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 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 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 genera 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.

CHAPTER V. LAWS OF VARIATION.

Effects of changed conditions--Use and disuse, combined with natural
selection; organs of flight and of vision--Acclimatisation--Correlated
variation--Compensation and economy of growth--False
correlations--Multiple, rudimentary, and lowly organised structures
variable--Parts developed in an unusual man