He argued that all the differences between humans and apes were explained by a combination of the selective pressures that came from our ancestors moving from the trees to the plains, and sexual selection.
Ia berpendapat bahwa semua perbedaan antara manusia dan kera dijelaskan oleh kombikasi tekanan selektif yang datang dari leluhur mereka yang bergerak dari pohon ke tanah, dan seleksi seksual.