126 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



the rate at which the smoke falls off in the outskirts is such that the 

 neighbouring country is not very smoky. In the North, it is more usual 

 to find iudustry in towns separated from another town by short 

 stretches of country. From farmland in these areas, it is often 

 possible to see industrial smoke on the horizon, in a way that does 

 not often happen near London. As a speculative hypothesis, I suggest 

 that the London anomaly is due to the absence of frog ponds where 

 there is much smoke, to an extent that does not happen in the mixture 

 of industry and farming in the North. In fact, my own area is one 

 of the nearest to London that has records of spawning, and I find 

 that the area just to the north, say five to ten miles away, is a httle 

 earher. 



From this wide survey of the conditions all over the country, let 

 us turn to the other extreme, and examine the records, first of some 

 interesting years in which the records within fifty miles of London 

 were analysed in detail, and then, making use of some fortunate 

 coincidences, to a few in other parts of the country where still greater 

 detail was available. 



In my opinion at the time, the number of records available in 1935 

 was not sufficient to justify the inclusion of those from the thinly 

 populated parts of the country. I also felt that such comphcations as 

 mountains and coastal areas would be better excluded. It was for these 

 reasons that I confined most of the inquiry to results within fifty miles 

 of London and used the weather data of Kew, at its centre. Kew is a 

 First Order Observatory, one of the five in the whole country where 

 every important element of weather is recorded at every hour of the 

 day and night. 



Temperature 



One of the difficulties of dealing with tliis factor is that a circular 

 argument can so easily be set up. Owing to the wide fluctuations in 

 temperature that are normal at this season, it is usually possible to look 

 back for a few days and find conditions as suitable as those considered 

 necessary for spawning. The following colloquy can then occur — 



Q. Why did frogs spawn on the 21st March 1929? 



A. Because it was warm on that day. 



Q. But it was warmer ten days earher — why did they not spawn 

 then? 



A. Because they were not then physiologically ripe. 



