RETARDING INFLUENCE OF CROWDING 135 



The explanation of Harnly's results is not necessarily obvious or 

 easy. It may be that there is actually a surface-population optimum 

 which stands below the largest surface and volume of food available. 

 A possible factor may be that with greater area and volume wild 

 yeasts or molds grow too rapidly for the Drosophila to control. Be- 

 fore coming to this conclusion, it is well to note the sizes of the dif- 

 ferent containers, which were: vials, size not stated, 118, 236.5, 473, 

 and 250 ml. The population curve may be a result of the available, 

 space rather than of the food surface acting alone. Such an interpre- 

 tation would be in line with the data of Pearl and Parker shown in 

 Figure 7. More work is needed, however, before one can draw as- 

 sured conclusions. 



The tendency toward universality of the effect of crowding upon 

 the rate of reproduction is shown by the fact that Hill (1926) and 

 Sarles (1929), working with hookworms, have reported counts on 

 population density of these parasites in relation to egg production 

 which show that as the number of worms in a given host increases, 

 the egg output per worm decreases. 



Pearl and Parker conclude the account of their work upon crowd- 

 ing and the rate of reproduction in Drosophila with the following 

 statement, which Pearl repeats in a later book: 'Tn general there 

 can be no question that this whole matter of influence of density of 

 population in all senses, upon biological phenomena, deserves a great 

 deal more attention than it has had. The indications all are that it 

 is the most important and significant element in the biological, as 

 distinguished from the physical, environment of organisms." With 

 this position I am in complete accord. 



