The Sarawak Law

Wallace set off on his mission to discover the origin of species with his friend Henry Bates in 1848. They journeyed to the Amazon Basin (Wallace, 1854), and over the following 4 years Wallace observed that individual species of birds, monkeys, and butterflies remained within natural boundaries. His collecting and theorizing over those first 4 years allowed him to formulate ideas regarding the geographical distribution of animals (Wallace, 1852). Before arriving in the Amazon Basin he had been completely unaware of the effectiveness of physical barriers in limiting the range of all kinds of species. In addition, his discoveries in the South American rainforests allowed him to state that since the banks of the lower Amazon were among the most recently formed regions of South America, it was possible to consider the butterflies that inhabited that district as amongst the youngest species to be found in the entire basin (Wallace, 1854). They were, he considered, in a phrase that outlined his own world view, ‘the latest in the long series of modifications which the forms of animal life have undergone’.

By the time Wallace reached the Malay Archipelago in 1854, only 5 years before the publication of On the Origin of Species, Darwin, close to finishing his study of barnacles, had paid little attention to geographical distribution, despite Lyell's insistence that such a study was of the utmost importance in any attempt to solve the species question (Stauffer, 1975).

Early the following year, Wallace sent the editor of the Annals an article based on evidence he had discovered in the Malay Archipelago that, for the first time, spelled out how the extinction of a parent population could leave strongly marked varieties as apparently new species of that family. It has long since been referred to as Wallace's ‘Sarawak Law’. It was published in September 1855 and dwelt on the importance of extinction and descent with modification – divergence – as vital elements in the process of species change over time (Wallace, 1855). His revolutionary ideas, however, seemingly attracted little attention at the time.

Roy Davies , “1 July 1858: what Wallace knew; what Lyell thought he knew; what both he and Hooker took on trust; and what Charles Darwin never told them” Biological Journal of the Linnean Society, Volume 109, Issue 3, 1 July 2013, Pages 725–736. https://doi.org/10.1111/bij.12081

Sarawak Law

The following propositions in Organic Geography and Geology give the main facts on which the hypothesis is founded.

Geography

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.

Geology

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.

10. The following law may be deduced from these facts:--Every species has come into existence coincident both in space and time with a pre-existing closely allied species.

This law agrees with, explains and illustrates all the facts connected with the following branches of the subject: — 1st. The system of natural affinities. 2nd. The distribution of animals and plants in space. 3rd. The same in time, including all the phaenomena of representative groups, and those which Professor Forbes supposed to manifest polarity. 4th. The phaenomena of rudimentary organs. We will briefly endeavour to show its bearing upon each of these.

Wallace, A. R. 1855. “On the law which has regulated the introduction of new species” Annals and Magazine of Natural History, 2nd Series, 16:184–196. http://www.esp.org/books/wallace/law.pdf