184 SMELL, TASTE, ALLIED SENSES 



the moist surfaces of the eye, judged by the sensations 

 they give rise to, are distinguishable from those in the 

 epithelium of the mouth and of the nose. But this special 

 differentiation is best seen in the gustatory organs. Here 

 three and probably four well defined senses can be dis- 

 tinguished, namely, sour, saline, sweet, and bitter. And 

 though separate receptors for these four senses have not 

 as yet been distinguished structurally, their functional 

 separation is beyond doubt. 



It is because of the repeated differentiations that 

 characterize the evolution not only of the chemoreceptors 

 but of the other groups of like organs that a classification 

 of them or even a simple enumeration proves to be so 

 unsatisfactory. For they are not unitary elements that 

 can be counted like the fingers on the hand nor are they 

 sufficiently co-ordinated to make classifications easy and 

 natural. They are like the whole organism itself in that 

 they exibit that kind of diversity that characterizes evo- 

 lutionary flux. 



6. BIBLIOGRAPHY 



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 CANNON, W. B. 1918. The Physiological Basis of Thirst. Proc. Roy. 



Soc., London, B, vol. 90, pp. 283-301. 

 CANNON, W. B., AND A. L. WASHBUEN. 1912. An Explanation of Hunger. 



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1'oreille chez les oiseaux. Ann. Sci. Nat., tome 15, pp. 113-124. 



