LIMITS OF A UDIBLE 'SOUND. 63 



mosquito much as it flics about, but when it passes close by my car I 

 hear a sudden "ping," which is very striking. Mr. Dalby, the 

 aurist, to whom I gave one of these instruments, tells me he uses 

 it for diagnoses. When the power of hearing high notes is lost, the 

 loss is commonly owing to failure in the nerves. On the other hand 

 we may find very deaf people who can hear shrill notes, in which 

 case the nerves are usually all right, but the fault is in the auditory 

 canal. I have tried experiments with all kinds of animals on their 

 powers of hearing shrill notes. I have gone through the whole of 

 the Zoological Gardens using a machine of the kind that I hold in my 

 hand. It consists of one of my little whistles at the end of a walking 

 stick, that is in reality a long tube ; it has a bit of india-rubber pipe 

 under the handle, a sudden squeeze upon which forces a little air into 

 the whistle and makes it sound. I hold it, as near as is safe, to the ears 

 of the animals, and when they are quite accustomed to its presence 

 and heedless of it, I make it sound, then if they prick their ears it 

 shows that they hear the whistle, if they do not, it is probably inaudible 

 to them. Still, it is very possible that in some cases they may hear but 

 not heed the sound. Of all creatures, I have found none superior to 

 cats in the power of hearing sharp sounds. It is perfectly remarkable 

 what a faculty they have in this way. Cats, of course, have to deal 

 in the dark with mice, and to find them out by their squealing. 

 Many people cannot hear any notes in the squeal of a mouse. Some 

 time ago, singing mice were exhibited in London, and of the people 

 who went to hear them, some could hear nothing, whilst others could 

 hear a little, and others again could hear much. Cats are differen- 

 tiated by natural selection until they have a power of hearing all 

 the high notes made by mice and other little creatures that they 

 have to catch. You can make a cat, who is at a very considerable 

 distance, turn its ear round by sounding a note that is too shrill to be 

 audible by any human ear. Small dogs also hear very shrill notes, 

 but large ones do not. You may pass through the streets of a 

 town with an instrument like that which I used in the Zoological 

 Gardens, and make nearly all the little dogs turn round, but not the 

 large ones. At Berne, where there are more large dogs lying idly 

 about the streets than in any other town in Europe, I tried this method 

 for hours together, on a great many large dogs, but could not find one 



