CHAPTER XXII 



EVOLUTION 

 Meaning and Theories of Evolution 



Meaning of Evolution. Throughout the preceding discus- 

 sion of plant groups evolution was assumed. It was assumed 

 that the more complex forms of plants have come from the 

 simpler ones by gradual changes which involved the modifi- 

 cation of structures and the introduction of new structures. 

 It was also assumed that some simpler plants are reduced forms 

 of more complex plants. Hence evolution, which is usually 

 forward, that is, leading to more advanced forms, may be 

 backward, leading to simpler forms. According to the theory 

 of evolution, the organisms which first inhabited the earth were 

 extremely simple, and the various forms of plants and animals 

 which we now have are their modified descendants. 



The evidences of evolution in both plants and animals are 

 obtained by studying the structure, development, and behavior 

 of living forms, and the structure of ancient forms preserved as 

 fossils. Evolution takes place too slowly to be observed or 

 demonstrated, and hence our conclusions about it are only 

 inferences. 



There are two kinds of evolution organic and inorganic. 

 Organic evolution is confined to living things including animals 

 as well as plants. However, inorganic things are also constantly 

 changing, and hence evolution applies to all nature and not 

 merely to living things. The physical features of the earth are 

 constantly changing. Mountains are being worn down, while 

 there are other regions where the land is becoming more ele- 

 vated. Areas that are now land were seas or a part of the ocean 

 at one time. A study of fossil plants shows that climates have 

 changed, so that regions once tropical now have a temperate 

 or arctic climate. Rivers and valleys are constantly changing 

 and thus altering the landscape. Also the planets, like our own 

 earth, and even the stars, are changing externally and internally. 



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