This sea border is only 22 miles across at its narrowest, but even birds and fish don't cross it. In 1859, British scientist Alfred Russel Wallace identified it and another scientist later named it the Wallace Line in his honor.
The animal life on either side of this line in the East Indies evolved separately because the Wallace Line marks the boundaries between tectonic plates. The straits along this border are narrow, but very deep. It's not completely unknown for animals to cross it, but modern scientists still see the Wallace Line as the site for an abrupt change in the distributions of many species.
-via Laughing Squid