78 MAN AN ADAPTIVE MECHANISM 



dangers, such as flying dust and debris, poisons, falling 

 bodies, heat and cold, have added the reflexes of wink- 

 ing, coughing, sneezing, vomiting, fainting, shivering 

 and sweating, which in like manner exemplify the adap- 

 tation of the organism as a whole or in part to specific 

 menaces in the environment. Of all the ceptor mech- 

 anisms for protection the most wonderfully adequate are 

 the mechanisms which guard the eye. In the primeval 

 struggle, the injury or loss of this member, so vitally 

 connected with all processes of nutrition and protection, 

 meant instant death. And there were always myriads 

 of agencies capable of accomplishing the destruction of 

 the eye. There were the menaces of flying twigs and 

 branches in the forest, of storm-driven dust on the plains, 

 of the dry pollen of ripening plants, of the sharp edges 

 of dead blossoms and leaves, of thorns and of stones. 

 As a protection against these manifold hazards, there 

 has been evolved a remarkable mechanism : a smooth, 

 round, elastic little ball, incased in a transparent, 

 glassy membrane, exquisitely sensitive to touch. This 

 ball is protected by the lid which fits around the curve 

 of the cornea and is rendered doubly effective by its 

 hard, smooth margin and fine fringe of delicate lashes. 

 This beautifully adapted lid closes instantly when a 

 foreign body touches the cornea and its moist inside 

 surface sweeps again and again across the surface 

 of the cornea in an effort to dislodge the offend- 

 ing particle, while the lachrymal glands, being simul- 

 taneously stimulated, expedite the process of removal 

 by pouring out a fluid which is admirably adapted to 

 carry away the invader. 



In like manner an especially delicate mechanism has 



