extinct species. At each period of growth all the growing twigs have tried
to branch out on all sides, and 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
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 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
favored 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 ramification."


It may also be noted that there is a significant correspondence between the