^"^^ THE NEW EVOLUTION "^f 



of the animals of fresh water excepting in the aquatic 

 young of certain Old World fire-flies. 



Luminescence certainly would not be of such general 

 occurrence in the creatures of the sea if there were not 

 some outstanding reason for it common to all groups 

 of animals. The only possible explanation is that 

 luminescence fulfils some function of importance for 

 the well being of every type of animal. In other 

 words, the type of light produced by luminescence 

 must be a physiological necessity. 



The black color so general in the deep sea fishes, 

 sometimes replaced by red, which is also common in 

 certain other deep sea creatures, probably serves pri- 

 marily to retain within their bodies the light from 

 the luminous things on which they feed. Were this 

 black color merely correlated with the darkness of the 

 regions where they live, we should expect to find it 

 also in cave living creatures. But all of these are 

 colorless or pink. 



A very striking illustration of the necessity of light 

 for certain animals is afforded by the hard shelled 

 (but not the soft shelled) pond and river turtles. All 

 of these, no matter where they live, spend a consider- 

 able portion of their time, especially in spring and 

 early summer, sunning themselves on logs or sand- 

 bars. Their skeleton is extremely heavy, the heaviest 

 in proportion to total body weight that is found 

 among the vertebrates, excepting in certain land 

 tortoises. The land tortoises all live in sunny 

 regions, and all of the larger ones in arid or semi-arid 

 regions. These creatures seem to have developed the 



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