no ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



the journey on a night when the conditions are not favourable. The 

 effect of this on the data will be large, and just what would result 

 in a poor correlation. If we had a complete picture of migration, 

 it could easily happen that these three factors turned out to be much 

 more important. Blanchard (1930) showed that, in the breeding 

 migration of the Spotted Salamander, Ambystoma maculatum, the 

 animals arrived at the pond in successive waves, according to the place 

 in which they were hibernating. 



It is impossible to account for the observed association of migration 

 with southerly winds by the hypothesis that the relation is due to 

 the well-known association of warm and wet weather with winds 

 from the south, although in the 1935 paper I raised this objection 

 myself. The most favourable condition for this hypothesis would be 

 complete correlation between these factors. In such an unlikely event, 

 there would be a state of contradiction between the high association 

 of wind direction with migration, and the low correlation with rainfall 

 and temperature. With any lower correlation between wind and 

 weather, the hypothesis would be even less tenable. I do not fmd it 

 possible to dismiss the evidence, at this particular pond, for migration 

 just as the smell hypothesis would require. It is unfortunate that no 

 other suitable opportunity has occurred to repeat the observations in 

 some quite different place, for I still doubt whether, in some places 

 with a different topography, wind would be so important, and feel 

 that other air currents may play a large part. My pond was situated 

 in a rather simple place, near the summit of one of two smooth parallel 

 ridges, and I should not expect much disturbances of the general 

 direction of the wind. 



I have now tried to convey a picture of migration. Nothing can 

 quite replace actual observation of this purposeful movement of 

 determined frogs. No one who has witnessed it can believe it to be due 

 to haphazard wandering over the countryside. In the more general 

 discussion which now follows, we can safely discard any idea that the 

 frogs merely come across their ponds by accident. 



Several have suggested that the animals fmd their ponds by going 

 downhill. This will not do at all, for frogs do not always go downhill 

 to the ponds, and ponds are often on the tops of hills. It is streams 

 that run at the bottom of hills, but tliis species does not usually breed 

 in streams. 



Hydrotaxis, the ability to move towards water at a distance, is 



