Re: How did life forms get distributed to widely separated islands?

You are actually asking two, somewhat distinct questions here. One is the 
question of biogeography; How did the current distribution of animals come 
to be? Depending on the circumstances and what you take to be an island (Is 
Australia the world's smallest continent or largest island?), current 
island biotas may be anywhere from a few thousand to tens of millions of 
years old. That is to say, they have been in the process of forming for 
that length of time. It all depends on when the island came into existence.

The other question is one of continental drift, which occurs over many 
millions of years. The question of how the biota of one continent is 
similar to or became different from that of another is almost, but not 
quite, the same as the question of how the biota of an island is similar to 
or became different from that of some other island or a related continent. 
I perceive that your question is more of the former and will dwell on that, 
especially considering that it is closer to my areas of specialty (I agree 
that it is hard to imagine Pangea, although most of the pacific islands are 
far too young to be considered derivatives of that formidable 
protocontinent).

There are actually two kinds of islands (we're talking islands here, not 
rocks in a lake), continental islands and oceanic islands. Continental 
islands share their origin and characteristics with the coast of a 
continent. They lie on a continental shelf, as opposed to sprouting up from 
the oceanic basin. Good examples of continental islands are Madagascar, 
Tasmania, and Bali.

Oceanic islands form by underwater vulcanism (they are the tops of huge 
volcanoes) or the accumulation of coral reefs, or, as if often the case, 
both. They are naturally younger than most continental islands, and 
consequently, have had less time to develop a characteristic biota. Good 
examples of oceanic islands include the Galapagos and Hawaiian Islands.

The biota of continental islands tends to have similarities with the 
continent to which they are connected. However, as continental islands may 
be separated from their associated continent by straits of varying widths 
and depths; shallow and wide, shallow and narrow, deep and wide, deep and 
narrow, it stands to reason that the degree of similarity will vary. 
Islands with shallow straits tend to be reconnected frequently by changing 
sea level. With each new reconnection and separation, migratory events 
allow relatively free mixing of the biotas, thereby altering the original 
condition established when the island was first separated from the 
continent with its allocated sub population of animals and plants.

Tasmania is a good example of an island with a shallow connection. It is 
the high end of a knob shaped peninsula which juts out from the southeast 
part of Australia. Thirteen or fourteen thousand years ago the sea level 
was several hundred feet lower than it is now and there was a broad land 
bridge more than 100 miles long connecting Tasmania to the Australian 
continent.

Continental islands with deep straits may have never been reconnected since 
the original rift. Madagascar is such an island. It literally fell away 
from the southeastern coast of Africa, perhaps as long ago as 90 million 
years, as an effect of continental drift. Consequently, the strait which 
separates it from Africa is very deep and not likely to be reconnected to 
the mainland by any sea level change.

How do animals and plants get to these islands other than walking over a 
land bridge (well, plants don't exactly walk but that's another story)? 
They fly, swim, raft, or are blown onto the islands. It may be a slow, 
tedious process but, over thousands of years, colonization happens. Plants 
can also raft or be carried stuck to rafting or flying animals. Seeds have 
even been known to be transported in the mud stuck to the foot of a bird.

Oceanic islands are colonized in the same way except for the fact that 
remoteness from continents may dictate any similarities of the biota of the 
given island to that of some other place. The Galapagos islands were 
clearly colonized from South America. The Hawaiian Islands, so far from any 
continent, have been colonized from multiple sources and have a very unique 
biota (much threatened on many of the islands by recent introductions of 
"exotic" plants and animals as well as rapid, modern development).

With this background in place, I would like to address your question 
directly. You stated: "Many land based animal and vegetable life forms, of 
similar and identical species even, live on widely scattered islands--so 
widely scattered, in fact, that it is hard to imagine they were ever 
connected in the past by land." Well, to put it bluntly, this is not true.

Yes, many of the species found on Tasmania are the same as those on 
Australia and some of the animals and plants that occupy Madagascar are 
similar to or the same as those on the African continent but these 
similarities are easily explained and the biotas of both islands have 
unique elements. Madagascar (in its natural state) is dominated by a flora 
and fauna almost completely unique to that island. Tasmania harbors a 
number of rare Australia animals which have gone completely extinct on the 
mainland. The general assertion that there are similar or identical species 
on widely scattered islands is simply not true.

Finally, you ask: "How did these life forms get from their supposedly 
unique places of origin to all the places where they live today?" I think 
I've explained how organisms move from continents to islands, but the truth 
is, its on islands were we see the origin of many new species. For example, 
there are about 14 species of finches on the Galapagos Islands that are 
found nowhere else in the world. The majority of "supposedly unique places 
of origin" are the islands themselves.

There are some common elements on far flung islands. Many are (or were) 
inhabited by giant land tortoises. These reflect ancient arrivals of 
ancestors on rafts, or by simply floating from nearby continents. The point 
is, each island has its own species which occurs in no other place. The 
Galapagos have several species, each on different islands.

There is a great book you should read for further information. Its "The 
Song of the Dodo: Island Biogeography in an Age of Extinction" by David 
Quammen. Its written in a very readable style. You can find it at any of 
the usual book sellers, such as Amazon:
 http://www.amazon.com/exec/obidos/search-handle-form/002-5762329-1842442