128 



ANIMAL AGGREGATIONS 



three seasons as is the difference between the 50- and the loo-bird 

 pens, where numbers only were varied. The experiments on the 

 whole indicate, as Pearl and Surface conclude, that the mean an- 

 nual egg production is much influenced by the differences in environ- 

 mental factors present in the experiment. 



so 



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Fig. 5. — A graphic summary of the relation between size of flock and mean annual 

 egg production in the domestic fowl. (From Pearl and Surface, 1909.) 



In an attempt to get at the underlying factors they suggest that 

 there is another element involved besides the physical density of the 

 population, which they are inclined to place on the psychological 

 level, and which works even when the amount of floor space per in- 

 dividual is equal. The conditioning of the surrounding medium is of 

 a different type from that of crowded aquatic animals, where the 



