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.
