136 THE SENSES OF ANIMALS 



and yet that we should not have learnt to use them to the 

 limit of their capabilities. Thus it is the use that animals can 

 make of their sense organs by means of their brains that is 

 all important, just as a telescope is a completely useless set 

 of brass tubes and pieces of glass unless someone is looking 

 through it. (See Plate 10.) 



The brains of men, dogs, frogs and other creatures all the 

 way down the scale of complexity to insects, worms, and even 

 jelly-fishes, receive messages from their sense organs, but the 

 responses to the messages depend not only on the sensitivity 

 of the sense organs but also on the use that the brain is able to 

 make of them. In ascending the scale from the simpler types 

 of sense organs and brains to the more complicated, the 

 awareness of the animal of the changes in its surroundings, as 

 signalled by the sense organs, seems to increase. It is probably 

 greatest in the warm-blooded animals and possibly reaches 

 its culmination in man, but the differences are those of 

 degree and not of kind. 



