Charles Darwin 15and 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 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 from being found in a
fossil state.... 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.
(129–130)Darwin still had a little more mop-up work to do. He had but scanty (and gen-
erally wrong) ideas about the principles of heredity — what we today call genetics
[Vorzimmer, 1970]. He knew that somehow features — new variations — had to
be transmitted from one generation to the next. But beyond a hodgepodge of
farmers’ lore and old wives’ tales and fanciers’ convictions, he had no real overall
picture. One basic problem was that Darwin thought that features would tend
to be blended from generation to generation — which of course is true in many
respects at the physical level – but at this time he had no underlying theory to
explain either the blending or the many exceptions. (A paradox often noted by
historians was, at this time, the obscure Moravian monk Gregor Mendel was un-
weaving the tangle, starting to grasp the true principle of heredity. However, it
seems generally agreed that even had Darwin had Mendel’s work, he would not
necessarily have recognized it for what it was. Mendel knew of Darwin and did
not think he (Mendel) had made a breakthrough. When Mendel was rediscov-
ered at the beginning of the twentieth century, it took a decade or more before
people really grasped that Mendel and Darwin are complementaries rather than
contradictories.)