i2o THE STRUCTURE AND LIFE OF BIRDS chap. 



front ccntr.il part of the skull bears witness to its 

 existence. In the 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 — i.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 



