STIMULATING EFFECTS OF CROWDING 167 



been reviewed at some length in the preceding chapter. Similar evi- 

 dence from the field of general bacteriology will be presented in 

 chapter xv under the heading proposed by Churchman (1920), 

 "Communal Activity of Bacteria." 



On the other hand, Cutler and Crump (1923), in cultures of Col- 

 pidium, failed to find the allelocatalytic effect when the volume of 

 the medium was reduced; and Greenleaf (1924), using Paramecia 

 and PleiirotricJia, in a brief note records his failure to confirm Robert- 

 son's work. Peskett (1924, 1924a) observed such stimulation to divi- 

 sion in but 3 cultures of yeast out of 128 examined. 



Robertson (1924a, 1924^) re-examined the problem in the light of 

 results published up to that date, and explained the lack of success 

 of other workers as being principally due to their failure to wash the 

 organisms before transfer. His reasoning here (1927) is as follows: 

 Presumably, if contiguous animals can affect each other's growth 

 without the occurrence of conjugation, they must do so through the 

 agency of some soluble substance which they emit and which is 

 transferred from one organism to another through the medium which 

 they inhabit. Presumably also, this soluble substance must be very 

 abundant in the thickly inhabited cultures from which subcultures 

 are usually prepared. This leads, of necessity, if Robertson's reason- 

 ing is sound, to the removal of the parent culture medium from the 

 animals by washing before transfer if allelocatalysis is to result. 

 Robertson reports (1924a) that allelocatalysis increases with pro- 

 gressive removal of preformed catalyst until a maximal effect is 

 reached just before its total removal. In a preceding paper (1924) he 

 presented evidence that washing not only removed adherent auto- 

 catalyst but also washed out the accumulation of this hypothetical 

 substance from the living cells themselves. 



Robertson also emphasizes (19246) an earlier statement that in 

 comparing the reproductive rate care must be taken to estimate the 

 population some time before it has attained maximal density. The 

 end result of introducing a second individual is to reduce the rate of 

 reproduction, since the final maximum is the same in all cases and is 

 independent of the size of the seeding. 



Peskett (1925, 1925a) returns to the problem of possible alleloca- 



