148 



ANIMAL AGGREGATIONS 



In only one of the three cases was there a clearly significant dif- 

 ference; yet Popovici-Baznosanu interpreted these results as mean- 

 ing that in the stagnant, snail-conditioned water, the higher plants 

 present, as well as the walls of the jar, are covered by growths of 

 microflora, which he regards as forming the chief food of the snails; 

 and that the snails therefore grew more rapidly in cultures contain- 

 ing a rich microflora than in those with only a scanty supply. Colton 

 had interpreted his results similarly. 



The observations of Eigenbrodt (1925) that Drosophila grow larg- 

 er in small culture vials when present in numbers of from 8 to 16 than 



TABLE V* 



* Data from Popovici-Baznosanu. 



at other population densities may be explained on the assumption 

 that too few Drosophila larvae per culture fail to control the growth 

 of harmful elements of the yeast or bacterial flora as well as optimal 

 numbers do, while overcrowding overcontrols the growth of the food 

 plant. This would result in a growth optimum occurring, as sug- 

 gested, at a relatively low population density but distinctly above 

 the minimum populations studied. These results should be compared 

 with the relation between numbers present and Drosophila survival 

 given in chapter xiv. 



Bilski (1926) tested the effect of crowding upon the rate of re- 

 generation of the tails of Rana esculenta tadpoles. Five of these tad- 

 poles were kept isolated, and five similar ones were placed together 

 in the same sort of dish and with the same amount of water which 

 was given to each of the singles. Although in most cases there was a 

 decrease in length of body from tip of head to the root of the tail, 

 there was growth both of the tail stump and of regenerated material. 



