Darwinism 



coming more specialized as they became more 

 completely adapted to their own special environ- 

 ments, their limited place and range of condi- 

 tions. Such a genealogical tree must show the 

 order of geological occurrence just as a human 

 genealogical tree must show the succession of 

 generations in the family. 



The theory offered the only reasonable ex- 

 planation why the tree showed the order of suc- 

 cession, and something concerning the character, 

 of embryonic stages. It explained von Baer's 

 and Agasslz's discovery that higher forms dur- 

 ing embryonic life passed through stages or con- 

 ditions of structure which remained permanent 

 in the adults of older and lower groups on the 

 same line or branch. 



The fowl in its early embryonic life passes 

 through a stage where it has many of the struc- 

 tures of a fish, and later another with reptilian 

 characters. Why is this? The first living be- 

 ings were minute masses of living substance 

 or protoplasm, each mass containing a central 

 spherical body — the nucleus. These single cells 

 or unicellular animals gradually formed groups 

 of cells, and from these again, after a long time 

 and with many intermediate stages, wormlike 



15 



