Illustrations 



PLATES 



FACING PAGE 



I A. A hibernating aggregation of ladybird beetles 32 



I B. A breeding aggregation of midges 32 



II. A grassland-bison community 38 



III. Aggregating behavior of brittle starfish 44 



IV. Diagrams showing the effect of population size 



on the rate of evolution 128 



V. Castes of a termite from British Guiana 266 



FIGURES 



PAGE 



1. Grasshopper nymphs on the march 36 



2. The effect of numbers present on rate of bio- 



logical processes 52 



3. Group protection from ultra-violet radiation 



for planarian worms 60 



4. Another aspect of group protection for plana- 



rians 62 



5. The small marine flatworm Procerodes 64 



6. Group protection from fresh water for Proce- 



rodes 66 



7. Bacteria frequently do not grow if inoculated 



in small numbers 67 



8. The common sea-urchin Arbacia 70 



9. Arbacia eggs cleave more rapidly in dense 



populations 72 



10. Robertson found that two protozoans placed 



together divided faster than if isolated 76 



9 



