tailing 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 {130} 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.

* * * * *

{131}

CHAPTER V.

LAWS OF VARIATION.

Effects of external conditions--U