EXTERNAL FACTORS ON THE SPAWN DATE I39 



The parts of the diagram to the right of the ridge resemble those of 

 the Mi diagram. There is some sign of an early valley at about o-5°C, 

 but these temperatures rarely occur in Mo, and, if this feature stood 

 alone, without the corresponding feature in the Mi diagram, it would 

 not be statistically significant. There is again a ridge of lateness at 

 about 4°-5°C, but it is not so well marked. At high temperatures, 

 there is again evidence that high temperatures delay spawning, hi 

 spite of popular impressions to the contrary, I fnid nothing surprising 

 in this. I have experienced seasons in which exceptionally warm 

 weather occurred with no sign of the frogs, until a date that was 

 unusually late, for example in 1949, when the weather at the usual 

 spawning time resembled that of the summer, but there were no frogs 

 in my ponds. 



Light 



It will be recalled that when the map was being discussed, the latest 

 areas, except the Crossfell district, were industrial. Moreover, as will be 

 shown later, there is a tendency for the North to be later than the 

 South, although the West is earlier. Now in winter, the temperature 

 of the North is not much different from that of the South; it is in 

 summer that the temperatures are different. Geologically, the North 

 resembles the West rather than the East. It was therefore rather 

 surprising to fmd the North so much later. The algal hypothesis has 

 all along suggested that there should be an effect of light. In winter, 

 the North is darker than the South, and there is less light still in 

 industrial districts. It is unfortunately impossible to use length of day 

 as a factor in this investigation, because there is an invariable astro- 

 nomically determined relation between day length, latitude and 

 date. Fix any two of these and the third is fixed. Since we use spawn 

 date as one variable, day length and latitude are merely alternative 

 expressions for the same fact. We can choose to use one but not both. 

 However, if we use the amount of bright sunshine, expressed as a 

 percentage of that possible at the particular place, then we have a 

 factor that is not rigidly connected with geographical position and 

 date, but varies with the weather and with local peculiarities. It is 

 therefore a factor we can use as easily as if it were temperature. 



Since it was already apparent that rainfall was acting in the form o£ 

 run-off, it was possible that sunshine would act in two opposing ways. 

 It is known that sunshine increases evaporation (Penman, 1948), so 



