CHAPTER XXVIII 

 THE EVOLUTION OF LIFE 



The brief survey of the animal kingdom to which the 

 first part of this book is devoted gives some idea, inadequate 

 though it be, of the variety of animal life on the surface 

 of the earth. Zoologists have described several hundred 

 thousand species, and the fact that new species are being 

 described at the rate of about ten thousand a year shows us 

 how far we still are from having a complete list of the earth's 

 fauna. The number of species of animals now on the 

 earth is certainly over a million and is possibly several 

 millions. But the number of species now living constitute 

 but a small fraction of the enormous number that formerly 

 peopled the earth. The science of Geology teaches us that 

 the crust of the earth is a great burial ground in which are 

 interred the remains of countless animals and plants, and 

 that new forms have constantly replaced the old during 

 the many millions of years involved in geological history. 



The question naturally arises: How did all this wealth 

 of plant and animal life come into existence? Formerly 

 it was generally held that each species was separately 

 created, but as students of life came to have more extensive 

 knowledge of the structure, distribution and relationships 

 of living forms, and as they traced the succession of extinct 

 species buried in the rocky strata of the earth's crust,. they 

 became convinced, almost without exception, that species 

 of plants and animals arose by a gradual process of devel- 

 opment or evolution. How life first began no one knows, 

 but the gap between the non-living and the living once 



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