SENSE ORGANS, 17 



wave-length and the sensation so imparted we call sight. 

 Lower vibrations than this we call heat, and we have, pro- 

 bably, sense-organs for the discernment of heat. Vibrations 

 of a still lower grade (from 40,000 to 30 per second) we per- 

 ceive by our ears and the sensation we term sound. Lastly, 

 the actual contact of particles upon a specially sensitive 

 surface gives us the closely allied senses of smell and taste. 



Sight involves the perception of light or shade and also, 

 as a higher faculty, the discernment of actual images. 

 The former alone exists in a number of low animals and 

 the latter is only added when the organ of sight has the 

 addition of an optical apparatus, known as the dioptric 

 mechanism. It is highly probable that many animals 

 have organs 'for the perception of vibrations higher or 

 lower than those of sight and the ^' sense" thus produced 

 is quite inconceivable to us. It may differ from our senses 

 as widely as sight from hearing. 



In the case of hearing much the same remarks hold. 

 There is little question that many aquatic animals have 

 organs of equilibrium or of motion which render them 

 cognisant of low mechanical vibrations of water produced 

 by the approach or proximity of a foreign object. In 

 certain land-animals {e.g.^ Bats) there appears to be much 

 the same kind of faculty, which enables their possessors to 

 avoid objects without the aid of eye, ear or nose. 



We may therefore divide sense-organs into three arbitrary 

 groups, as follows : — 



1. High-vibration organs. — 



(i) Possible organs for perception of vibrations above 

 A.760 wave-length. 



(2) Eyes for perceptions of vibration A760 to A390 wave- 



length. 



(3) Possible heat- organs for vibrations of lower frequency. 



2. Low-vibration organs. — 



(i) "Auditory" organs for perception of vibrations above 

 40,000 per second. 



(2) Auditory organs for perception of vibrations 40,000 to 



30 per second. 



(3) Motion-organs for perception of vibrations 30 per 



second to a single vibration. 



3. Contact-organs — 



( 1 ) Olfactory organs for finely divided particles. 



(2) Taste-organs for food. 



(3) Touch-organs for mechanical contact. 



M. .3 



