VISION. 83 
points are scattered, while if it is near they are 
crowded together; and probably in this way a 
sense of distance is obtained. We may judge also 
that this form of eye is suitable principally for seeing 
very near objects. 
In all the higher kinds of animals—namely, in all 
the vertebrates, from fishes up to man, and likewise 
in the highest group of invertebrates, including the 
nautilus and cuttle fishes, the arrangement exists of 
a camera, a lens, a bacillary layer and a retina or 
membranous expansion of brain-matter. So far, 
the eye of the cuttle fish, together with the other 
less developed camerate eyes found in the inverte- 
brata is similar to the eye of vertebrate animals ; 
but while the optical contrivances in the two sets of 
structures are similar, the sources of their origin are 
totally different; so that it is impossible to con- 
ceive that by any process of modification in succes- 
sive ages the one kind of eye could have grown out 
of the other. This is particularly the case as 
regards the retina or sensitive curtain on which the 
light is thrown. In both vertebrate and cuttle fish 
eye, it consists of a sheet of nervous substance con- 
nected with a covering of microscopically minute 
rods which receive the rays of light and are affected 
by them. In both instances it is an inverted pic- 
ture which is cast on the retina, not an erect picture 
