Geography Reference
In-Depth Information
sary to explain geographic distribution. On the contrary, the known facts
point distinctly to a general permanency of continental outlines during the
later epochs of geologic time, provided that due allowance be made for the
known or probable gaps in our knowledge. (Matthews 1915:3)
The implications of Matthew's (1915) idea are rather simple. At any given
period, the most advanced species are those inhabiting the original area
where they evolved, and the most primitive species are those remote from
the center of origin. This remoteness is not a matter of geographic distance
but of inaccessibility to invasion, conditioned by the habitat and facilities for
migration and dispersal. Classic dispersalism, especially that of the twenti-
eth century, is a reflection of Matthew's (1915) work, which simplified and
codified Darwin's and Wallace's ideas (Heads 2005a). Matthew reaffirmed
the permanence of the great features of the earth, the origin of most taxa
in the Northern Hemisphere, and the importance of climatic changes as the
principal cause of the distribution of land vertebrates (Bowler 1996).
George Gaylord Simpson (1902-1984), a U.S. mammalogist and pa-
leontologist, worked as one of Matthew's assistants and succeeded him
in the position of curator of vertebrate paleontology at the American Mu-
seum of Natural History. In 1940, he published the essay
Mammals and
Land Bridges
, in which he began developing his concepts of corridors, filter
bridges, and sweepstake routes. He discussed the ideas about biotic expan-
sions and contractions by Willis, Rosa, and Matthew. He dismissed Willis's
“age and area,” although he found this theory to involve “various interest-
ing corollaries” (Simpson 1940:140) because conclusions reached were un-
satisfactory for mammals, and he dismissed Rosa's hologenesis because
it “seems at first sight so fantastic as hardly to warrant serious discussion”
(Simpson 1940:141). With respect to Matthew (1915), he found that his
theses were typical of those about mammals.
Simpson (1940) postulated that almost any taxon appears on a center of
origin, then it spreads steadily in all directions until it encounters insuperable
barriers, and after a time it begins to contract, often splitting into disjunct
even to some places where their immediate ancestor has never been. If no
barrier exists between two areas, their faunas will be very similar. When two
areas are separated by a strong barrier, they develop quite different faunas,
their differences being roughly proportional to the lapse of time since the
areas were connected. Simpson (1940) postulated that what is a barrier for
one taxon may be an open route for another and that wide-open, nonselect-
