8o THE SOCIAL LIFE OF ANIMALS 



Now he was ready for the grand Robertson test, 

 except that by this time nearly all the factors were 

 controlled. The results are shown in the following 

 figure (Figure 12). With X concentration it made 

 no difference whether he started his small cultures 

 with one or with two sterile animals. With 2X con- 

 centration, the cultures started with two individuals 

 did as well as in X concentration, but those which 

 were started with only one individual lagged defi- 

 nitely, producing only 80 per cent as many animals 

 in twenty-four hours. With 4X concentration even 

 the culture started with two Oxytricha was slowed 

 down, but not so much as that started with only 

 one. He had shown that in the presence of an ex- 

 cess number of bacteria, cultures seeded with more 

 than one bacterium-eating protozoan thrive better 

 than if but one is introduced. Not content with this 

 Johnson took another species and tried it all over 

 again with the same results. 



From all this careful work we judge that the facts 

 on this particular aspect of the effects of numbers 

 present on the rate of asexual reproduction seem 

 now to be straight; but what about their expla- 

 nation? This, as it turns out, also interests us. 

 Robertson advanced the following hypothesis to 

 explain the results which he had observed. Dur- 

 ing division each nucleus retains as much as pos- 



