

ZOOGENESIS 



other living things, it naturally follows that all the 

 present life upon the earth is descended from other 

 life that flourished in the ages that are past, and that 

 from still earlier life, and so on back to the time when 

 life first appeared. 



In the second place, all living things, no matter 

 what they are, begin their life as a single cell in which 

 no trace whatever of the adult form is discernible, 

 and all germ cells no matter from what form of life 

 they are produced are alw^ays strictly comparable. 

 The bodies of all living things are composed either of 

 a single cell or, more commonly, of vast numbers of 

 cells which are all alike in their fundamental struc- 

 ture. No matter how many cells there may be in 

 the body of an animal, and no matter how diversified 

 they are, all of the cells in any individual arise 

 through the division of the original germ cell. 



So as the starting point for our search for order in 

 the living world we hsivc first the unbroken continuity 

 of life from the very earliest times until the present 

 day, and second the necessity of interpreting or explain- 

 ing the origin of all forms of life in terms of the 

 single cell. 



These two important facts, the unbroken continuity 

 of life and the necessity for explaining and interpret- 

 ing all forms of life in terms of the single cell, must be 

 considered in their relation to the further obvious 

 fact that all living things must eat. The single cell 

 in order to grow and to develop and to produce other 

 cells must be supplied with food, and the continuity 

 of life from one generation to the next indefinitely is 



[2.09] 



