112 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



quanta of light and sound energy — that is near the hmit of the physically 

 possible. We may often be inattentive to noises and sights of no 

 importance to us, as other animals are, but in favourable circum- 

 stances, what v^e cannot see or hear is probably invisible or inaudible 

 to other animals. It has been shown above that there is conclusive 

 evidence that, in R. temporaria, migrations can go on in complete 

 silence, so that we may eliminate the sense of hearing as a possible 

 guide for migration. 



Sight, of course, is out of the question, A frog has eyes only an 

 inch or so above ground level, and in the rough country over which 

 it usually migrates, its horizon must be limited to a few yards. 



Of the known senses capable of acting at a distance, there is now 

 only one left: the sense of smell. But, before I deal with this specific- 

 ally, I must answer a criticism that may well be rising in the reader's 

 mind. A great deal of v/ork has been done on migration in other 

 animals, and there are still very many obscure points. We have to 

 admit that we often have very little idea of the clues that animals use 

 to find their way. It does seem, however, that things are getting a 

 Httle clearer. In the now classical case of bees, we know that elaborate 

 social instincts are combined with navigation using the sun. In birds, 

 too, there is an increasing probability that astronomical navigation aids 

 are used, much as we do at sea or in the air. It is noteworthy that in 

 these cases what has been discovered is not some new organ of navi- 

 gation or some special sense or som^e new clue, that we did not know 

 existed, but the means by which the animals use those senses and 

 existing clues we knew were there all the time. We stand amazed at 

 delicacy, precision and resources unimagined before, but it is famihar 

 all the same. I feel, therefore, that before we invent some special pond- 

 fmding sense, or give up the problem as beyond the reach of any 

 hypothesis so far suggested, we ought to exhaust the possibilities of the 

 known senses. There is now only one left: does it fit? 



It will be remembered from the last chapter that frogs prefer one 

 pond to another, do not spawn in every pond and that ponds with 

 frogs in one year may have none in the next. There is evidently some- 

 thing specific, yet temporary about a frog-pond. What property is 

 there that is specific, temporary, and can be perceived at a distance? 

 It is quite certainly not water, for that is common to all ponds and at 

 times to much of the land as well. Can it be that the salts in the water, 

 which I have shown to be associated with the. preferences of frogs, are 



