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Charles Darwin on Phylogeny and "Tree-thinking"
From Chapter 4 of The Origin of Species, Natural
Section "Descendants of a Common Ancestor"
It is a truly wonderful fact--the wonder of which we are apt to
overlook from familiarity--that all animals and all plants throughout all time
and space should be related to each other in groups, subordinate to groups, in
the manner which we everywhere behold--namely, varieties of the same species
most closely related, species of the same genus less closely and unequally
related, forming sections and sub-genera, species of distinct genera much less
closely related, and genera related in different degrees, forming sub-families,
families, orders, sub-classes, and classes. The several subordinate groups in
any class cannot be ranked in a single file, but seem clustered round points,
and these round other points, and so on in almost endless cycles. If species had
been independently created, no explanation would have been possible of this kind
of classification; but 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 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.
Darwin, C. (1872), pp. 170-171. The Origin of Species. Sixth Edition.
The Modern Library, New York.