244 PRACTICAL ZOOLOGY 



The Origin of Life. Scientists have speculated for centuries 

 regarding the place where life originated upon the earth. Ac- 

 cording to the theory of spontaneous generation animals were 

 supposed to originate directly from inorganic substances; for ex- 

 ample, frogs and toads from the muddy bottom of ponds under the 

 influence of the sun, and insects from dew. The brilliant experi- 

 ments of Redi (1668), Pasteur (1864), and Tyndall (1876) over- 

 threw this theory completely, and scientists now believe that 

 living organisms originate only from preexisting organisms. 

 Where life first began is still unknown, but the meeting point of 

 sea and land is the most probable place of origin. From here the 

 fresh water, deep sea, and land were gradually peopled. 



This general view of protoplasm, cells, tissues, organs, and 

 systems of organs will make more intelligible the discussion of the 

 life processes of the frog presented in the next chapter. 



REFERENCES 



Comparative Anatomy of Vertebrates, by J. S. Kingsley. Henry Holt and 



Co., N. Y. City. 



The Cell, by E. B. Wilson. The Macmillan Co., N. Y. City. 

 Histology, by Dahlgren and Kepner. The Macmillan Co., N. Y. City. 

 College Zoology, by R. W. Hegner. The Macmillan Co., N. Y. City. 



