THE ORIGINS OF MIND III 



highly efficient and gifted organs, and more will be said of 

 them in Chapter 11. Many modifications of these kinds of 

 eyes are in existence; in fact, we have for study a whole 

 evolutionary series at all stages, from a primitive pigment 

 spot upwards. 



From a certain hollow kind of eye, which is a multiple 

 arrangement of pigment cells found in lower forms, we get 

 the camera eye of the vertebrates and man. A dimple of 

 sensitive skin tucks itself inward to form an optical cup, the 

 eyeball, and a focusing structure is formed over this; thus, the 

 retina and the lens arise. Later, in higher forms, nature elab- 

 orates on this plan, and we have the cornea and the iris and 

 the appearance of muscles to move the eyeball and to con- 

 trol the shape of the lens. Centers in the brain and the brain 

 itself are all the while improving under the drive of nature's 

 genius for organization, and it all culminates and is corre- 

 lated in man who looks out and begins to understand even 

 to the great depths of space. 



One can find similar series in the evolutionary history of 

 other sensory organs. All start from the generalized proper- 

 ties of protoplasm such as sensitivity to touch, to shock, to 

 heat and cold, to vibrations in water or air, to chemical sub- 

 stances. Each is organized by imperceptible degrees, by 

 trial-and-error use, worked over by natural selection from a 

 long line of genetic change and rearrangement. In the end 

 each becomes highly efficient in giving to the higher organ- 

 isms a greater and greater receptivity and a more complete 

 and accurate account of the objective world about them. 

 Here nature reaches out with every device she can possibly 

 invent to feel, to taste, to smell, to hear, to see— the better to 

 know. 



Motility is another property of protoplasm. Responses to 

 stimuli produce an activity, a movement, a secretion, a 

 change in blood pressure, and so on. Specialization for 

 greater efficiency again involves the organization of particu- 

 lar kinds of cells and organs, but the property is present even 



