100 THE FITNESS OF THE ENVIRONMENT 



that in the morning the surface temperature 

 rises about 0.6° per hour. This, however, 

 appears to account for but a small fraction 

 of the solar heat which the lake had taken 

 up ; the rest must have been expended in 

 evaporation. Another element of great im- 

 portance is the transparency of water. As a 

 result the rays of the sun are not absorbed 



