OF THE 



UNIVERSITY 



OF 



PARAMECIUM AURELIA AND PARAMECIUM CAU- 

 DATUM.* 

 GARY N. CALKINS, PH.D. 



AT the present time, when the subject of mutations and species 

 is discussed on every hand, and when every eye is keenly on the 

 alert for new evidence among animals and plants, a sudden trans- 

 formation of one known species into another known species is of 

 interest. Such an incident has recently come under my observa- 

 tion; a Paramecium caudatum became P. aurelia, and remained 

 so for about 45 generations, when it reverted to P. caudatum. 

 Apart from the facts of the change, which in itself is of obvious 

 importance from the standpoint of cellular biology, the essential 

 question to consider is whether these two species are sufficiently well 

 defined to justify their separation. If not, then the experiments and 

 the changes indicated have less bearing on the general problem of mu- 

 tation than upon the problems of cell physiology. If they are suffi- 

 ciently distinct, then we have in this incident an interesting case of mu- 

 tation. I personally believe that the slight differences that distinguish 

 the two types of Paramecium are not of specific value, and hold that 

 P. caudatum should be regarded as a mere variant of P. aurelia. 



Paramecium aurelia was the name given by Miiller, in his general 

 work on the Protozoa in 1773,. to the ciliated organism which had 

 been known as the " slipper animalcule." Several different species 

 of Paramecium were created by Ehrenberg in 1838, and described 

 in his work on the Infusionsthierchen. Most of these have been 

 sifted out into other genera, and only three have remained, P. bur- 

 saria, P. aurelia, and P. caudatum. Paramecium caudatum and 

 P. aurelia have been united into a single species by the majority 

 of observers subsequent to Ehrenberg, on the ground that the differ- 

 ences upon which Ehrenberg had based his species were inadequate. 

 The number of species was thus reduced to two, and the names 

 used were P. aurelia and P. bursaria, the former having been given 

 originally by Miiller. Maupas, however, in 1887-89, and R. Hert- 



* Received for publication March 17, 1906. 



I 



