106 THE SOCIAL LIFE OF ANIMALS 



random through their floury universe. They eat the 

 flour, but they may also eat their own eggs as they 

 encounter these on their travels. This habit of egg- 

 eating tends to reduce the rate of population growth, 

 the more so the denser the population. 



The second factor is the experimentally proven 

 fact that up to a certain point copulation and suc- 

 cessive re-copulation stimulate the female Tribolium 

 beetles to lay more eggs, and eggs with a higher per- 

 centage of fertility. Thus the more dense the beetle 

 population the more rapid its rate of increase. The 

 interaction of these two opposing tendencies results 

 in an intermediate optimal population in which 

 more offspring are produced per adult animal than 

 in either more or less dense populations. 



It may be felt that I have been keeping too closely 

 to the more or less artificial conditions found in the 

 laboratory. It is true that in an attempt to bring the 

 various aspects of the population problem under ex- 

 perimental control we have avoided those field obser- 

 vations which can only be recorded as more or less 

 interesting anecdotes. We have now come to a point 

 in our inquiry, however, at which it is necessary to 

 move directly into the field. 



Given the evidence at hand, that optimal numbers 

 present in a given situation have certain positive 

 survival values and some definitely stimulating effects 



