PHYSIOLOGICAL EFFECTS 125 



water if they are not fed and that they will starve 

 to death in practically the same time as if placed 

 in unconditioned water. 



Similarly salamander tadpoles will grow more 

 rapidly in such conditioned water than in raw, 

 unconditioned water and both fishes and tad- 

 poles will regenerate tails that have been cut off 

 more rapidly if several fishes or tadpoles are 

 present together than if they are isolated in the 

 same volume of water. Evidently in these last 

 cases the exudates from the cut tails condition 

 the water so that the wounds heal more rapidly 

 and growth takes place faster than if one animal 

 must do the whole conditioning by itself. 



As has already been stated it is easy to demon- 

 strate that crowding, especially overcrowding, 

 decreases the rate of growth of crowded animals. 

 Even in this field more recent experiments have 

 shown that the common fruit-flies, much used in 

 experiments on heredity, grow larger in small 

 culture vials when present in numbers of from 

 eight to sixteen than at other population densities 

 either smaller or larger. This is usually explained 

 by suggesting that with too few flies present, 

 harmful sorts of yeasts and bacteria which are 

 usually present in the culture media are not kept 

 under control; but with about eight to sixteen 

 flies in the cultures, the number is about right 



