THE EGGS AND YOUNG TADPOLES 9 



than other ponds, and of the seventeen clumps, seven had spoiled eggs 

 in them. 



It is, however, rather unusual in biology for a simple explanation 

 such as this to account for the whole of the facts of a complicated 

 matter hke the geographical distribution of an animal. We do not, 

 for example, know that this frog, regarded by taxonomists as one 

 species, has the same thermal physiology in different parts of its range. 

 Volpe (1953) has shown that, in the case of Bufo americanus, the eggs 

 from toads coming from Oklahoma were less tolerant of cold than 

 those of toads from Wisconsin or North Carolina. Volpe does not 

 think that the upper lethal temperature, 3i°C, is a limiting factor, 

 because he doubts whether this temperature is reached in the breeding 

 ponds, although he says that there are no temperature readings to 

 confirm this view. It will be recalled that a temperature only two 

 degrees less than this has been found in England, not a very hot 

 country, and Dickerson's remarks suggest that in Texas, 40°C may be 

 reached. Spallanzani in the eighteenth century (from Daudin, 1802) 

 was interested in this problem and put the thermal death point of the 

 eggs of a species that was probably R. esculenta as high as 35°R, 

 approximately 43 °C, and has an account, given him by a friend, of 

 hving frogs in the Baths of Pisa in a temperature of 37°R (46^C). 

 Zoologists in the eighteenth century made many excellent field 

 observations and these results differ by only i°-2°C from observations 

 of Seurat (1922), who found R. esculenta and its tadpoles in a hot 

 spring in North Africa at 44°-45°C. For all we know at present, the 

 eggs of R. temporaria from the extreme southern parts of its range may 

 withstand higher temperatures than those of the frogs used by Douglas, 

 taken in the neighbourhood of London, or those of Moore, from 

 north-east France and Belgium. A slight change of habit, such as a 

 tendency to lay eggs in the shade, rather than in full sun as is usual 

 in England, could extend the range by hundreds of miles. 



It is much easier to say that field observations are needed to settle this 

 question than it is to provide them. By definition, the observations 

 would have to be carried out at the southern limits of the range, and 

 it is almost certain that there, the frogs would be rare. Local knowledge 

 would be necessary to fmd the breeding ponds of the rare animal, and 

 only in some seasons could it be expected that climatic conditions 

 would be suitable. It is the sort of work that only residents in the 

 areas would be likely to carry out successfully. All they would need, 



2— (T.914) 



