THE WONDERS OF LIFE 



pendent on sunlight. If it acts in a one-sided way, it 

 causes the particular form of stimulation which we 

 call phototaxis or heliotropism. This is of a positive 

 character — that is to say, they turn towards the source 

 of the light — in the great majority of organisms, both 

 protists and histona. Everybody knows that flowers 

 that are growing in the window of a room turn to the 

 light. However, many organisms which have grown 

 accustomed to living in the dark are heliotropically nega- 

 tive ; they shun the light and seek darkness, such as the 

 fungi, many lucifugous mosses and ferns, and many 

 deep-sea animals. 



The principal organs of light-sensation in the higher 

 animals are the eyes; they are wanting in many of the 

 lower animals as well as the plants. 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 in 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 extensive and 

 remarkable than the historical succession of artificial 

 optical instruments from the simple lens to the com- 

 plicated iflodern telescope or microscope. This great 

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

 eye— ^has an interesting bearing on many important 

 questions of general physiology and phylogeny. We can, 

 in this case, see clearly how a very complicated and 

 purposive apparatus can arise in a purely mechanical 

 way, without any preconceived design or plan. In'other 

 words, we can see how an entirely new function — and 

 one of its principal functions, vision — has arisen in the 

 organism by mechanical means. 



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