34 

 The Changing World of Life 



It has been said that the only invariable law of nature is variation. 

 Everything" in the universe seems to be subject to change. Astronomers 

 tell us that the planets, the stars, and the nebulae are constantly under- 

 going changes in their relative positions, their illumination, and other 

 characteristics. Geological studies show that the face of the earth has 

 been greatly altered in the past and is now undergoing changes that will, 

 in time, make our present-day maps of the continents obsolete. Fossils 

 of deep-sea forms of life found hundreds of miles from present-day 

 oceans enable us to trace the presence of prehistoric seas that once ex- 

 isted where some of our great cities now stand. Paleontology and 

 geology show the great climatic changes that have occurred in the past. 

 Fossils of tropical plants indicate that the United States at one time had 

 a tropical climate, while geological studies show that huge glaciers have 

 covered a great part of our country at other times. The social sciences 

 show that there have been great changes in the social, political, and 

 economic relationships of the people of the earth. 



With such cosmic, geographical, climatic, and social evolution estab- 

 lished, it would indeed be strange if there were not corresponding bio- 

 logical changes to keep pace with a constantly changing environment. 

 Studies in almost all of the phases of zoology indicate that extensive 

 changes in animal life have taken place in the past, are now taking place, 

 and will continue to take place as long as animal life continues to exist. 

 In fact, no species of animal is able to exist for long without such 

 changes. The passenger pigeon, the dodo bird, and the dinosaurs are 

 but a few of the many species of animals that have become extinct be- 

 cause they could not change fast enough to accommodate themselves to 

 a changing environment. 



Natural Selection 



As has been brought out repeatedly in the course of our study of 

 zoology, life is an adaptable thing. Individual adaptations and adjust- 

 ments are constantly taking place as animals respond to the environ- 

 ment in which they live. These have no evolutionary significance, be- 



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