i6o ANIMAL AGGREGATIONS 



material may strongly affect the growth processes of organisms pres- 

 ent. Relatively slight conditioning by the products of plant or ani- 

 mal metabolism produces marked results, which are not necessarily 

 increased by increasing the amount of material present. One reason 

 for the failure to recognize the presence and effectiveness of such 

 compounds hes in the exceedingly minute minimal quantities neces- 

 sary to produce maximal effects. Unless especial care is taken, traces 

 of such materials will be present as contamination and will produce 

 as great an effect as if more were added. 



