HARMFUL EFFECTS OF CROWDING UPON GROWTH 115 



can say at present is that they are unproved. We shall find the sug- 

 gestion of an .Y-substance made in many different connections be- 

 fore we have finished this discussion. It is useful as a hypothesis but 

 is not to be confused with concrete fact. However, the recent de- 

 velopments concerning the importance of small traces of vitamins, 

 and the work upon "bios" and upon tissue-culture inhibitions, will 

 keep us from dismissing this hypothesis too hastily. 



Of the definite factors suggested, we have lack of sufficient aera- 

 tion, in addition to undernutrition, reported as operating in crowded 

 tadpoles (Yung) and among snails (Willem, Colton, Crabb). There 

 can be little doubt but that insufficient aeration is an effective factor 

 under many conditions. The suggested harmful effects of lack of ex- 

 ercise in snails (De Varigny) now appear groundless. The accumu- 

 lation of excretory products reported as an effective agent by many 

 workers appears to have undoubted and marked influence, whether 

 in echinoderm larvae (Vernon, Peebles), in Daphnia (Warren), in 

 snails (Legendre, Colton, Crabb), in planarians (Goetsch), or in fish 

 (Church, Wilier and Schnigenberg, Shaw). Evidence in favor of this 

 conclusion will accumulate as we proceed. 



The reduction of available food correlated with crowding, whether 

 caused by increase in numbers or decrease in volume, is another un- 

 doubted factor in the situation, as shown for snails by Colton and 

 Popovici-Baznasanu and for Hydra by Goetsch. With some animals, 

 such as Hydra, it may be that this is the only factor operating. With 

 rapidly moving animals, the effect of frequent contacts resulting in 

 overstimulation of some sort also contributes to the retardation of 

 growth in crowded animals, as in tadpoles (Bilski, Goetsch) and in 

 fish (Church). 



