"The affinities of all the beings of the same class
have sometimes been 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 former years
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 at all times overmastered 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 young, budding twigs;
and this connection 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 generations 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."

~ Charles Darwin

Source: Moonlight Serenade♥