Second expedition – Malayan Archipelago 1854-1862
Keen to extend his biogeographic experience Wallace now headed for the East Indies in Malaysia across to Indonesia, still hoping to fund his studies through sale of specimens.
‘ . . . With a young assistant, Charles Allen Wallace would spend nearly eight years in the region, undertaking sixty or seventy separate journeys resulting in a combined total of around 14,000 miles of travel. He visited every important island in the archipelago at least once, and several on multiple occasions, and collected almost 110,000 insects, 7500 shells, 8050 bird skins, and 410 mammal and reptile specimens, including probably more than 5000 species new to science. His best known zoological discoveries are Wallace’s Golden Birdwing Butterfly (Ornithoptera croesus) and Wallace’s Standard-Wing Bird of Paradise (Semioptera wallacei), both from Bacan island, and Rajah Brooke’s Birdwing Butterfly (Trogonoptera brookiana) from Borneo. The book he later wrote describing his work and experiences there, The Malay Archipelago, is the most celebrated of all travel writings on this region, and ranks with a few other works as one of the best scientific travel books of the nineteenth century.
Beetles were his great passion but it was during this period that his cogitations on natural selection were taking shape, expressed in the epoch-shattering letter to Darwin in 1858. His record of this period was eventually published as The Malay Archipelago (1869) and an instant best-seller, which it remains today.
While staying in a small house in Sarawak, Borneo, Wallace sketched his famous ‘Sarawak Law’ paper On the Law Which Has regulated the Introduction of New Species, which summarised his views on geology and its relation to biogeography as well as his cogitations on organic evolution. His final paragraph read:
It has now been shown, though most briefly and imperfectly, how the law that “Every species has come into existence coincident both in time and space with a pre-existing closely allied species,” connects together and renders intelligible a vast number of independent and hitherto unexplained facts. The natural system of arrangement of organic beings, their geographical distribution, their geological sequence, the phænomena of representative and substituted groups in all their modifications, and the most singular peculiarities of anatomical structure, are all explained and illustrated by it, in perfect accordance with the vast mass of facts which the researches of modern naturalists have brought together, and, it is believed, not materially opposed to any of them. It also claims a superiority over previous hypotheses, on the ground that it not merely explains, but necessitates what exists. Granted the law, and many of the most important facts in Nature could not have been otherwise, but are almost as necessary deductions from it, as are the elliptic orbits of the planets from the law of gravitation. Sarawak, Borneo, Feb. 1855.
Reading this paper Charles Lyell was prompted to visit (Darwin) at Down House in April 1856 and hearing his theory of natural selection recommended that Darwin immediately publish his ideas. Darwin was indeed forced into action but he eventually decided on a book rather than a paper.
The precise mechanism of evolution still evaded Wallace until in February 1858 during an attack of fever in the village of Dodinga on the remote Indonesian island of Halmahera his ideas crystallised sufficiently for him to send a draft of his theory to Darwin via the island of Ternate. His essay titled On the Tendency of Varieties to Depart Indefinitely from the Original Type was read to the Linnean Society at the same meeting as Darwin’s exposition on natural selection thus giving the pair equal credit.