STIMULATING EFFECTS OF CROWDING 165 



this method to the results listed in Table VIII, we find that there 

 are 128 chances in 10,000 of random sampling giving as large dif- 

 ferences in the ratios per individual animals as those given in the 

 first eight comparisons listed, which is well within the range of 

 statistical significance. It will be noted that these include all the 

 experiments cited by Robertson (1921a) that were run in normal hay 

 infusion with the parent culture 24 hours old. If one extends the 

 inquiry with Student's method to include all the cases listed in 

 Table VIII, one finds that the results are almost statistically per- 

 fect; that under no conditions would random sampling be expected 

 to give the results shown in that table. 



But the fact that the results are statistically significant does not 

 necessarily mean that they support a given hypothesis. It merely 

 means that in the case of Robertson's experiments here cited, 

 Enchelys divided significantly faster when 2 individuals were placed 

 together in a drop of culture medium, as compared with i individual 

 of similar history similarly treated. 



Robertson thought at first (1922) that old Enchelys culture fluid 

 contained no substances that were toxic to the Infusoria isolated 

 into it from young cultures; but as evidence accumulated, he 

 changed his opinion to conclude (1924) that "the ultimate cessation 

 of reproduction in old cultures is attributable also to the accumula- 

 tion of a product of growth, and possibly of the same product that 

 was originally responsible for the acceleration." The presence of an 

 accelerator Robertson finds indicated by: {a) the accelerative effect 

 due to the addition to the culture medium of a small proportion of 

 culture fluid which was previously inhabited by the infusorian En- 

 chelys; (b) the increase of reproductive rate when the volume of 

 fluid is reduced so that less of the accelerator passes into the fluid and 

 more remains within the organism ; and (c) the fall of the reproduc- 

 tive rate when one of two recently divided individuals is isolated into 

 fresh culture medium, in comparison with the reproductive rate of 

 the other left in the original medium. 



Originally Robertson did not believe this phenomenon to be due 

 to a heavier growth of bacteria, although the results are said to de- 

 pend on the presence of an optimum amount of bacteria in the cul- 



