I found all Alfred Russel Wallace‘s writings online. Not only all the original works by Wallace are available there; the webmaster, Charles H. Smith, includes some summaries and comments that help understand the texts.
Wallace’s wrote that a traveller in the Malay Archipielago finds himself sailing for days or even for weeks along the shores of one of these great islands, often so great that the inhabitants believe it to be a boundless continent.
A few toponyms mentioned in his travelogues can be found on online maps: Sadong River, Sarawak, Borneo and Macassar, Celebes, for instance. I failed to find other places whose names, like Ternate and Arru Islands, are probably spelled differently nowadays, like Wallace’s Lombock island, near Bali, now Lombok.
Locating the places that Wallace visited in Borneo is exciting, but the findings are sometimes sad. I believe for instance that most of the Sarawak that Wallace describes in his work “On the Orang-Utan or Mias of Borneo” is nowadays transformed into cultivated forests in intensive regime.
The map of the distribution of the Orangutan shows some surviving distribution in Borneo:
Sarawak, the north west region of Borneo well known to Wallace, and where he hunted a number of Orangutans, shows almost no presence of them nowadays. The state targets naturalits and tourits however: Sarawak Forestry Corporation and Forest Department.
A particularly interesting writing is On the Law which has Regulated the Introduction of New Species. Its content is known as the ‘Sarawak Law‘ since Wallace wrote it while in Sarawak in 1855. It is a preliminary enunciation of the theory of the evolution of species, written was three years before Wallace’s evolutionary theory based on natural selection and four years earlier than the publication of Darwin’s On the Origin of Species.
The evidence offered to support the law is composed by nine facts:
1. Large groups, such as classes and orders, are generally spread over the whole earth, while smaller ones, such as families and genera, are frequently confined to one portion, often to a very limited district.
2. In widely distributed families the genera are often limited in range; in widely distributed genera, well-marked groups of species are peculiar to each geographical district.
3. When a group is confined to one district, and is rich in species, it is almost invariably the case that the most closely allied species are found in the same locality or in closely adjoining localities, and that therefore the natural sequence of the species by affinity is also geographical.
4. In countries of a similar climate, but separated by a wide sea or lofty mountains, the families, genera and species of the one are often represented by closely allied families, genera and species peculiar to the other.
5. The distribution of the organic world in time is very similar to its present distribution in space.
6. Most of the larger and some small groups extend through several geological periods.
7. In each period, however, there are peculiar groups, found nowhere else, and extending through one or several formations.
8. Species of one genus, or genera of one family occurring in the same geological time are more closely allied than those separated in time.
9. As generally in geography no species or genus occurs in two very distant localities without being also found in intermediate places, so in geology the life of a species or genus has not been interrupted. In other words, no group or species has come into existence twice.
As a consequence of the nine propositions above, the Sarawak law is:
Every species has come into existence coincident both in space and time with a pre-existing closely allied species.
The law retains the power of its simplicity and elegance to these days.