178 ANIMAL AGGREGATIONS 



negative results at will, depending on her manipulation of volume 

 relations. Her work and discussion make it impossible for one to 

 lump together all volumes in testing for the possible stimulating 

 effect of the presence of more than i cell in a limited amount of 

 medium. 



Here we shall have to let the discussion of Robertson's phenome- 

 non rest for the time being, pending the accumulation of further 

 data. So far, the evidence demonstrates that reproductive rate does 

 not always depend on the number of the original transplants; that, 

 in fact, under many conditions and with many organisms it has not 

 yet been shown to have any positive correlation. All are agreed that 

 in the end the rate of reproduction falls sooner in small subcultures 

 seeded with more than one organism and more rapidly than it does 

 when a single individual is isolated. However, the repeated experi- 

 ence of Robertson, supported now by Yocum's work in so far as it 

 covers the same ground, and by the work of Petersen with larger 

 volumes of culture medium, demonstrates that true acceleration of 

 the rate of reproduction may occur associated with the introduction 

 of more than one organism into a limited amount of medium. 



When the very establishment of the phenomenon is a matter of 

 such great difficulty, it is to be expected that the explanation of the 

 phenomenon, when it does occur, is still uncertain. At present we 

 cannot accept Robertson's hypothesis of the action of an autocata- 

 lyst; and while there is evidence for the production of some condi- 

 tioning agent, some A''-substance, which renders the culture more 

 favorable to growth, more light is needed, particularly concerning 

 the role of bacteria in the phenomenon among other possible factors, 

 before much progress can be made toward a solution of this aspect 

 of the problem. Perhaps when we have examined the relations of 

 masses of larger animals to survival under adverse conditions, a sub- 

 ject to be taken up soon, we can better understand the complexities 

 of the present problem. 



An effect strikingly similar to Robertson's phenomenon has been 

 described by Chapman (1928), though not with that in mind. In his 

 work upon the effect of a limited environment, in this case whole 

 wheat flour, upon the number of confused flour beetles, Triholium 



