PARAMAECIUM AURELIA AND PARAMAECIUM CAUDATUM 231 
example, even when the aurelia and caudatum cultures are sub- 
jected to unfavorable environmental conditions, as, for example, 
scarcity of food, the very great majority of individuals retain 
the shape which is characteristic of the race. 
Size. As has frequently been pointed out, size alone is an 
entirely inadequate character on which to base species. It is 
significant, however, I believe, that during the long life of my 
pure strains, I have never observed the relative size of the indi- 
viduals of the aurelia and caudatum forms, when bred under 
identical conditions, to change greatly during any single period. 
Experiments have shown that even when the two forms have 
been bred under diverse conditions, for example, aurelia in a 
medium rich in food and caudatum in a medium with a very 
small amount of bacterial growth, the size of the caudatum form 
always has remained sufficiently great to render it distinguish- 
able from the aurelia form. On the basis of size alone, then, it 
has been possible, with great accuracy, to separate the two forms 
when mingled together. It is probable, of course, that I began 
my pedigree cultures with very typical specimens of the aurelia?® 
and caudatum groups as described by Jennings. If such be the 
case, then my cultures add considerable evidence in favor of 
the different strains which Jennings has isolated. It appears 
to me, however, that what that author has done for Paramaecium, 
can probably be done for many closely related species of infusoria, 
and the very fact that he did find it difficult to secure an inter- 
mediate race between the aurelia and the caudatum groups is a 
strong point in favor of the. distinctness of the forms. 
Vitality. It has been customary to regard the rate of repro- 
duction of infusoria in culture as a just criterion of vitality. 
Maupas wrote: ‘‘Cette faculté de reproduction (aurelia) resem- 
ble beaucoup 4 celle de la précédente espéce (caudatum).”” My 
cultures completely corroborate this statement, for during the 
six and one half months of the life of the caudatum culture, 324 
generations have been attained, while during the same period, 
20For further details of the culture see: L. L. Woodruff, Two thousand genera- 
tions of Paramaecium; Archiv fiir Protistenkunde, 21, 3, 1911. 
21Sur la multiplication des Infusoires cilies, loc. cit., p. 234. 
