CONTENTS. vii 



CHAPTER VI. 

 CERTAIN LAWS OF VARIATION. 



PAGE 



Effect of environment on growth diminishes rapidly from 

 time of impregnation onwards Reaction of an organism to 

 environment dependent on nature of organism Rapidly 

 diminishing rate of growth in man and in the guinea-pig 

 with progress of development Variability also diminishes 

 with growth Effect on growth once produced, probably 

 never eradicated Increased variability of sparrow and of 

 periwinkle in America Relation between variability and 

 want of adaptation to environment Variability of migra 

 tory and non-migratory birds Does domestication increase 

 variability? 193 



CHAPTER VII. 

 THE EFFECT OF TEMPERATURE AND OF LIGHT. 



Variations and modifications Effect of temperature on growth 

 of frog Optimum temperature of growth in plants 

 Effect of temperature on size of sea-urchin larvae, of 

 Lepidoptera, and of Mollusca Seasonal dimorphism in 

 certain Lepidoptera in its relation to temperature Temper- 

 ature differences giving rise also to local races, sports, 

 and phylogenetic forms Critical period of reaction to 

 temperature Effect of Arctic climate on coat of mammals 

 Effect of darkness and of light on growth of plants Effect 

 of sunlight and of diffused light How far does pigmen- 

 tation of animals depend on exposure to light? Cave 

 animals Illumination of under surface of flounder Effect 

 of light and of darkness on Molluscs Variable protective 

 resemblance in the frog, in fish, and in larvae and pupae of 

 certain Lepidoptera, 223 



CHAPTER VIII. 

 THE EFFECT OF MOISTURE AND OF SALINITY. 



Effect of humidity of soil on plant growth Effect of dry 

 and moist surroundings on characters of plants Desert 

 plants and Aquatic plants Effect of moisture on Lepi- 



