THE SENSE-ORGANS 503 



of the original connection. We seem here almost able to re- 

 produce an important bit of lost history, but the proofs are not 

 forthcoming and may always be wanting, since the early phy- 

 logenetic stages were probably passed in those lost forms be- 

 tween Amphioxus and the selachians, and concern soft parts, 

 no trace of which is likely to be found in fossil remains. 



The absolute size of the eyeball is very variable. In gen- 

 eral it is somewhat in proportion to the size of the body, yet 

 the eyeballs of the elephant or the whale, although large in 

 both cases, are not proportionate to their enormous bulk when 

 compared with those of Man, for instance. Again there is a 

 certain proportion between the size of the eyeball and the 

 sharpness of vision, as, for example, the enormous eyes of 

 birds ; but here, again, must be mentioned the small but exceed- 

 ingly acute eyes of rodents where the decrease of size seems to 

 be due to the excessive development of the masseter muscles, 

 and appears to have no direct influence upon the vision. The 

 eyes are apt to be large in animals with nocturnal vision, like 

 the lemurs, and it is possible that the relatively large eyes of 

 Man, which have encroached upon the nasal cavities, and thus 

 reduced the power of smelling, may be the result of a nocturnal 

 habit in some not very remote ancestors. 



Of the organs external to the eyeball which are accessory to 

 the sense of sight the muscles have been treated in a preceding 1 

 chapter (Chap. VI). There thus remain for consideration 

 only the eyelids and the glands, two sets of structures closely 

 associated with one another. They are both employed in pre- 

 venting the surface of the eyeball from becoming dry upon 

 exposure to the air, and belong to that series of changes necessi- 

 tated by the change of environment from water to air. They 

 are consequently found only in the higher vertebrates, and are 

 absent in fishes, and but poorly developed in aquatic urodeles. 



In fishes the integument fits smoothly over the region sur- 

 rounding the eyeball, and is continuous over the latter as a 

 thin skin, usually transparent, but occasionally ornamented in 

 places with pigmented areas, which continue the color scheme 

 of the rest of the skin. The eyelids, which appear first in 



