Ever heard of the Wallace Line?
The Wallace Line (or Wallace's Line) separates the ecozones of Asia and Wallacea, a transitional zone between Asia and Australia. West of the line are found organisms related to Asiatic species; to the east, a mixture of species of Asian and Australian origin is present. The line is named after Alfred Russel Wallace, who noticed this clear division during his travels through the East Indies in the 19th century. The line runs through Indonesia, between Borneo and Sulawesi (Celebes), and through the Lombok Strait between Bali and Lombok. Antonio Pigafetta had also recorded the biological contrasts between the Philippines and the Maluku Islands (Spice Islands) (on opposite sides of the line) in 1521 during the continuation of the voyage of Ferdinand Magellan, after Magellan had been killed on Mactan.
The distance between Bali and Lombok is small, about 35 kilometers. The distributions of many bird species observe the line, since many birds do not cross even the smallest stretches of open ocean water. Some bats have distributions that cross the line, but other mammals are generally limited to one side or the other; an exception is the Crab-eating Macaque. Other groups of plants and animals show differing patterns, but the overall pattern is striking and reasonably consistent.
The only reason I know about it is that one of my older relatives told me years ago that he studied it when he did Geology at high school in the late 1930s. The reason he remembered it so well was because the settled science of the day was that "oil does not exist south of the Wallace Line". That was stated as geological gospel. It was thought that the different flora below the Wallace Line in prehistoric times was not conducive to the creation of oil. I think.
So much for that theory. At least the geologists who promoted it had the good grace to ditch it when drillers actually discovered oil south of that line. In fact it has been ditched so thoroughly, I can find no sign of it via a quick Google search. You'd probably have to dig up an actual 1930s text book to find evidence that it ever existed.