BEGINNINGS OF CO-OPERATION 51 



creasing population. Under these conditions only 

 the ill effects of overcrowding are visible, and the 

 optimum population is the lowest possible. This is 

 the modern expression of what used to be called the 

 struggle for existence. In the more poetic post-Dar- 

 winian days this struggle was thought of as so in- 

 tense and so personal that an improved fork in a 

 bristle or a sharper claw or an oilier feather might 

 turn the balance toward the favored animal. Now 

 we find the struggle for existence mainly a matter 

 of populations, measured in the long run only, and 

 then by slight shifts in the ratio of births to deaths. 



A second type of phenomena is represented by a 

 curve with a hump near the middle (97) as shown in 

 Figure 2B. 



Again, height above the base line measures the 

 speed of some essential biological process or proc- 

 esses, such as longevity; distance to the right gives 

 increasing population densities. The harmful effects 

 of overcrowding, indicated by the long slope to the 

 right, are still plainly evident, but there is also ap- 

 parent a set of ill effects associated with undercrowd- 

 ing which are shown by the downward slope to the 

 left. Many have written pointedly about overcrowd- 

 ing, and while there is still much to be learned in 

 that field, it is in the recently demonstrated exist- 

 ence of undercrowding, its mechanisms and its im- 



