The Origin
of Species: By Means of Natural Selection of the Preservation of Favoured Races in the Struggle for Life.
– Summary by
Susan Fleck
Summary of Chapter
4 – Natural Selection; or The Survival of the Fittest
1.
If under changing
conditions of life organic beings present individual differences in almost
every part of their structure, and this cannot be disputed; if there be, owing
to their geometrical rate of increase, a severe struggle for life at some age,
season, or year, and this certainly cannot be disputed; then, considering the
infinite complexity of the relations of all organic beings to each other and to
their conditions of life, causing an infinite diversity in structure,
constitution, and habits, to be advantageous to them, it would be a most
extraordinary fact if no variations had ever occurred useful to each being’s
own welfare, in the same manner as so many variations have occurred useful to
man [in domestic productions].
2.
But if variations
useful to any organic being ever do occur, assuredly individuals thus characterized
will have the best chance of being preserved in the struggle for life; and from
the strong principle of inheritance, these will tend to produce offspring
similarly characterized.
3.
This principle of
preservation, or the survival of the fittest, I have called Natural Selection.
It leads to the improvement of each creature in relation to its organic and
inorganic conditions of life; and consequently, in most cases, to what must be
regarded as an advancement in organization.
4.
Nevertheless, low
and simple forms will long endure if well fitted for their simple conditions in
life.
5.
Natural selection,
on the principle of qualities being inherited at corresponding ages, can modify
the egg, seed, or young, as easily as the adult.
6.
Amongst many
animals, sexual selection will have given its aid to ordinary selection, by
assuring to the most vigorous and best adapted males the greatest number of
offspring.
7.
Sexual selection
will also give characters useful to the males alone, in their struggles or
rivalry with other males; and these characters will be transmitted to one sex
or to both sexes, according to the form of inheritance which prevails.
8.
Whether natural
selection has really thus acted in adapting the various forms of life to their
several conditions and stations, must be judged by the general tenor and
balance of evidence given in the following chapters. But we have already seen
how it entails extinction and how largely extinction has acted in the world’s
history, geology plainly declares.
9.
Natural selection
leads to divergence of character; for the more organic beings diverge in
structure, habits, and constitution, by so much the more can a large number be
supported on the area,--of which we see proof by looking to the inhabitants of
any small spot, and to the productions naturalized in foreign lands.
10.Therefore,
during the modification of the descendants of any one species, and during the
incessant struggle of all species to increase in numbers, the more diversified
the descendants become, the better will be their chance of success in the
battle for life.
11.Thus
the small differences distinguishing varieties of the same species, steadily
tend to increase, till they equal the greater differences between species of
the same genus, or even of distinct genera.
12.It is
the common, the widely-diffused and widely-ranging species, belonging to the
larger genera within each class, which vary most; and these tend to transmit to
their modified offspring that superiority which now makes them dominant in
their own countries. [Note: ‘larger’ does not mean physical size; it means
those genera that have the most number of species i.e., the most divergent.]
13.Those
species that have diverged widely, leads to much extinction of the less
improved and intermediate forms of life.
14.On
these principles, the nature of the affinities, and the generally well-defined
distinctions between the innumerable organic beings in each class throughout
the world, may be explained. It is a wonderful fact 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, etc. –to
forming families, orders, sub-classes and classes.
15.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.
16.The
affinities of all the beings of the same class have sometimes been represented
by a great tree. 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, wee themselves once, when the tree was young, 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 and 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, 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. Occasionally we see a straggling branch, which by some conditions of
life has been favored and is still alive. . . 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.