SENSE ORGANS. x ^j 



Origin of the Compound Eye. — The spider has many 

 very small simple eyes in two groups, one on each 

 side of the head. Now imagine the number greatly in- 

 creased, the size correspondingly diminished, and then 

 the whole group crowded together until by mutual 

 pressure they are squeezed and elongated into pris- 

 matic tubes, and we have a general idea of the prob- 

 able process of change. 



EVOLUTION OF THE EYE. 



The exquisite beauty of the mechanism of the eye 

 makes its evolution extremely interesting ; but hereto- 

 fore it has seemed an insoluble mystery. Recently, how- 

 ever, much light has been thrown on the subject. 



I. Invertebrate Eye. — General sensibility to light 

 is coextensive with life itself. But it is a law in biology 

 that any useful function will be gradually separated 

 from other functions, localized in an organ, and then im- 

 proved indefinitely. How did a light-perceiving organ 

 begin ? It probably began to be formed under the 

 stimulus of light itself, as follows : 



(i) On the exposed epithelial surface certain spots 

 became pigmented, and thus more absorbent of light ; 

 the nerves to these spots became specialized to respond 

 to the light ; the epithelial cells of these spots became 

 slightly modified by elongation into rodlike form; and 

 already we have an eye-spot, the simplest beginnings of 

 an eye. Why this effect should occur only in spots we 

 know not, any more than we know why freckles should 

 come in spots. Such eye-spots may occur anywhere in 

 exposed surfaces, but more commonly on the most 

 sensitive part, viz., the head, when there is a head. 

 This first step is found in very many lowest animals, 

 especially in lowest mollusks (Fig. 107, a). 



