WHAT EVOLUTION IS 35 



true backboned animals a notochord, 

 or supporting rod, precedes in devel- 

 opment the real backbone of these 

 forms and is replaced by this bone, 

 except in the very lowest fishes where 

 the notocord is the permanent and 

 only organ of support. Another ex- 

 ample may be found in the embryonic 

 human being where small ribs occur 

 attached to the neck vertebrae. As 

 development advances these ribs fuse 

 with the vertebrae and are thus lost 

 to view, but in lower animals, like the 

 alligator, neck or cervical ribs are 

 persistent throughout life. Again 

 all animals that reproduce sexually 

 pass through an tgg stage in which 

 they are, for the time being, a single 

 cell. This state is a permanent condi- 

 tion in the simplest animals, the pro- 

 tozoans, which are very usually only 

 single cells. Innumerable examples 

 such as these might easily be given. 



