ii8 ANIMAL AGGREGATIONS 



general situation long ago and left the impression that the suggest- 

 ed relationship was either not proved or only indirectly related 

 to the suggested space factor. The idea that there is a direct con- 

 nection between available space, and size attained in land animals, 

 still has, however, considerable vitality, as is shown by Bilski's 

 suggestion (1921), following his careful statistical analysis of the re- 

 lations between available space and growth in tadpoles, that the 

 smaller size of children reared in slums, as compared with that of the 

 children of more fortunate parents, is to be accounted for by the 

 smaller space available per child for the former and the resulting 

 greater degree of stimulation by repeated contacts, such as have been 

 shown to result in decreased growth in tadpoles, fish, and other 

 rapidly moving animals. 



There can be no doubt that crowding decreases the rate of growth 

 in many instances, and any interpretation of the facts to be present- 

 ed later concerning beneficial effects of crowding up to an optimum 

 population must take this fact into consideration. When one at- 

 tempts to summarize the evidence concerning the factors causing the 

 retarded growth in crowded conditions, he finds a decided lack of 

 unanimity among the different investigators, indicating that in all 

 probability there are many factors which may produce the same 

 result. 



It is instructive to review the retarding factors suggested to date. 

 They are of two kinds: the vague and the definite. In the former 

 category one must put the suggestion of Hogg, working with snails 

 in 1854, that they adapt themselves to the necessities of their exist- 

 ence, which Davenport, 45 years later, said still summarized the 

 state of knowledge on the subject at that time. There is also Sem- 

 per's postulated X-substance necessary for growth in snails and 

 water isopods (1874, 1881) ; the autotoxins of the bacteriologists; and 

 the growth-inhibiting substances of the tissue culturists (Heaton, 

 1926) and of Peebles (1929) for echinoderm larvae; as well as a 

 "space factor" seriously discussed by many observers (cf. Wilier and 

 Schnigenberg, 1927). As commonly used, this space factor is about 

 equivalent to Hogg's conception. 



Regarding this group of suggested retarding factors, the best we 



