THE EGGS AND YOUNG TADPOLES 5 



part of the warmest pond in a season with a fatally low temperature 

 elsewhere were at even a slightly higher temperature, the species would 

 survive. It is important to note that the time scale in this argument 

 is a long one. The fact that in one lifetime, or even in recorded history, 

 no such catastrophe has occurred to our knowledge is not quite the 

 point, for in ten or twenty thousand years a very rare event may happen 

 several times. 



Dickerson (1906) considered that the jelly absorbed and held the 

 warmth from the sun's rays, and provided a somewhat higher and 

 more equable temperature than that of the surrounding water, and she 

 was imdoubtedly right, although she gave no measurements to show 

 that her opinion was based on fact. 



Douglas (1948) for European frogs and Moore for the North Ameri- 

 can species (1939) as well as for R. temporaria (195 1) have studied the 

 relation between the highest temperature that the eggs can withstand 

 without any abnormal development occurring, and the geographical 

 distribution of the species. They conclude from laboratory experiments 

 that those species that breed early in the year and have compact masses 

 of spawn, among them R. temporaria^ are unable to survive temperatures 

 as high as those species that breed later and lay their eggs in films or 

 small bunches of spawn are able to do, and they think that the southern 

 limits of the species may well be set by these biochemical limitations. 

 Balcells (1956), in a study of R. temporaria on the Spanish side of the 

 Pyrenees, where, as mentioned already, this frog occurs in isolation, 

 concludes that the July isotherm of 2i°C is the southern limit. Douglas 

 gives figures to show that the eggs of this species do not survive long 

 above 24°-25°C in close agreement with Moore's estimate of 25°C. 

 On the other hand, the more southern species, R. esculetita, survives 

 about 28°C. Moore provides similar data for the American species, 

 but in a later paper (1940) suggests that the lethal effects of these high 

 temperatures may be due to the high rate of metabolism produced by 

 these high temperatures, which in turn so increases the demand for 

 oxygen that the embryos are unable to get enough through the compact 

 masses of jelly. It seems, however, that although he may be quite 

 right in attributing to temperature the failure of those species with 

 compact masses of spawn to colonize the southern regions, and even 

 right in thinking that there may be difficulties in gaseous diffusion, the 

 assumption that the gases have to diffuse through five to ten centi- 

 metres of jelly is probably not correct when the eggs are in their native 



