The
limbs divided into great branches, and these into lesser and lesser
branches, were themselves once, when the tree was young, budding twigs; and
this connexion of the former and present buds by ramifying branches may
well represent the classification of all extinct and living species in
groups subordinate to groups. Of the many twigs which flourished when the
tree was a mere bush, only two or three, now grown into great branches, yet
survive and bear the other branches; so with the species which lived during
long-past geological periods, very few have left living and modified
descendants. From the first growth of the tree, many a limb and branch has
decayed and dropped off; and these fallen branches of various sizes may
represent those whole orders, families, and genera which have now no living
representatives, and which are known to us only in a fossil state. As we
here and there see a thin, straggling branch springing from a fork low down
in a tree, and which by some chance has been favoured and is still alive on
its summit, so we occasionally see an animal like the Ornithorhynchus or
Lepidosiren, which in some small degree connects by its affinities two
large branches of life, and which has apparently been saved from fatal
competition by having inhabited a protected station. As buds give rise by
growth to fresh buds, and these, if vigorous, branch out and overtop on all
sides many a feebler branch, so by generation I believe it has been with
the great Tree of Life, which fills with its dead and broken branches the
crust of the earth, and covers the surface with its ever-branching and
beautiful ramifications.
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