Travel Reference
In-Depth Information
arm of the ocean still forms a boundary between Borneo on the Southeast
Asian side to the west and Sulawesi on the Australasian side to the east. But
this arm is zipping up, starting from the south. Bali and Lombok, just south
of Borneo and Sulawesi, used to be separated by a great southern extension
of the oceanic arm, but this has now closed up and the islands are only sepa-
rated by a narrow and relatively shallow strait through which strong cur-
rents pour from north to south.
Worlds in collision
Many of the animals and plants living on these two colliding plates have fol-
lowed dif erent evolutionary paths since Gondwana parted from Laurasia a
quarter of a billion years ago. Now the gap between these two great biologi-
cal provinces is a mere 25 kilometers wide at its narrowest point.
The birds, animals, and plants that rafted toward Southeast Asia on the
Australasian plate were easily able to spread and colonize the new islands
that had been thrust up to the north and east of New Guinea. These new
islands, arising smoking from the sea, were empty of terrestrial life before
the Australasian animals and plants arrived. But the Southeast Asian part
of the vast Eurasian plate that lay to the west was already densely populated
with well-adapted creatures. As a result, neither the Australasian nor the
Southeast Asian biological province has been able to replace the other.
The pioneering biogeographer Alfred Russel Wallace was the fi rst to doc-
ument the existence of a line of demarcation between the two provinces. The
discontinuity that he observed in the Malay islands was even more dramatic
than the dif erences Darwin saw between the animals and plants on the west
and east sides of the Andes, because its origins lay much further in the past.
The abruptness of what became known as “Wallace's line” puzzled Wallace
more than Darwin's Andean separation, because the geological reason for it
is so much less obvious. The Southeast Asian and Indo-Australian biologi-
cal provinces are divided, not by a towering mountain range, but by a far less
intimidating narrow passage of ocean water between islands. Wallace could
not have imagined the true reason for these dif erences, the long history of
