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 werc 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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