The Evolution of Optics. 279 



As we ascend in the animal scale we find the pineal gland 

 becomes more and more rudimentary, although present in 

 the higher animal forms up to man, in whom it is a small, 

 apparently functionless remnant. It is interesting to note 

 that the pineal eye when found is formed after the inverte- 

 brate type, thus differing from the two lateral eyes in the 

 same animal. 



Certain flat fishes, notably the Pleuronectidce, furnish an 

 interesting example of adaptation to changing condition of 

 environment. The young of these fishes are symmetrical, 

 swim in an upright position, and have an eye on either side 

 of the head ; but, from the habit of groveling upon the bot- 

 tom in search of food, by which the lower eye is exposed 

 to constant danger of injury, the skull undergoes such a 

 modification that in the adult fish, which swims on its side, 

 the eyes have swung around, so that both eyes appear on 

 the upper surface of the head. And we sometimes see in 

 man, when the upright position of the head is interfered 

 with by the scars of burns or other contractions, a some- 

 what similar modification of the bones of the cranium. We 

 have referred several times to a nictitating membrane, a 

 third eyelid which is found in many lower animals, where it 

 exists in a state of perfection. In man and the Quadrumana 

 this is present only as a rudimentary organ, a small func- 

 tionless fold at the inner corner of the eye, an ever-present 

 reminder of our relation to the lower animals. 



While discussing the subject of the developmental stages 

 through which the eye has already passed, the question 

 naturally arises, " Has our own eye reached its final and 

 most perfect form, or is it still in the process of evolution ? " 

 As we have seen when considering the human eye, the 

 ideally perfect organ when in a state of rest is adapted for 

 vision at infinite distances. It is the eye best suited for the 

 savage in' the chase or in war, to the sailor, to any man who 

 leads an outdoor, country life ; but in the process of civil- 

 ization the eye is put under conditions to which it is by no 

 means so well adapted. In the crowded city the eye is con- 

 stantly directed through all the waking moments to objects 

 at short range. Its longest distance is across the narrow 

 street, while its most usual range is from the desk or book 

 to the limits of a room. The result is that the focusing 

 power is called constantly into play. The muscles which 

 converge the eye exercise a constant tension upon the globe, 

 while the enervating habits of city life do not tend to fit 



