30 BIOLOGY: GENERAL AHD MEDICAL 



Thus, conditions of temperature during past periods of 

 the world's evolution are believed by many to have 

 been responsible for molecular combinations impossible 

 at the present time. This is, however, conjectural, and 

 not demonstrable. The chief argument in its favor 

 is that all of our endeavors to see protoplasm come 

 into being independently of antecedent life, have failed. 

 We are thus obliged to conclude either that life never 

 did arise spontaneously, or that it can no longer be gen- 

 erated spontaneously, or that we are at present unable 

 to recognize the most primitive forms in which it occurs, 

 being acquainted only with what are by comparison 

 highly evolved forms. 



REFERENCES. 



JOHN TYNDALL: "Floating Matter in the Air," N. Y., 1882. 



J. BUTLER BURKE: "The Origin of Life," N. Y., 1906. 



H. CARLTON BASTIAN: "The Modes of Origin of Lowest Organ- 

 isms, etc.," Lond., 1871. "The Nature and Origin of 

 Living Matter," Lond., 1905. 



