In Darwin's Shadow: The Life and Science of Alfred Russel Wallace ...

122 / In Darwin’s Shadow
Malay Archipelago, Wallace noticed that some are more allied than others as
a function of the geography in which they are found. One train of thought,
of course, led to the Ternate paper. But a related strain led him to think more
on a sharp division he noticed that existed between species allied to Asia and
species allied to Australia. He remembered his earlier work on the differential
distribution of monkeys along the Lower Amazon and Rio Negro, where he
had noticed that different species inhabited different sides of the river, and
that the river was acting as a reproductive isolating mechanism. Then, in the
February 1858 issue of the Proceedings of the Linnean Society, Wallace read
a paper by the British ornithologist Philip Lutley Sclater, on the “Geographical
Distribution of Birds,” in which the author noted a break in bird distribution
between the western and eastern islands of the Malay Archipelago. This
caught Wallace’s attention, because he too had noticed a similar division. As
he wrote Bates in 1859: “In this archipelago there are two distinct faunas
rigidly circumscribed, which differ as much as do those of Africa and South
America, and more than those of Europe and North America: yet there is
nothing on the map or on the face of the islands to mark their limits. The
boundary line often passes between islands closer than others in the same
group.” 34 This faunal split between Asia and Australia led Wallace to conclude
“that the same division will hold good in every branch of Zoology,” and he
promptly penned a paper on the subject, “On the Zoological Geography of
the Malay Archipelago,” published in the Zoological Proceedings of the Linnean
Society in November 1859, the same month that Darwin’s Origin of
Species was published.
This paper is, arguably, the second or third most important paper on natural
history that Wallace ever wrote, for it led to his identification of a line that
can be drawn “among the islands, which shall so divide them that one-half
shall truly belong to Asia, while the other shall no less certainly be allied to
Australia.” This biogeographical line that would eventually (and still) bear his
name (“The Wallace Line,” initially labeled as such by Thomas Huxley) was
further evidence for Wallace of the geographical isolation and biological transformation
that species may experience over space and time. “The Australian
and Indian regions of Zoology are very strongly contrasted. In one the Marsupial
order constitutes the great mass of the mammalia,—in the other not a
solitary marsupial animal exists. Marsupials of at least two genera (Cuscus
and Belideus) are found all over the Moluccas and in Celebes; but none have
been detected in the adjacent islands of Java and Borneo.” 35
As far as Wallace knew—and few in science had more time in the field
than he to make such observations—this distribution was “the most anomalous
yet known, and in fact altogether unique. I am aware of no other spot
upon the earth which contains a number of species, in several distinct classes