THE LIVING STRUCTURES 161 



human race. The bird is made to sail through the air. 

 Consider how perfectly everything is calculated and ad- 

 justed to effect that purpose. The bones are all made 

 hollow and extremely light, even lighter than aluminum. 

 Those little animals we call cells build and produce these 

 structures and attend to every detail in their production. 

 Think of the factors entering into the construction of the 

 eye. It is made to receive an image like a photographic 

 plate with the purpose of transferring impressions and 

 images received to the brain cells. Every step in the pro- 

 duction of the eye must be exactly calculated, and every 

 act performed with a purpose in view. Think of the per- 

 fect adjustment of the crystalline lens, vitreous humor, 

 and cornea of the eye and how the light is continually 

 regulated by those cells who have charge of the opening 

 and closing of the pupil of the eye to adjust the intensity 

 of the light falling on the retina. In reference to the de- 

 velopment of the eye Mr. Haeckel has the following to 

 say: 



"The essential difference between the real eye and a 

 part of the skin that is merely sensitive to light is that 

 the eye can form a picture of objects of the outer world. 

 This faculty of vision begins with the formation of a small 

 convergent lens, a bi-convex refracting body at a certain 

 spot on the surface. Dark pigment cells, which surround 

 it absorb the light rays. From this first phylogenetic 

 form of the organ of vision up to the elaborate human eye, 

 there is a long scale of evolutionary stages not less ex- 

 tensive and remarkable than the historical succession of 

 artificial optical instruments from the simple lens to the 

 complicated modern telescope or microscope. This great 

 "wonder of life" the long scale of the evolution of the 

 eye has an interesting bearing on many important ques- 

 tions of general physiology and phylogeny." 



