Answer:
air
Explanation: without air you can cant even live 
 
        
                    
             
        
        
        
In terms of function, bird wings are used to flap transversely, whereas butterfly wings are used to flap laterally. In terms of structure, bird wings are filled with hollow bones for easier flight, whereas butterfly wings are structured by scales and tiny hairs.
        
             
        
        
        
In this question, austin turk and many other conflict criminologists see crime as a natural consequence of inter group struggles over control. although crime is commonly define as an unlawful act of someone and is punishable by the state or other authority. And it is also define as not just an act but as an illegal act. 
        
             
        
        
        
The two substances needed in this situation are:
potato extract and catechol<span />
        
             
        
        
        
Darwin's idea of the survival of the fittest means that certain organisms are more likely to survive with they traits in particular environments. This is because some organisms are more fit to live in a given environment whether it be due to temperature, weather, or predators.
One example is that dark moths are more fit for survival than light moths in many areas because they are able to camouflage against dark tree bark.
Hope this helps.