In the 19th century the small island of Gilolo (today Halmahera), located in the Moluccas archipelago, was still one of the most remote places on earth. In march 1858 a letter delivered to the nearest post office, located on the island of Ternate, was first sent to Singapore. From there a ship of the "British P & O Steamship Company", connecting Hong Kong with Suez, transported the mail to Africa. From Alexandria the mail was transported again with a ship over the Mediterranean Sea, reaching Paris, then Rotterdam and finally London. After three months the letter was finally delivered at the address of Down House, town of Bromley, County Kent, 26 km south-east of London and 12.000 km north-west of New Guinea.

The letter contained an 20 pages long article entitled "On the Tendency of Varieties to Depart Indefinitely from the Original Type" - presenting some concepts that maybe could explain the biodiversity and peculiar geographical distribution of related species on the islands of Indonesia. The author argued that if in an inhomogeneous population only some specimens, with certain characteristics, survive, these characteristics tend to become more common and more pronounced, until we would classify the modified specimens as a new species.

The author of the letter, send to gentleman geologist C. Darwin, was a certain Alfred Russel Wallace, a self-educated naturalist born January 8, 1823 in the Welsh town of Usk.

Fig.1. A.R. Wallace during his expedition to Singapore in 1862 (image in public domain).

Wallace discovered his passion for botany and natural sciences as young man and working as surveyor and teacher he saved some money for a planned expedition to South America. The expedition lasted from 1848 to 1852. In four years he collected an incredible variety of plant and animal species, intended to be sold to rich collectors and museums back in England. He also planned to publish his notes and observations.

But then the disaster, the steamer "Helen", on which he was returning back home, caught fire and sank in the North Atlantic. Wallace was able to save only some drawings of plant- and fish-species. He had lost almost everything - his collection and so his income, most disappointing all his scientific notes and the hope to achieve some fame as naturalist. Only the sum paid by his insurance saved him from the financial ruin. He published a summary of his journey, but due the lack of exact information the book was not received well by the scientific establishment.

Wallace swore never to travel again, but in April 1854 he arrived to Singapore, ready again for an expedition that this time would last for eight years. During this expedition Wallace collected 125.660 specimens of plants and animals and discovered 1.500 new species of insects and birds. He will publish his results, adventures and observations in the acclaimed book "The Malay Archipelago".

Fig.2. Between 1854 and 1862 Wallace explored almost the entire Indonesian archipelago, travelling for more than 22.000 km.

One of his most important discoveries resulted in part by his bad luck with ships. January 1856 Wallace missed the ship to Sulawesi. After waiting for four months in Singapore, he decided to make a detour on the islands of Bali and Lombok. Here Wallace noted something important, as the species of animals and plants differ significantly on the two islands, even if the islands are separated only by a narrow strait of sea. In fact he will later emphasize that the fauna differs more than the fauna of the British Islands if compared to the Japanese Islands! He will describe these two distinct faunal communities - on the one side the continent of Asia and islands dominated by tigers, rhinos and primates and on the other side Australia and islands characterized by kangaroos, koalas and the birds of paradise - in his essay "On the Zoological Geography of the Malay Archipelago" (1859) and later in his book "Distribution of animals" (1876).

Fig.3. Faunal regions in Indonesia, as proposed by Wallace in 1876 (image in public domain).

Wallace couldn't know of the tectonic forces modifying the islands of Indonesia and he could only cautionary speculate about sea level variations, but he deduced correctly that the observed distribution of animals is explained only by large changes of earth's surface in the geological past.

During the last ice age the level of the sea was much lower than today, exposing the continental shelf between Asia and Australia. Asia formed with the mountain ranges of Borneo, Sumatra, Java and Bali the lost continent of Sunda, Australia was connected to New Guinea in the lost continent of Sahul.

Between these two continents, colonized by animals coming respectively from Asia and Australia, a narrow strait of sea formed a natural barrier even during the last glacial maximum. In the strait some isolated islands persisted, like Celebes, Timor and Flores, and organisms evolved even more peculiar adaptations to survive there. The former natural barrier was named in 1868 by the naturalist Thomas Henry Huxley "Wallace Line" and the region of isolated islands, never reached by large tetrapods from Asia or Australia, is known today as "Wallacea".

Fig.4. Topographic map of Sunda, Sahul and Wallacea

But back to England in 1858. Darwin was prompted by Wallace's letter and manuscript to finish his summary on his theory "On the Origin of Species by means of natural selection, or the preservation of favored races in the struggle for life" (published in November 1859). It is interesting to note that one of the main mechanisms of his theory, the selection acting on the variations between specimens, was significantly improved in Darwin's writings between May and June 1858. This prompted some authors to claim that Darwin adopted this fundamental principle of evolution from Wallace's letter. Apart the question when exactly the letter from Ternate was delivered to Darwin (early May or late June, there are no exact records in Darwin's documents), there is no reason to doubt that Darwin had collected enough data since his Voyage on the Beagle to realize and deduce the mechanisms of evolution for himself - nevertheless Wallace's letter gave him the final push to finally publish his theory.

But reducing Wallace's contribution to the theory of evolution to a simple bystander would do injustice to this extraordinary naturalist, who was not afraid to speculate even on the habitability of other worlds. Wallace' experience with the variations in a single population (as salesman he was familiar with hundred of specimens of a single species) made him an important ally to promote this idea in the scientific establishment.

Along with Darwin, Alfred Russel Wallace must be considered one of the pioneers of evolution, a fundamental principle to truly understand the history of earth.

Bibliography:

GLAUBRECHT, M. (2008): Alfred Russel Wallace und der Wettlauf um die Evolutionstheorie Teil 12 . Naturwissenschaftliche Rundschau 61(7): 346-353

GLAUBRECHT, M. (2008): Alfred Russel Wallace und der Wettlauf um die Evolutionstheorie Teil 2 . Naturwissenschaftliche Rundschau 61(8): 403-408