EVOLUTION 225 



and by the mud-covered ocean floor. The present appear- 

 ance of the earth is only one stage in a long succession of 

 changes; indeed, it is believed that the earth was once a 

 body much hotter than the sun a mass of incandescent 

 gases in fact and that it has reached its present shape 

 through a cooling and shrinking process extending over such 

 vast stretches of time that a million years is but a unit of 

 measurement. 



190. Organic Evolution. It is very evident that the first 

 plants and animals did not appear on the earth until some parts 

 of it at least had cooled sufficiently to assume the solid state 

 with a temperature not much higher than is found at the 

 equator at present. That the earth had not reached its 

 present form when such organisms appeared is shown by the 

 fact that we find the remains of both animals and plants in 

 all but the oldest rocks. Coal, as everybody knows, is com- 

 posed of plant remains, and yet it is found everywhere deep 

 in the earth where no plants could grow. Nearly every 

 museum has a collection of fossil plants and animals taken from 

 the rocks. The great changes in the earth which have un- 

 doubtedly taken place since the coal beds were formed must 

 have sufficed to exterminate an immense number of species, 

 genera, or even larger groups, but in the process making new 

 habitats in which other races of animals and plants could 

 exist. 



191. Variation in Nature. If we assume that the animals 

 and plants of today have descended from earlier and less 

 complex species by gradual changes in their form and struc- 

 ture, we shall find it necessary to show that living things are 

 capable of making such changes. This, however, is not a 

 difficult matter. No two objects in nature are exactly alike. 

 Whether we are gathering flowers, choosing an apple, or select- 

 ing a kitten or puppy from among its brothers and sisters, 

 there is always room for a choice because of the small dif- 



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