RETARDING INFLUENCE OF CROWDING 131 



reached, crowding has the opposite effect, for reasons not yet clear. 

 The nights are still cold, frequently colder than in December, when 

 the opposite results were obtained. Probably the differential effect 

 of crowding is associated with acclimatization to the cold ; the gen- 

 eral physiological condition of the hens must be different at the 

 height of the laying season from that at its beginning, and this shift 

 in physiological state may account for the reversed effect of crowd- 

 ing; still, perhaps the psychological factor invoked by Pearl and Sur- 

 face to cover admitted ignorance may be the only feasible suggestion 

 as yet. Third, following the approach of warm weather and the com- 

 ing of the hot summer months, the birds of the crowded pens prob- 

 ably have difficulty in maintaining comfortable temperatures, par- 

 ticularly while roosting. 



In concluding this discussion of the effect of crowding on the rate 

 of egg production in chickens, it is of interest to note that the varia- 

 bility in the rate of egg production increases with crowding when the 

 annual egg production is taken as the unit. When this is broken into 

 monthly periods, it is seen that the greatest effect of crowding is to 

 be found at the beginning and the end of the laying-year, at a time 

 of low production. From February to July, at the time of heaviest 

 laying, the environmental differences implied by flock size as used in 

 these experiments do not affect the relative variability of produc- 

 tion. Unfortunately there are no data concerning egg production of 

 chickens isolated in pens with the floor areas per individual used in 

 these experiments. 



EFFECT OF CROWDING ON RATE OF REPRODUCTION IN DROSOPHILA 



Pearl and Parker (1922) have contributed another bit of signifi- 

 cant evidence to our problem by their work upon the influence of the 

 density of population upon the rate of reproduction in Drosophila. 

 In this work mass matings were made from a given line. The off- 

 spring from this mass mating were used in making up the matings in 

 the experiments to be described. Half-pint milk bottles were used as 

 containers. The procedure was definitely standardized throughout. 

 Sets of four bottles were started, each containing i, 2, 3, .... 9, 

 mated pairs of flies. Sets of three bottles contained, respectively, 10, 



