PREFACE 



from the fact that life existed on this globe 

 for untold millions of years in very specific 

 and hereditary forms, before there ap- 

 peared on earth a single living thing which 

 could be visible to the naked eye. During 

 this period some of these forms materially 

 altered the earth's surface. In this vast 

 living world of microscopic forms, there- 

 fore, was the origin of life here. This 

 inference is further borne out by the sig- 

 nificant fact that every visible living thing 

 now, however gigantic in size, yet begins 

 with only a single microscopic cell to its 

 physical being, and likewise must every 

 descendant from it so begin. Without a 

 microbic beginning no form of life, is a 

 universal law. But the ancient and per- 

 manently unicellular living forms still con- 

 stitute the largest and most powerful divi- 

 sion of the living kingdom, with moreover 

 such important relations to all visible 

 forms that we ourselves can postpone the 



