lO ILLUSTRATIONS 



PAGE 



11. Other protozoa reproduce more rapidly when a 



certain number of bacteria are present 78 



12. Some protozoans divide more rapidly in dense 



bacterial suspensions if more than one is 

 present 79 



13. A and B. A recent suggestion concerning the 



ancestral relations within the animal king- 

 dom 86 



14. Goldfish grow more rapidly if placed in slightly 



contaminated water 95 



15. An extract from the skin of goldfish frequently 



has growth-promoting power 97 



16. White mice grow faster in small groups than in 



large ones 101 



17. Flour beetles reproduce more rapidly if more 



than one pair is present 105 



18. The "spread" of time in which eggs are laid in 



a colony of herring gulls affects the per- 

 centage that survive ii2 



19. In small populations, genes drift into fixation 



or loss largely irrespective of selection 121 



20. In medium populations complete fixation or 



loss is less likely to occur 123 



21. In large populations, gene frequency is held to 



a certain equilibrium value by the opposing 

 pressures of mutation and selection 124 



22. As intensity of selection increases it becomes 



more and more dominant in determining the 

 end result 126 



23. Manakin males establish rows of mating courts 



in the Panamanian rain-forest 134 



24. Many kinds of fishes eat more if several are 



present 136 



