This is a fascinating video on the Wallace Line, which separates to biogeographic regions:

The wildlife on each side differ tremendously from one another, even the line cuts through straits that aren’t wide at all.

Naturalist Alfred Russel Wallace (a contemporary of Darwin), noticed the distinction and defined the line. But what we now know is that he discovered the effects of plate tectonics decades before the theory was formalized.

So it’s not that different species mysteriously won’t cross the line — it’s that the separated landmasses with their own distinct biological lineages are now closer than they’ve been for millions of years.