114 ANIMAL AGGREGATIONS 



These were 4.5 cm. in diameter and were covered with coarse scrim 

 at the bottom. They were suspended by wires so that each extended 

 1.5 cm. below the surface of the water, thus giving to the contained 

 fish a volume of 24 cc. in which to move about, as contrasted with 

 the 4,000, 1,000, or 500 cc. volume per fish to which the 2, 8, or 16 

 fish were exposed in the surrounding aquaria. A single medium-sized 

 fish was transferred to each of these tubes, regardless of whether 

 those in the surrounding aquaria were large-, small-, or medium- 

 sized. 



Under these conditions the fish within the small tubes grew less 

 than did those in the aquaria. At the end of the first 10 days the 

 average length of the 34 fish in the tubes showed 1.35 per cent in- 

 crease, while the medium-sized fish in the surrounding aquaria grew 

 6.51 per cent. At the end of 20 days the difference was still more 

 striking. The 25 fish in the tubes had grown in this period on the 

 average 2.78 per cent, while those of the same original size in the 

 larger volume of water had grown 12.83 per cent. So far as known, 

 the size of the container was the only variable. The meshes of the 

 scrim cloth were open throughout the experiment; but to guard 

 against the possibiUty of lack of adequate diffusion, the tubes were 

 raised once daily to insure a complete change of water. Such results 

 are similar to those Goetsch secured for the relatively swiftly moving 

 tadpoles, and are probably due to the effect of overstimulation 

 caused by frequent contact with the walls of the small tube. 



As stated above, Bilski (192 1) points out that when limitation of 

 growth rate is caused primarily by stimulation from repeated con- 

 tacts, and when the number of individuals present is proportional to 

 the different sizes of the vessels, the rate of growth is not the same. 

 If we take two vessels of different sizes, a and h, and populate them 

 with a and h number of animals respectively, so that each animal has 

 the same amount of space available, in the simplest case the stimu- 

 lation will come from the contact, or near approach, of two animals. 

 The relation of the size of the two containers will be a:h, which in a 

 simple case might be 2:3. The stimulation possibilities from group 

 interference would he a{a—i):h{h—i). Substituting the values sug- 

 gested above, we get a stimulation possibility of 2:6. Under such 



