Explanation:
What is the name of this website 
or the book? 
 
        
             
        
        
        
Fossil record- species evolve differently in different environments . different species evolve similarly in the same type of environment
homologous structures of living organisms- 2 variations; similar body structures that don't necessarily have similar functions 
homologous- diff mature forms but develop from embryonic tissues
vestigial- similar structures with reduced size and lesser or no functions
embryology- early stages of many animals are very similar- suggesting that organisms share common ancestry
        
             
        
        
        
The human heart has only 4 chambers. Left and right atria and left and right ventricles.
        
                    
             
        
        
        
Answer:
the paired bases( nitrogenous bases )