DISTRIBUTION AND ABUNDANCE 83 



natural regulation of the numbers of tadpoles in an area, I should say 

 that, within any pond, they are regulated by predation or shortage 

 of food in these parts of the range where heat death is uncommon, 

 but in the extreme south, possibly by heat death as well. Now, the 

 number of metamorphosing frogs in any area depends not only on the 

 numbers of tadpoles that the frog-ponds can support, but also on the 

 number of frog-ponds. Tliis is not nearly as obvious as it sounds, for 

 an area may be well supplied with ponds, none of wliich is a frog- 

 pond. It is only a certain kind of pond that will do for the frogs. It is 

 unlikely, in my opinion, that the tadpoles have any reciprocal effect 

 on the ponds. They are only there for about three months, and the 

 reproductive processes of the algae and other food organisms are 

 mostly so rapid that, even if the tadpoles made serious inroads on them, 

 it is probable that by next spring this effect would not be noticeable. 

 A quantity of water with no life in it, exposed to the air for nine 

 months, will be teeming with hfe if it is chemically rich enough to 

 support it. I think that the continual passage of ponds through phases 

 in which they are used by frogs followed by periods when no frogs 

 visit them has a very important influence on the numbers of frogs 

 living on the land near the ponds. It is probably much more important 

 than any density-controlled events in the population of tadpoles, and 

 corresponds to the effect on birds of the felling of a wood, or in insects 

 to the disappearance of a food plant. It differs in being far less obvious 

 to our senses, for the ponds do not disappear from sight. They do 

 seem to disappear from the notice of frogs. 



Table i, already discussed, shows the irregular nature of the 

 phenomenon, using merely the presence or absence of spawn as the 

 distinguishing feature. If the numbers of frogs using the pond is 

 considered, there are even more striking changes. From about 1927 

 to 1935, Large Totteridge was used by frogs, and the numbers at the 

 peak of popularity were about one thousand. They then began to 

 decline in number in the middle thirties, and dropped to about fifty 

 in 1938. They then were reduced to zero, and have been absent ever 

 since, although I have not kept observations every year, and it is 

 possible that there have been exceptions. I did not know of Hospital 

 Pond until 1948, when the tadpoles had already hatched, and I could 

 not estimate the quantity of spawn. In 1949, the number of clumps 

 was about three thousand, more in one place than I have ever seen. 

 In the next few years, there was a rapid decline, and in 1958 there 



