William Morse  wrote:

> A famous criticism of "On the Origin of Species" is that it
doesn't say
> much about the origin of species. The classic (over half a century old)
> neoDarwinian explanation is allopatric speciation. The mountain rises, the
> populations are cut off, they adapt separately. Another classic assumption
> is that gene flow between populations is significant, so that effective
> population sizes are large. There are some problems with this:  birds
> (which can fly over the mountain) should all be one species; a recently
> appeared spatially small geologic feature, like Lake Victoria, should have
> only a few species, while  a widely dispersed long lived species such as
> horseshoe crabs should show great variation. This may just prove Orgel's
> Second Law - "evolution is cleverer than you are" - or it
may mean that it
> is time for a Post Modern Synthesis (sorry Josh - sometimes I can't help
> myself).

Darwin was quite able to account for speciation - it was, he thought,
caused by selection on favourable subspecific variants. Moritz Wagner
and George Romanes both proposed that isolation would cause speciation
during Darwin's lifetime, and debated the matter with him.

Darwin said:

Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow from the struggle for life. Owing to this struggle,
variations, however slight and from whatever cause proceeding, if they
be in any degree profitable to the individuals of a species, in their
infinitely complex relations to other organic beings and to their
physical conditions of life, will tend to the preservation of such
individuals, and will generally be inherited by the offspring. The
offspring, also, will thus have a better chance of surviving, for, of
the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which each
slight variation, if useful, is preserved, by the term Natural
Selection, in order to mark its relation to man's power of selection.
[Chapter III, p51f]

and on the isolation view he says (both from the Sixth Edition):

Isolation, also, is an important element in the modification of species
through natural selection. In a confined or isolated area, if not very
large, the organic and inorganic conditions of life will generally be
almost uniform; so that natural selection will tend to modify all the
varying individuals of the same species in the same manner.
Intercrossing with the inhabitants of the surrounding districts will,
also, be thus prevented. Moritz Wagner has lately published an
interesting essay on this subject, and has shown that the service
rendered by isolation in preventing crosses between newly-formed
varieties is probably greater even than I supposed. But from reasons
already assigned I can by no means agree with this naturalist, that
migration and isolation are necessary elements for the formation of new
species. The importance of isolation is likewise great in preventing,
after any physical change in the conditions such as of climate,
elevation of the land, &c., the immigration of better adapted organisms;
and thus new places in the natural economy of the district will be left
open to be filled up by the modification of the old inhabitants. Lastly,
isolation will give time for a new variety to be improved at a slow
rate; and this may sometimes be of much importance. If, however, an
isolated area be very small, either from being surrounded by barriers,
or from having very peculiar physical conditions, the total number of
the inhabitants will be small; and this will retard the production of
new species through natural selection, by decreasing the chances of
favourable variations arising. [p79f] 

Although isolation is of great importance in the production of new
species, on the whole I am inclined to believe that largeness of area is
still more important, especially for the production of species which
shall prove capable of enduring for a long period, and of spreading
widely. Throughout a great and open area, not only will there be a
better chance of favourable variations, arising from the large number of
individuals of the same species there supported, but the conditions of
life are much more complex from the large number of already existing
species; and if some of these many species become modified and improved,
others will have to be improved in a corresponding degree, or they will
be exterminated. Each new form, also, as soon as it has been much
improved, will be able to spread over the open and continuous area, and
will thus come into competition with many other forms. Moreover, great
areas, though now continuous, will often, owing to former oscillations
of level, have existed in a broken condition; so that the good effects
of isolation will generally, to a certain extent, have concurred.
Finally, I conclude that, although small isolated areas have been in
some respects highly favourable for the production of new species, yet
that the course of modification will generally have been more rapid on
large areas; and what is more important, that the new forms produced on
large areas, which already have been victorious over many competitors,
will be those that will spread most widely, and will give rise to the
greatest number of new varieties and species. They will thus play a more
important part in the changing history of the organic world. [p80f]

Kottler in 1978 noted that Darwin's early views *were* isolationist, in
the NoteBooks...

A short excerpt from my thesis :-)

Wagner (1813-1887) was a celebrated explorer and geographer, and his
writings were influential. He had proposed, in opposition to Darwin's
notion that species are formed from racial types through selection, that
species must be isolated geographically (Wagner 1889). Mayr (1982
p562-566) discusses the reaction of Darwin and his contemporaries to
Wagner's isolation model. Dismissed by Weismann and Wallace, as we shall
see, the geographical isolation thesis was nevertheless adopted by the
Rev. Gulick, whose work on Hawaiian landsnails (gen. Achatinella) led
him also to claim that much evolutionary variation was due to chance
(Amundson 1996; Mayr 1982 p555). Thereafter, there seemed to be two
camps; those who thought that isolation was the sine qua non of
speciation and that chance was the cause of variation between
populations, and those who thought that speciation occurred equally if
not entirely through the action of natural and perhaps sexual selection.
After the later work of Sewall Wright had been accepted and promoted in
the work of Dobzhansky during the mid-twentieth century, and following
Poulton's and Mayr's coinages, this view came to be known as the
allopatric theory of speciation, and Darwin's published view as the
sympatric theory (Depew et al. 1995 p275-278).

Let's hear no more about how Darwin did not explain the origin of
species in The Origin of Species, then. I don't know where that came
from, but I suspect the Duke of Argyll's _Reign of Law_ (1867)
http://www.scholars.nus.edu.sg/landow/victorian/science/science_texts/a
rgyll/argyll_intro.htm> p 230.
-- 
John Wilkins
john_SPAM{at}wilkins.id.au   http://www.wilkins.id.au
"Men mark it when they hit, but do not mark it when they miss" 
                                               - Francis Bacon
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