EXTERNAL FACTORS ON THE SPAWN DATE 



141 



this region of the diagram (Fig. 38), and nowhere else, sunshine acceler- 

 ates spawning, In the other parts of the diagram, where there would 

 be some rim-off, sunshine delays spawning. These effects, I must 

 point out, occur in this month, when the frogs are liibcrnating, mosdy 

 in the dark. 



Mi, Sunshine and Temperature. Much the same occurs when 

 sunsliinc and temperature are used as joint factors (Fig, 39). At low 



ISOPHENES IN YEAR-DAYS 



Fig. 39. Joint Functional Regression Diagram, Showing the 



Relation between Sunshine and Temperature and Spawn Date, 



for the Month Mi 



Below about 14 per cent of sunshine, spawning is earlier when there is 



more sunshine, but above these low amounts, sunshine always retards 



spawning. Note the late dates when it is hot and sunny, in the top right 



hand part of the diagram. 



temperatures, the evaporating property of sunshine is less prominent 

 than at high temperatures. At low temperatures, sunshine accelerates 

 spawning. At all other temperatures, it retards. Taken in conjunction 

 with the other evidence, this seems convincing support for the 



