EXTERNAL FACTORS ON THE SPAWN DATE I37 



importance. If a pond is at 4°C, water running in at any other tempera- 

 ture will tend to remain at the surface until it is stirred in by the wind. 

 If, on the other hand, the water running in is at 4°C, and the pond is 

 at any other temperature, then the run-off will go to the bottom. It is 

 difficult to decide wliich of these alternatives is the more likely, for the 

 temperature of a pond is closely correlated with that of the air at a 

 slightly lower level, and follows it in time. During most of the daylight 

 hours, the pond is colder than the air; at night, the reverse occurs. 

 It will be recalled that a similar ridge exists, in a much less developed 

 form, in the M2 diagram. 



Near the top of the diagram, at about the 7°C level, another change 

 occurs. Above these temperatures, the spawn date gets later the warmer 

 the weather, and at the low rainfall area, the late isophenes appear again. 

 In the extreme comer, where the temperature is very high for the time 

 of the year and there is httle or no rain, the 90-day isophenc makes a 

 hmited reappearance, so that the latest dates are to be found at opposite 

 margins of the whole diagram. 



Mo, Temperature and Rainfall. It is obvious that the weather 

 near the end of Mo carmot affect the actions of frogs spawning at the 

 beginning, and, for that reason, the use of a mean value for this 

 month must introduce errors. It is, however, a property of the weather 

 that it runs in spells, so that the Meteorological Office can head its 

 monthly reports with a condensed summary in a short phrase, such as 

 "Warm in the west, colder and windy in the rest of the country." 

 The mean values used in this work are therefore not so bad a measure 

 as would seem at first sight. Moreover, spawn dates near the end of 

 the month can clearly be quite reasonably correlated with the mean 

 values. A computation of accumulated values, as was done in 1935, 

 would have been very laborious with the 2,734 results of this chapter, 

 and, moreover, could not have been done for many of the meteoro- 

 logical stations, because many of the details are not published. 



The Mo diagram resembles that for Mi in many respects, and 

 contains only one quite new feature. This is a well-marked ridge of 

 lateness running vertically through the diagram at about the forty- 

 miUimetre level of rain. There are so many records in tliis region, 

 1,478 of them, that there is htde doubt about the reality of the effect. 

 It must be concluded that very hght rain in the spawning month 

 results in a later date than if no rain fell at all. It should now be recalled 

 that some peculiar circumstances attended rain in the daytime at 

 10— (T.914) 



