Biology Reference
In-Depth Information
Fig. 4.16
Transitional Fossils
Unlike microevolution, macroevolution is much too slow to be seen directly but
its occurrence is inferred - just as electrons cannot be observed directly but their
existence is inferred. Macroevolution is inferred from the fossil record and from
the principle of uniformitarianism (Fig. 4.17). This principle was formulated by the
Scottish geologist James Hutton in the late eighteenth century. He proposed that
rock is being continually eroded and washed down into the seas, where it sediments
into layers and is compressed back into rock. This sedimentary rock is then uplifted
out of the water by earthquakes so that the erosion cycle is repeated. The observed
thickness of sedimentary rocks suggested that this process has been continuing for
very long periods of time i.e. it was a “uniform” process. This idea led to the gener-
alisation that the continued operation of processes observable today could account
for the geology of the planet if prolonged over millions of years.
Figure 4.17 defines this principle - the idea that the present is the key to the past.
This view was very influential in the development of both biology and geology in
the eighteenth and nineteenth centuries. This principle stems from the naturalistic
view that the world is governed by unvarying regularities, and is also an exam-
ple of Occam's razor in action, because it is based on the simplest hypothesis -
that processes acting now also operated in the past. But the principle does not rule
