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 all the other branches; so with the species which lived
during long-past geological periods, very few now have living and
modified descendants. From the first growth of the tree, many a limb
and branch has decayed and dropped off; and these lost 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 from having been found 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.
CHAPTER 5. LAWS OF VARIATION.
Effects of external conditions.
Use and disuse, combined with natural selection; organs of flight and
of vision.
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