CHAPTEE 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 tem- 

 perature on size of sea-urchin larvae, of Lepidoptera, and of Mol- 

 lusca Seasonal dimorphism in certain Lepidoptera in its rela- 

 tion to temperature Temperature 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 pigmenta- 

 tion 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 pupse of certain Lepidoptera. 



HAVING discussed blastogenic or germinal variations 

 as fully as our very limited knowledge of the subject 

 permits, we will now proceed to enquire into the other 

 great class of variations, namely, those produced by the 

 action of the environmental conditions on the soma or 

 body tissues. These were termed by Weismann somato- 

 genic variations, but Lloyd Morgan, Mark Baldwin, and 

 others, with a view to distinguishing them more point- 

 edly from the former class of variations, have referred 

 to them as somatic modifications, or more simply as 

 modifications. The term " variations ' they reserve 

 exclusively for blastogenic variations. Though doubt- 

 less there is much to be said for this system of classify- 

 ing variations, yet there is also something against it. 

 It is frequently difficult or impossible to decide whether 



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