Fig. 1.

Red lines show model parameters estimated from the network of published chemical relationships over historical time, 1975–2010, every 5 y. The preference for more central chemicals (${\alpha }_{\mu }$, ${\alpha }_{\iota }$) increases consistently over time. The parameters controlling preference for walk length ($\beta ,\gamma$) and for jumping to disconnected network components (δ) also decrease consistently between 1975 and 2010, although the interpretation is somewhat subtle (main text). The green lines illustrate the optimal 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100% strategies against the historical trend and highlight the contrast between the trajectories.