Two other papers, dealing with parrots and pigeons respectively (1864-5), were thought by Wallace himself to be among the most important of his studies of geographical distribution.  Writing of them he says:  “These peculiarities of distribution and coloration in two such very diverse groups of birds interested me greatly, and I endeavoured to explain them in accordance with the laws of Natural Selection.”

In March, 1864, having begun to make a special study of his collection of butterflies, he prepared a paper for the Linnean Society on “The Malayan Papilionidae, as illustrating the Theory of Natural Selection.”  The introductory portion of this paper appeared in the first edition of his volume entitled “Contributions to the Theory of Natural Selection” (1870), but it was omitted in later editions as being too technical for the general reader.  From certain remarks found here and there, both in “My Life” and other works, butterflies would appear to have had a special charm and attraction for Wallace.  Their varied and gorgeous colourings were a ceaseless delight to his eye, and when describing them one feels the sense of pleasure which this gave him, together with the recollection of the far-off haunts in which he had first discovered them.

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: