54 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



Strawinski worked on R. esculenta, and in transferring his results to 

 other species, it is necessary to remember that this is an act of extra- 

 polation, and that, in the respiratory and digestive systems, we are 

 dealing with structures in very close contact with the environment, 

 and so liable to considerable modifications according to the ecology of 

 the particular species. In fact these structures are avoided by systema- 

 tists, precisely because, for their purposes, such systems may be un- 

 reliable guides to relationships. Nevertheless, it seems unlikely that 

 R. esculenta differs so much from R. temporaria, or even more remotely 

 related species, as to invalidate completely Strawinski's results when 

 applied to them. What is needed is a cautious approach when some- 

 thing appears to be different. 



Strawinski fmds that the principal respiratory surface is the skin. 

 He considers that the internal gills are only a supplementary respiratory 

 organ, and that, late in development, the lungs take over the first place. 

 The inner surface of the operculum is important, but the filtering 

 apparatus contributes very little to respiration. External gills are so 

 poorly developed in this species that they do not play any important 

 part. The tail, which others (Medvedev, 1937) have thought respira- 

 tory, is poorly supphed by respiratory capillaries. 



The relation of the above facts to the ecology of the animals can 

 now be considered. It is interesting to see the low place given to the 

 external gills, because this is much what would be expected from the 

 different ecology of the two species. R. esculenta does not lay its eggs 

 in one large mass, but in comparatively small packets in separated 

 places. I am not very famihar with the species in the field but, from 

 what I have seen, I should be surprised if the dense aggregations of 

 R. temporaria tadpoles are usually formed. The conditions I suggested 

 in the last chapter as being those that make external gills important 

 probably do not occur in this species. Tadpoles, hving in unstable 

 environments, must be versatile, and R. temporaria uses its lungs from 

 a very early stage, but I think that this is an adaptation to hfe in ponds 

 that would be too dangerous for species without lungs, or to food- 

 finding in parts of a pond too lacking in oxygen for such species. For 

 example, B. bufo, which is lungless until just before metamorphosis, 

 does not occur in the small polluted ponds that are sometimes used by 

 R. temporaria, but favours larger and better-oxygenated ponds. In 

 Upper Parkfield, both species occurred. The toad tadpoles spent the 

 whole season milling round in shoals just beneath the surface of the 



