THE HISTORY OF ANIMAL LIFE 391 



ness of vision has been developed and is maintained, for if gulls 

 are hatched in which the acuity of vision is less than that required 

 for the prompt discernment of prey, such individuals are at great 

 disadvantage in the competition for food. Thus unfit individuals 

 may be eliminated and a stock of genetic characters including sharp- 

 ness of vision becomes the species character of the race. 



Natural selection also operates in other ways to determine the 

 character of the animals found in various areas. It acts to determine 

 the coloration of animals. Where the background is sandy as on 

 shore lines and in deserts, animals have a tawny color. In forests and 

 among brush striped animals are found. In polar regions animals 

 with white coats prevail. It seems hardly necessary to explain in 

 detail how these inconspicuous colors make for safety of the species, 

 and how variations that increase visibility are eliminated. Kipling's 

 fable of the origin of stripes on the zebra and the spots on the leop- 

 ard illustrates the usefulness of the color pattern. Mimicry, sexual 

 selection, and the complicated life cycles of parasites are also re- 

 garded as further examples of natural selection and the operation of 

 the law of the survival of the fittest. 



Darwin's theory of the mechanism of evolution is distinctly dif- 

 ferent from that of Lamarck. The difference becomes clear if we 

 cite an illustration, for example the webbed feet of certain aquatic 

 birds. According to Lamarck's interpretation, the webbing between 

 the toes is developed in response to a need, an adaptation imposed 

 directly by the circumstances in which the bird finds itself. Darwin 

 assumed the occurrence of variations without definitely explaining 

 their origin. Some birds with webbing which appeared as a type of 

 variation were better fitted for their environment and this inherit- 

 able variation (or mutation, in modern terms), then became the 

 preponderant character of the race because of the elimination of 

 those without webbing by the operation of the law of the survival 

 of the fittest. 



