This series of papers on birds and insects, with others on the physical
geography of the Archipelago and its various races of man, furnished all
the necessary materials for the general sketch of the natural history of
these islands, and the many problems arising therefrom, which made the
"Malay Archipelago" the most popular of his books. In addition to his
own personal knowledge, however, some interesting comparisons are drawn
between the accounts given by early explorers and the impressions left
on his own mind by the same places and people. On the publication of
this work, in 1869, extensive and highly appreciative reviews appeared
in all the leading papers and journals, and to-day it is still looked
upon as one of the most trustworthy and informative books of travel.

When the "Malay Archipelago" was in progress, a lengthy article on
"Geological Climates and the Origin of Species" (which formed the
foundation for "Island Life" twelve years later) appeared in the
_Quarterly Review_ (April, 1869). Several references in this to the
"Principles of Geology"--Sir Charles Lyell's great work--gave much
satisfaction both to Lyell and to Darwin. The underlying argument was a
combination of the views held by Sir Charles Lyell and Mr. Croll
respectively in relation to the glacial epoch, and the great effect of
changed distribution of sea and land, or of differences of altitude, and
how by combining the two a better explanation could be arrived at than
by accepting each theory on its own basis.

His next publication of importance was the volume entitled
"Contributions to the Theory of Natural Selection," consisting of ten
essays (all of which had previously appeared in various periodicals)
arranged in the following order: