PARAMAECIUM AURELIA AND PARAMAECIUM CAUDATUM 237 
SUMMARY 
Briefly stated, I am convinced from my study of paramaecia 
that— 
1. A very great majority of individuals of aurelia and caudatum 
can be distinguished on the basis of shape alone; 
2. Avery great majority of individuals of aurelia and cau- 
datum can be distinguished on the basis of size alone; 
3. The power of reproduction, or general vitality, of aurelia 
and caudatum is practically identical; 
4. The macronucleus of aurelia and caudatum is subject to 
such great variation that it affords no diagnostic feature; 
5. The micronuclear apparatus of aurelia and caudatum affords 
crucial diagnostic characters. 
I have summarized the various characters of the two forms as 
they have shown themselves in my long pedigree cultures, and it 
is evident that they have conformed practically identically to 
the Maupasian types—such variations as have appeared not being 
so great as have been observed to occur in undisputed species, 
or as one would expect to find when the intimate relation of the 
unicellular organism to the environment is considered. Therefore, 
I believe, that since one of the crucial tests of a species is 
its ability to breed true to its type indefinitely, aurelia and cau- 
datum have adequately met this test during more generations 
than any other animal under observation, and accordingly 
Paramaecium aurelia O.F.M.and Paramaecium caudatum Ehrbg. 
should be regarded as distinct species.*: * 
22TIn this paper I have followed the spelling of the name of the genus as given 
by its founder, except in direct quotations from other authors. 
337 have the satisfaction to note that my conclusions are in accord with the 
final results published by Jennings and Hargitt in the last number of this 
journal, which was received when this paper was in press. MHargitt says, 
“There is cytological warrant for distinguishing caudatum races from aurelia 
races, and it seems probable that it will continue to be convenient to distinguish- 
these as two species.’’ 
