DISTRIBUTION AND ABUNDANCE 



71 



intentions. In this book, expressions such as "I think" mean just what 

 they mean in conversation: what follows has been carefully considered 

 but I do not expect anyone to believe that I have proved the point. 

 I hope that they will not follow disbehef of the hypothesis by rejecting 

 the facts on which the hypothesis is based. 



Walking across the fields from one pond to another in the early 

 thirties, I could not reconcile what I saw with the current hypotheses 

 about migration, pond-fmding or the relation between the weather and 

 the activities of the frogs. I came to the conclusion that, in the study 

 of frogs, it was the events in the ponds rather than those in the frogs 

 that determined the actions of the animals. I felt that the animals were 

 for much of the time responding to changes in the enviroimient, but not 

 to changes in their own physiology, except in the limited sense that, un- 

 less the animal is in a suitable physiological condition, it cannot respond. 



In detail, the hypothesis that seemed to me to fit the facts (Savage, 

 1935) was that the frogs responded to some change or difference in the 

 algal population of the ponds, so that the effects of physical factors 

 such as the weather were indirect. At this stage, the reasons for this 

 hypothesis may seem obscure, but I hope that, as the tale unfolds to 

 its end, things will become clearer. I therefore now give merely a very 

 short summary of the ideas that led to the hypothesis — 



1. The explosive nature of breeding in this species ; all frogs breeding 

 in a pond do so within a few days of each other. 



2. The wide differences in the date of spawning between different 

 ponds in the same area. 



3 . An apparent influence of rain in accelerating spawning. 



4. The impossibility of accounting for the selection of ponds by any 

 obvious character. 



5. The precision of migration, so that the frogs seem to know where 

 the ponds are long before they could see them. 



6. The parallelisms with the known properties of algae, as follows — 



7. Algae exhibit phases of growth or development often of an 

 explosive character. 



8. In a small area scarcely any two ponds have the same algal flora. 

 The algal population is highly specific. 



9. Algal growth and development are under the influence of the 

 weather, especially the rainfall, and are strongly related to the geology 

 and water composition. Algal events are often simultaneous in one 

 pond, but occur on different dates in different ponds. 



