Thank you to everyone who came to the talk at Stanfords in Bristol last night, great crowd.
An additional thanks to Maria, from Spain. When I explained that the Dayak are forensically, passionately interested in gradient and see route choices in terms of uphill, downhill or upriver, downriver, she mentioned they did the same in her home town, near Seville in Spain.
On the way to Bristol I took this picture at Barnham train station. The train tracks run east away from us. Notice how the ragworts and other sun-loving weeds only thrive on the sunnier, north side of the track. The same is often true by motorways too and in each case the seeds hitch a lift with passing trains and cars.