96 MENTAL EVOLUTION IN ANIMALS. 



extent of the receiving surface, the greater is the impression ; 

 so that if tlie whole hand be immersed in water at 102°, the 

 temperature of the water will be erroneously judged to be 

 higher than that of another body of water at 104°, the tem- 

 perature of which is simultaneously estimated by a single 

 finger of the other hand ; and, similarly, smaller differences 

 of temperature can be appreciated by the whole hand 

 than by a single finger. According to Weber, the left hand 

 is considerably more sensitive to temperature than the right ; 

 and it is certain that different parts of the body differ greatly 

 in this respect. The more sudden the change of temperature, 

 the greater is the sensory effect. We have no means of 

 testing the truth of any of these statements with reference 

 to any of the Invertebrata, or even with reference to the 

 cold-blooded Vertebrata ; but we can scarcely doubt that 

 they apply in a general way to all the warm-blooded. The 

 facts certainly show an elaborate provision for appreciating 

 local changes of temperature occurring upon this and that 

 part of the external surface (the general comfort or discom- 

 fort arising from the body being kept at a normal tempera- 

 ture or not is another matter, and one wdth w^hich the special 

 mechanism we are considering is not concerned) ; and there- 



Ifore we have to contemplate the probable cause of its origin 



land development. 



At first sight we appear to encounter a difficulty which I 

 wonder never to have seen adduced by opponents of evolu- 

 tion. For in nature the only differences of temperature 

 which normally occur in objects with which animals have 

 any opportunity of coming into contact, are those between 

 ice and objects heated by a tropical sun ; and no one animal 

 ever has the opportunity of experiencing changes of tem- 

 perature extending through anything like so great a range ; 

 for in the arctic regions there is no tropical sun, in the 

 tropics there is no ice, and in the temperate zones the solar 

 heat is moderate. Of course since the introduction of fire by 

 man, the sense of temperature has become of much use to 

 sundry species of animals for the examination of food, &c., and 

 in this connection is of almost indispensable service to man 

 himself ; but, looking to the antecedents of these animals 

 and also to the antecedents of man, it may at first sight 

 seem remarkable that such an elaborate provision should 

 have been developed, and, as I have said, I wonder that no 



