82) recognizes but 

 one of these species, under the name P. aurelia. 



