140 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



that it must diminish run~ofF. If so, it should delay spawning. But if 

 there is not much rain in any period, there may be no run-off to 

 diminish, for this ceases at low rainfalls. It was therefore possible that 

 when the rain was low, a direct action of sunshine would be revealed 



ISOPHENES IN YEAR-DAYS 



100 200 



RAINFALL IN MILLIMETRES 



Fig. 38. Joint Functional Regression Diagram, Showing the 



Relation between Sunshine and Rainfall, and Spawn Date, 



for the Month Mi 



The most important part is between about 80 mm of rainfall and 18 per cent 



of sunshine, where alone sunshine accelerates spawning. Under all other 



conditions, sunshine retards spawning. Note particularly the very late 



dates when it has been both sunny and dry. 



as an accelerating influence. Which of these effects would predominate 

 could not be forecast, and the fmal graphs provided another of the 

 dramatic episodes of the investigation, for both effects appeared at the 

 correct places on the same diagrams. 



Mi, Sunshine and Rainfall. At low rainfalls, so low that there 

 would be Httle or no run-off, the effect of this factor disappears. In 



