SEXSATIOX. 95 



sense-organs as it occurs in the sight of the vulture or the 

 smell of the dog. Indeed, Haeckel and others have specu- 

 lated whether the facts in this case do not call for the suppo- 

 sition of some additional and unknown sense, different in 

 kind from any that we ourselves possess. But I think it is 

 safer not to run into any such obscure hypothesis unless 

 actually driven to do so, and therefore I shall not here enter- 

 tain it. For this reason, also, I shall not follow Haeckel in 

 his view that the " homing " faculty of certain animals is due 

 to some additional and inexplical3le sense, and therefore I 

 shall reserve my treatment of this topic for my chapters on 

 Instinct. 



After this rapid survey of the powders of Special Sense as 

 they severally occur in different classes of the animal king- 

 dom, I shall conclude the present chapter by briefly consider- 

 ing certain general principles connected with Sensation. 



The muscular sense, the sense of hunger, thirst, satiety, 

 and others of the like general kind need not detain us; for 

 although their causation is somewhat obscure, we know at 

 least that they are dependent upon nervous adjustments, 

 and, being of so much importance to animals, we infer that 

 they have been developed under the general principles of 

 neuro-muscular evolution already considered in previous 

 chapters. My object here is rather to consider the mecha- 

 nisms of certain more special senses from the point of view 

 of those general principles. 



First as to the sense of Temperature, there is good evi- 

 dence that in ourselves and at least in all the higher animals, 

 thermal sensations can only be received by the nerve-endings 

 in the skin and adjacent parts of the mucous membranes ; if 

 the nerve-tibres immediately above their terminations in 

 these localities (as in the raw surface of a wound) be stimu- 

 lated by heat or cold, the sensation produced is merely one 

 of pain. There is strong evidence that not only the nerve- 

 endings, but even the whole of the nerve-tracts of which 

 they are the endings, are specialized for the purpose of re- 

 ceiving thermal impressions. These impressions, when 

 received, are not absolute, but relative to tlie temperature of 

 the part receiving them — the greater the difference of tem- 

 perature between the part and the object touching it, the 

 greater being the impression. Moreover, the greater the 



CT- 



