AGGREGATIONS OF HIGHER ANIMALS QQ 



This may not be the whole of the story but it is prob- 

 ably a significant part of it. 



In both of these cases the additional growth of 

 aquatic animals, which occurs as a result of the pres- 

 ence of other animals of the same species, is produced 

 in response to some sort of chemical which has been 

 given off into the surrounding water. This may be 

 nothing more than the unswallowing of surplus food 

 by the conditioning fish. With animals whose tails 

 have been freshly cut off the addition of salts to the 

 water by the group may balance the osmotic tension 

 at the cut surfaces and so favor re-growth. The excit- 

 ing result of these studies lies in the suggestion that 

 some less obvious growth-promoting substances may 

 also be secreted into the surrounding water. 



Animal aggregations frequently produce physical 

 as well as chemical changes, and while we are con- 

 sidering the effect of numbers of animals present on 

 the rate of growth of individuals it is interesting to 

 examine one case in which growth-promotion appears 

 to have been produced largely by changes in tempera- 

 ture. Such an effect has been reported more than 

 once; it is most simply illustrated in a warm-blooded 

 animal, this time the white mouse. The experiment 

 was first performed in Poland, but the causal factors 

 were then only partially recognized. It has been re- 



