He added that he believed the most likely cause for the rapid extinctions to have been glaciation, but by the time he wrote World of Life (1911) he had come to believe those extinctions were "due to man's agency".
Ia menambahkan keyakinannya bahwa penyebab yang paling mungkin dari kepunahan cepat tersebut adalah glasiasi, tetapi pada saat ia menulis World of Life (1911) ia sampai pada keyakinan bahwa kepunahan itu "karena intervensi manusia".