Origin of the scaling law of C ( k ). (A) C(k) for metabolic network of E.coli (green dot) and a randomized network (black circle). (B) The C(k) curves averaged over 43 organisms (red circle) and over 43 randomized networks (black circle). (C) The degree distribution of a random network with 4 super-hubs added. (D) The variation of C(k) of the random network before and after 4 super-hubs added (corresponds to black and red circles respectively). (E) Number of newly added triangles, △N as a function of k. (F) C(k) for random networks with large degree nodes added. In (E) and (F), black circles correspond to the original network, red circles correspond to the network after 2 nodes with degree 2,000 added and purple squares corresponds to the network after 200 nodes with degree 20 added.