PHYSIOLOGICAL EFFECTS 127 



that can readily be seen with the unaided eye; 

 it reproduces asexually. If similar individuals 

 are isolated or are placed in groups of two or more 

 in a small drop of culture medium, the isolated 

 individuals will divide more rapidly from the 

 start of the experiment than will the two or more 

 Paramecia crowded together in the same small 

 drop. With larger amounts of medium, say 

 approximately one or two cubic centimeters, the 

 early division rate of each animal is greater if 

 more than a single individual is present. 



These animals change the medium in some 

 manner not yet understood so that single individ- 

 uals isolated into the conditioned medium will 

 divide faster in the early stages of the culture 

 than will wholly similar individuals isolated in 

 wholly similar media which have not previously 

 contained other individuals, provided, of course, 

 that the volume used is sufficiently large. 



Similarly with the flour beetle Tribolium (See 

 Chapt. VI) which flourishes in ordinary flour, 

 independent experiments in three laboratories 

 have shown that two pairs of beetles introduced 

 into 32 grams of flour reproduce more rapidly in 

 the first 11 and 25 days of the life of the new 

 population, than does a single pan* on the one 

 hand or a greater number of pairs on the other. 

 In some experiments 16 pairs and even 32 pairs 



