Land and water have been continually shifting their
positions; some regions are undergoing subsidence with diminution of
area, others elevation with extension of area; dry land has been
converted into marshes, while marshes have been drained or have even
been elevated into plateaux. Climate too has changed again and again,
either through the elevation of mountains in high latitudes leading to
the accumulation of snow and ice, or by a change in the direction of
winds and ocean currents produced by the subsidence or elevation of
lands which connected continents and divided oceans. Again, along with
all these changes have come not less important changes in the
distribution of species. Vegetation has been greatly modified by changes
of climate and of altitude; while every union of lands before separated
has led to extensive migrations of animals into new countries,
disturbing the balance that before existed among its forms of life,
leading to the extermination of some species and the increase of others.
When such physical changes as these have taken place, it is evident that
many species must either become modified or cease to exist.
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