112 ANIMAL AGGREGATIONS 



from the same egg mass would be expected to have similar genetic 

 constitution. He concludes that food insufficiency and foul media 

 are the most common growth-inhibiting factors in snails reared in 

 otherwise favorable media. Extreme crowding markedly retards 

 growth, but the individuals rapidly reach normal size after transfer 

 to standard conditions, unless they are too old. The volume of me- 

 dium has little effect on the growth of isolated snails providing foul- 

 ness is not permitted. Aeration promoted growth through reducing 

 foulness rather than by increasing the respiration of the snails. 

 Daphnia introduced into the culture are beneficial to snail growth, 

 since they retard fouling of the medium. He found no evidence that 

 environmentally induced dwarfing is transmitted, though on this 

 the experiments were not continued through enough generations to 

 be conclusive. 



Crabb, in his work, continued the general methods of study of this 

 problem which have been used since the time of Hogg, adding re- 

 finements which make his conclusions the more trustworthy. Un- 

 fortunately, he did not take advantage of the method originated by 

 De Varigny (1894) and used extensively by Goetsch (1924), which 

 allows a separation of the factor of available space from that of 

 available volume. In this procedure Goetsch placed animals in the 

 experimental aquaria in separate tubes thrust through corks to keep 

 them afloat and covered at the lower end with gauze, which allowed 

 diffusion connection with the entire aquarium while limiting the 

 amount of available space. 



Goetsch was led to this method by the experience of Bilski (192 1), 

 who found that the relatively active tadpoles of Bufo and of Rana 

 esculenta grew less rapidly when subjected to frequent changes of 

 water than they did when metabolic wastes were allowed to accumu- 

 late. Bilski also found that an increase in numbers slowed down the 

 rate of growth more than would be expected by the change in vol- 

 ume relations involved, when the rate of growth was compared with 

 that given by an equal number of animals placed in different aquaria. 



Goetsch experimented upon sessile Hydra, upon the relatively 

 slow-moving flatworms, and upon amphibian larvae which are capa- 

 ble of rapid locomotion. As might be expected, he finds different 



