The Blakiston Line
It was an Englishman, Thomas Blakiston, who first noticed that the native animals of Hokkaidō are different species from those on the southern side of the Tsugaru straits on Honshū. Blakiston lived in Japan from 1861 to 1884, spending most of his time in Hokkaidō in Hakodate, and his name is now used to describe the border in the distribution of animal species between Hokkaidō and the rest of Japan – 'the Blakiston Line'.
While Hokkaidō had land bridges to north Asia via Sakhalin and the Kuril Islands, southern Japan's land bridges primarily connected it to the Korean peninsula. Bears found on Honshū are Asiatic black bears while Hokkaidō's bears are Ussuri brown bears, found in northern Asia. On the southern side of the straits, Japanese macaque monkeys are found on Honshū as far north as Aomori, but not in Hokkaidō. Among other species north of the Blakiston Line are Siberian chipmunks, Hokkaidō red squirrels, the ezo-jika (Hokkaidō deer), kita-kitsune (northern fox), northern pika and Blakiston's Fish Owl.