136 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



that the heavy statistical labour of this investigation was all drudgery 

 — from time to time there was the excitement of discovery.) o°C is a 

 minimum date. At both higher and lower temperatures, spawning in 

 the next month is later. When it is reahzed that we are dealing with 

 an aquatic animal, and an event that takes place in the water, it is 

 impossible to regard the existence of this minimum at o°C as a co- 

 incidence; it must be connected with the freezing point of water. 

 Since it is not shown when there has been little rain or snow, it must 

 surely be due to the conditions of alternate freeze and thaw that such 

 a mean temperature must imply, for, of course, a mean temperature 

 of o°C impUes that the temperature has been rising and falling through 

 the freezing point, either because of the diurnal range of temperature 

 or because the temperature in different parts of the month varied about 

 this mean. In either case, flooding is to be expected. There is other 

 evidence that flooding accelerates spawning, so that it is probable that 

 this minimum is due to the flooding from melted snow. This, of 

 course, as in the case of other forms of "run-off," brings plant 

 nutrients into the ponds. Barnes and Barnes (1932) suggested that 

 water rich in trihydrol from melting ice was more favourable to 

 the hfe of algae than dihydrol water from condensed steam. 



Passing now still further up the diagram, we find an area between 

 3°C and 4°C and o and 80 mm of rain where the changes of spawn 

 date induced by these factors are very shght. Since this area comprises 

 some very common conditions, there are parts of the country where 

 the influence of rain and temperature are only observable in exceptional 

 seasons: for examples aheady discussed, in the London area in 1926 

 and 1929. 



At high rainfalls, still keeping within the temperature range 2°C 

 to 4°C, we fmd another remarkable feature. A strongly marked ridge 

 of lateness crosses the diagram horizontally. Having already found one 

 of the fixed points of water, the freezing point, on this diagram, it is 

 perhaps possible that here is another : the point of maximum density 

 at 4°C. This temperature is of great importance in the study of lakes, 

 for there it determines whether the upper layers of water will lie for 

 a whole season without mixing with the lower layers. Twice a year, 

 when the temperature reaches this figure, there is so httle resistance 

 to mixing that the whole contents of the lake overturns. This cannot 

 happen in a small pond, for a certain depth is necessary for the pheno- 

 menon to occur, but it remains true that this temperature has a unique 



