DISTRIBUTION AND ABUNDANCE 75 



it, and I now know of no exception to the rule that frogs do not spawn 

 in any pond every year. Ponds range from those in wliich spawn is 

 almost always present to those in wliich it has never been seen, with 

 all gradations in between. 



It must not be thought that the choice of ponds by frogs is random. 

 Fig. 23 is a map of the area, in wliich the popularity of ponds is shown 

 against a backgroimd consisting of a geological map. From this, it 

 can be seen that there is a conspicuous association of popularity with 

 geological formation, most of the popular ponds being on any for- 

 mation except London Clay. The favoured formation is the Drift, 

 such as Glacial Gravel and Pebble Gravel, but Reading Beds, Flood 

 Plain Gravel and AUuvium are also used. The London Clay has many 

 ponds and in the summer frogs live on it, and in the winter may 

 hibernate there. When the time comes for them to lay their eggs, they 

 leave it for the other formations. Chapter lo. Appendix i d, provides 

 statistical evidence for the significance of the association. 



This dislike of the London Clay is not absolute. The ponds there 

 do sometimes have spawn, and in fact Dagger Lane is a London Clay 

 pond, but it is not a frog-pond every year. 



The proportion of ponds used by frogs varies from year to year. In 

 1938, 22 ponds out of 86 observed contained spawn, but in 1937 only 

 II out of 78. In 1958, I toured the area, visiting not only the familiar 

 ponds, but many others, and found only two with spawn, one on 

 Pebble Gravel and the other on Glacial Gravel. 



The investigation included many analyses of the waters for the seven 

 principal ions: calcium, magnesium, sodium, potassium, carbonate, 

 sulphate and chloride. As might be expected there was a strong 

 association between the geological formation and the composition of 

 the water, but it also became apparent that ponds very near to one 

 another might sometimes have water very different in composition. 

 It is unusual for a London Clay pond to have water much different 

 from the usual type, but the more pervious formations yielded many 

 unexpected results: with them, it is quite difficult to guess from the 

 position of the pond what the water will contain. Statistical analysis 

 of the association of spawn with water-composition showed that the 

 only ions having significant associations were potassium, phosphate, 

 and to a lesser extent, carbonate. Frogs undoubtedly prefer ponds 

 richer in potassium than the average for the district. This association 

 was independent of the geology, for there was no significant association 



