Figure 2.

Organization of levels in the biological hierarchy. The organization of elements at each level may be similar across the biological hierarchy. Because elements or parts (indicated by the numbers) behave or interact (indicated by arrows connecting the parts) in a specific way, they acquire (in the case of dynamic interactions between the parts) or have (in the case of static interactions) a form with a defined function. (a) A level may be a protein, composed of amino acids having a specific spatial organization or, (b) in a behavioural display with an arm, a determined coordination in time and space of parts of the arm (hand, forearm). A key question is understanding how the levels are connected (vertical double arrow with question mark).