EXTERNAL FACTORS ON THE SPAWN DATE I25 



on rainfall maps as wet areas. Rainfall acts as an clement of "run-off," 

 the proportion of rain that is not evaporated and eventually reaches 

 watercourses. It is interesting to see that the pervious soils of the 

 South-East have hills, but that these are not early spawning areas. 

 There is no sign of earliness in spawning over the Nortli or South 

 Downs, the Cliiltern Hills, or the East Anghan Heights. There are no 

 ponds on the Chalk itself, but this formation often has inliers of Drift, 

 or patches of retentive clay, and frogs do occur in these areas. 



Passing now to the late spawning areas, we see that East Anglia is 

 well known for its dry climate. Rainfall maps show patches of dryness 

 to the south of the two Scottish Firths, just where the frogs indicate. 

 South- West Scotland is not very dry, so that the patch shown seems 

 anomalous, as is the Thames Estuary, the driest part of the country, 

 but not remarkably late. The Crossfell area is one of the most inter- 

 esting. Besides rainfall, temperature is a factor, not in the short-term 

 sense, but as a long-acting factor, especially important two months 

 before the spawn date. This area has been investigated by Manley 

 (1936) in a meteorological paper. He found that it had an exceptional 

 cHmate. It is the bleakest inhabited area in England, and its winter 

 climate resembles that of Iceland at sea level. 



Ireland is interesting. The whole island is earlier in spawning than 

 parts of England corresponding in latitude. The "Emerald Isle" is 

 renowned for its rain, but the meteorological peculiarity is not so 

 much the quantity of rain as its frequency, so that the ground is 

 generally moist (Bilham, 1938, p. no). It is when rain falls on dry 

 ground that there is no "run-off." 



It now remains to account for the lateness of the industrial areas. 

 This can perhaps be attributed to the fact that, as will be shown later, 

 light is a factor, and these areas havfe smoke palls. It is probable that 

 all Lancashire east of the early area near the coast would also be early 

 if it were not for the smoke, for in that county the early area in the 

 west is not smoky, and in the more eastern areas, high rainfall counter- 

 acts the smoke and results in average dates. London is again anomalous, 

 for it is a large and smoky city, but from the map it does not appear 

 later than the country round it. It is often risky to attempt to explain 

 such anomahes, but it does seem possible at least to suggest an 

 explanation. London differs from the industrial North by being a 

 concentrated area. It is, it is true, very large, but it is a unit, in which 

 the inner areas are too thickly populated to have frog ponds at all, and 



