represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during each former year may represent the long
succession of extinct species. At each period of growth all the growing
twigs have tried to branch out on all sides, and to overtop and kill
the surrounding twigs and branches, in the same manner as species and
groups of species have tried to overmaster other species in the great
battle for life. The limbs divided into great branches, and these into
lesser and lesser branches, were themselves once, when the tree was
small, 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 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 V.
LAWS OF VARIATION.
103