THE EVOLUTION OF ANIMAL COMMUNITIES 83 



They are instrumental in the distribution of seeds 

 as in the case of the sand cherry; in pollination 

 of many of the flowers and in the destruction of 

 plants which in other environments may dominate 

 the community life. Thus the cottonwoods have 

 more insect enemies when they grow late in the 

 dune succession than when they are in the exposed 

 fore-dunes. 



As the climate changes, the climax communities 

 change; comparatively recently, just following the 

 last ice age, the climax forest in the Chicago 

 region was a conifer one such as exists today in 

 the Canadian forests. With the gradual change 

 in climate, this forest community could no longer 

 maintain itself except in favored spots, such as 

 have allowed the relic grove of white pines near 

 Oregon, Illinois, to persist; gradually this old 

 conifer forest gave way before the advancing 

 beech and maple community. 



Such slow climatic changes are as characteristic 

 of community evolution, as are the more rapid 

 changes toward the existing climax in the case of 

 successions such as those of the dunes. If as 

 persistent spirits we could hover over the Chi- 

 cago area as Hardy in The Dynasts imagines 

 the ethereal spirits watch the creeping of the 

 human armies in the Napoleonic struggle in 

 Europe, so in centuries of time we would doubtless 



