120 THE STRUCTURE AND LIFE OF BIRDS cuap. 
front central part of the skull bears witness to its 
existence. Inthe Hatteria, the now rare New Zealand 
lizard, this hole is very large. As long ago as 1829 
it was noticed that in the Sand Lizard (Lacerta agilis) 
one of the scales at this point was quite unlike the 
rest. In 1884 it was first suggested that the pineal 
body was a rudimentary eye—v.e.,an eye that had 
become functionless. It has now been examined 
in various reptiles; and partly in one, partly in 
another, the lens, the retina, and the nerves, all the 
chief characters of an eye, have been identified. But 
in one important point, which I shall explain when 
I come to what are commonly known as eyes, it is 
the eye of an invertebrate, not of a vertebrate animal. 
We must go to insects or to crustaceans to find its 
fellow. In birds it has lost all resemblance to an eye, 
and it has been covered by the hemispheres which 
extend over and in front of it. In man it is also 
present, and Descartes suggested that this mysterious 
object, about the size of a hazel-nut, might- be the 
seat of the soul. 
If the question be asked what any animal wants 
with two different kinds of eyes, it is not easy to 
answer positively. We can say that many insects 
have compound eyes with hundreds of facets as well 
as simple eyes (ocelli), the latter having, probably, 
very defective sight, extending only to the very 
nearest objects. The lower crustaceans have eyes 
and a central ocellus; but in the higher members of 
the class, such as the crayfish, the ocellus has been 
lost. Possibly in vertebrates, before the two eyes as 
we know them had attained to their present perfection 
