540 JOHNSON. [Vol. VIII. 



the 20th, but no further fission took place. The Stentors fed 

 upon the algae, but obtained very little else. On Feb. 3 the 

 culture was examined, and the Stentors were found to be much 

 increased in size and finely developed. In the interval of two 

 weeks during which there had been no increase in number, a 

 slow growth had taken place. 



B. Relative Vitality of Large and Small Stentors. 



It is a fact familiar to all who have observed Infusoria that 

 there are many species individuals of which vary much in size. 

 These differences are due to various causes, such as rapid 

 multiplication without intermediate growth-periods, multiple 

 fission within the cyst, bud-formation, scarcity of food, etc. 

 Hardly any forms show such an astonishing variation in size as 

 the Stentors, and as we have seen (p. 537), this is especially 

 true of ^. ccsruleus. 



It seemed to me probable that the dwarf Stentors were not 

 necessarily enfeebled and dying members of the colony, and I 

 surmised that under favorable conditions (which they certainly 

 do not have in an aquarium inhabited by multitudes of their 

 larger kindred, which overreach them and obtain most of the 

 food), they would increase in size and prove as prolific as the 

 larger ones. In order to test the truth of my supposition, I 

 arranged the following simple experiment. On the 21st of 

 March, 1891, I started eight colonies in small beakers, each 

 containing 20 cc. of water, taken from a gathering made less 

 than a week previously. Great care was taken not to introduce 

 any Stentors with the water. Into each of four beakers ten 

 of the largest Stentors obtainable were put, and into each of 

 the remaining four, ten of the smallest. No Stentors in 

 process of fission were introduced. The beakers were placed 

 upon a table out of direct sunlight, and those containing 

 Stentors of different sizes were set abreast, two and two. 

 The relative positions of the pairs were changed at intervals, 

 those towards the light being placed away from it, and vice 

 versa. Thus nearly all the conditions — light, temperature, 

 size and shape of dish, quantity, quality, and depth of water — 

 were made as uniform as possible. But one most important 



