NIBIBERS : STATISTICS 53 



has been brought together by Allee (1931), who has 

 made two generahzatioiis of great importance on the 

 subject. Observations and experiments on a number 

 of different animals have shown that they possess 

 reactions which lead them to group together into 

 aggregations, although the latter do not have any 

 obvious significance or adaptive value socially, e.g. 

 for defence or attack or the rearing of the young, 

 and are not brought about simply by the mechanical 

 influence of some common attractive factor such as 

 food. Allee has carried out extensive studies, espe- 

 cially upon aggregations among freshwater isopods 

 {Asellus). At the same time a great deal of other 

 experimental work has been done on such widely 

 different organisms as insects, isopods, echinoderms, 

 protozoa, bacteria, yeasts, and also on spermatozoa. 

 The results prove that slight crowding has marked 

 effects in increasing the growth rates, survival rates 

 or reproductive rates. Ecologists have become 

 familiar with the effects of overcrowding in causing 

 increased mortahty from starvation or disease. 

 These experiments on animal aggregations have 

 proved beyond any doubt that undercrowding has 

 also definite effects which are disadvantageous physio- 

 logically. For example, Pearl, Mner and Parker 

 (1927) proved that the length of life of fruit-flies 

 {Drosophila) was not greatest at the lowest densities 

 but at densities higher than this. But still higher 

 densities again caused lower average lengths of 

 Hfe. 



These experimental studies by various workers 

 agree in showing that there is usually an optimum 

 population density for greatest physiological effi- 

 cienc}". There is still much discussion as to the 

 reasons for this phenomenon in different instances. 

 Among aquatic organisms discussion centres round 

 the existence of various growth-promoting substances 

 that may be produced by the organisms themselves, 

 while other explanations have to be sought for terres- 



