134 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



meteorological data, Mo, the spawn month, would be sometimes 

 December or January, often February or March, and sometimes April 

 or even May. Clearly the weather of March cannot influence the 

 frogs that spawn in February. Numbering the months gets over this 

 difficulty, for Mi is always the month before that in which spawning 

 takes place, M2 always the month before this. 



In Fig. 35 there is an example. The temperature and rainfall of M2 

 are arranged along the axes, and the mean dates of spawning corre- 

 sponding to any particular combination of temperature and rainfall 

 are plotted as contour lines. The diagram is therefore a representation 

 of the surface of a soHd figure, just as a contour map represents the 

 variations in the altitude in relation to the latitude and longitude. 

 What would be hills on a map are regions of lateness on the diagrams, 

 and the valleys are areas of earliness in spawning. The area has bounda- 

 ries, just as a country has, where, because there are no records, it is 

 impossible to plot the contours. With this explanation, which apphes 

 to all, mutatis mutandis, we can pass to a detailed consideration of the 

 effects that the diagrams have revealed. 



M2, Temperature and Rainfall, In the diagram just mentioned, 

 it can be seen that up to a temperature of about 6°C this factor is not 

 important. The surface slopes slightly, and, as one goes towards the 

 top of the diagram, one passes almost parallel to the contours. The 

 flat area at about i°-2°C will become more easily understood when the 

 Mi diagram is described. Above 6°C, the surface becomes steeper, and 

 it is clear that, at these relatively high temperatures for winter months, 

 two months before spawning, warm weather does accelerate spawning. 

 6°C (42°F) is the temperature from which the official meteorological 

 calculations of accumulated temperatures are reckoned, although 

 43 °F has been suggested as more suitable. These temperatures were 

 chosen because they are approximately those at which plant growth 

 begins to be appreciable. Rainfall has very httle influence, for, over 

 most of the diagram, the contours are parallel to the rainfall axis. 

 There is little sign of any joint effect, that is an effect that depends not 

 on a mere sum of both rainfall and temperature effects, but on an 

 increased effect when both act together. The distinctive sign of a 

 joint effect is that the contours are curved, indicating a warped surface. 

 The diagram is thus quite simple. It shows a temperature acceleration, 

 setting in suddenly at about 6°-7°C, very httle rainfall effect, and not 

 much joint effect. 



