lanation of this great fact in the classification of all
organic beings; but, to the best of my judgment, it is explained through
inheritance and the complex action of natural selection, entailing
extinction and divergence of character, as we have seen illustrated in
the diagram.

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 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 surfac