14 THE SENSES OF ANIMALS 



more on — say — sight than anything else. Their hearing may 

 be as sensitive as their eyes, but this sense may require much 

 more investigation both in the laboratory and in the field 

 before a true assessment can be made. 



However, from a practical point of view it is possible and 

 desirable to have some idea as to which sense is most probably 

 the dominant one. We humans are so much dependent upon 

 our eyes that it is often difficult to appreciate that there are 

 many animals which have little or no seeing abilities; yet 

 they conduct their lives most efficiently, making up for the 

 lack of sight or poorly developed sight by an enormously 

 increased sense of hearing or smell. 



It is almost impossible to place animals neatly into some 

 convenient category as regards their senses ; but one can give 

 what is certainly more than a rough outline as to which senses 

 play the most important part in the lives of the various 

 Classes, Orders and Families. 



Before attempting to do this, it is probably wise to remind 

 students that no animal possesses an effective sense that is of 

 no practical use to it. Hence one can approach the subject in 

 two different ways : one can observe an animal's behaviour, 

 and from some action or actions one can deduce that a 

 certain sense is clearly present or absent. For example, frogs 

 croak ; and even without going deeply into the physiological 

 side of the frog's vocal behaviour, it is obvious that frogs 

 would not croak unless other frogs could detect the sounds 

 uttered. This may seem all too obvious ; but it is surprising 

 how often the question is asked, "Do frogs have ears?" There 

 is a pitfall here for those who cannot rid themselves of con- 

 stant comparisons between human beings and other animals. 

 If hearing is mentioned, we too often think of an outer ear 

 like our own, and fail to realize that what we call hearing is 

 a matter of receiving and interpreting vibrations. These 

 vibrations are conducted to our brains, in the first instance, 

 by means of our outer ear ; but this does not mean that other 

 creatures cannot detect sounds and react to them even when 

 they do not possess an outer ear at all. 



Be this as it may, observation in the field can draw our 

 attention to the fact that frogs utter sounds, and by deduction 

 we can soon appreciate that these sounds would not be made 



