i8 9f ] THE BRITISH NATURALIST. 231 



nerve cells soon present themselves as we ascend the scale of devel- 

 opment. They are found in the Medusae, in which in some form a 

 mass of such cells occurs opposite each sense organ on the margin 

 of the bell. They are found in connection with cells, evidently 

 having tactile functions, in some of the echinoderms, and, in fact, 

 are familiar to us as the little masses of nervous tissue known 

 as ganglia, found in almost all animals, from the lowest worm to the 

 vertebrates. 



It is a familiar fact that light has a powerful influence on the 

 processes of animal and vegetable life. Granted this effect of light, 

 it will be seen that if at any part of an organism there were 

 developed a little mass of tissue more sensitive to light than the 

 general tissue of the organism, there would be the first suggestion 

 of a special eye, and if this special bit of tissue proved of service 

 to the animal the probability exists of its being further perfected 

 by its descendants. 



Such a specialized tissue is perhaps seen in the pigment spot found in 

 so many of the Protozoa. In this class we can scarcely expect to find 

 anything more elaborate than this, but if we pass on to multicellular 

 animals we can trace up the progress from the simple pigment spot to 

 the elaborate eye in an almost uninterrupted series. The same 

 is true to a certain extent of auditory organs. From a condition 

 in which the whole surface of the organism is equally sensitive 

 to vibration we pass to one in which certain parts become more 

 sensitive ; sometimes rigid hairs are found in intimate relation with 

 nerves, sometimes a sac containing fragments of calcareous matter 

 — otoliths — is found, sometimes a combination of the two, but whatever 

 form the organ takes we have the same essential features as in other 

 sense organs, namely, a sensitive area to " multiply disturbances," a 

 nervous system more or less developed to transmit these disturbances, 

 and a contractile element to produce the necessary movements. 

 With smell and taste we do not find any great elaboration of the 

 sense organs as compared with sight and hearing, in fact, in the highest 

 animals the condition is not so very far removed from the conditions in 

 the lowest ; in both cases all that is required is that a certain sensitive 

 area should come in direct contact with substances having a certain 

 chemical composition, and a rather simple improvement of the 

 mechanism by which this is effected is all the change w§ see in the 

 advance from the lowest to the highest forms. Our organs of smell 

 and taste do not give us anything like such elaborate information 

 about the surrounding world as do our organs of hearing and 

 sight, so we can understand that their structure should be relatively 

 simple. 



