THE DOMINANT SENSES I5 



unless they could be received by other individuals of the 

 species ; and therefore it may be said that frogs can "hear" 

 or, if one puts it more pedantically, frogs have some auditory 

 apparatus. 



The other way of reaching the same conclusion is to study, 

 with the help of an expert, the actual auditory mechanism 

 and nervous system of a frog. Once demonstrated this will 

 show, by quite different means, that there really is some 

 hearing apparatus by which the frog receives the sounds 

 made by other frogs. 



Now as to the senses themselves: these are sight, smell, 

 taste, hearing and touch. Nearly all actions performed by 

 animals are governed by these, sometimes in combination 

 and sometimes even singly. In implying some reservation by 

 using the words, "Nearly all actions . . .", we must remember 

 that there are some kinds of animal behaviour which at 

 present we cannot explain. The way in which dogs and cats 

 can find their way home over what is quite unfamiliar ground 

 is a good example of this. It may well be that in the course 

 of time these extraordinary journeys will be explained in 

 terms of the senses we already understand, but at present 

 students of animal behaviour are baffled by the many well- 

 proven accounts of these amazing performances. More will 

 be said about this subject later on. 



Let us now consider the matter of the dominance of 

 certain senses in various groups of animals and see in what 

 kind of world such animals live. 



The lowest forms of animal life, the Protozoa, being uni- 

 cellular (one-celled) will not be expected to possess anything 

 but the simplest of senses — if indeed they can be truly said 

 to have any at all. They respond to certain stinmli : light and 

 touch (or vibrations in their environment) but as they have 

 neither brains nor proper nervous systems, they are very 

 limited in their sensitivity. 



The still simple, but somewhat higher kinds of animals — 

 mostly aquatic — such as Rotifers, Hydra, Planarians (flat- 

 worms), though not unicellular, are also very limited in their 

 sensory equipment. They cannot "see" even if they react 

 positively or negatively to light ; they cannot "hear" but they 

 will react to vibrations both from the air and the water in 



