Fig. 3.
Impact of geography and biogeography on present-day log per capita incomes. Geography and biogeography is the sum (linear combination) of the variables log geography, (T - 
) and (T - 
)2 after each variable was weighted by its estimated coefficient in regression model 5 (Table 3). Because the time since transition term (T - 
) is determined by biogeographic conditions Ãn, the solid slope line in the figure represents the estimated response of log GDP per capita to geography and biogeography combined.