CHAPTER IV - NATURAL SELECTION;
OR THE SURVIVAL OF THE FITTEST
Summary of Chapter
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. But if variations
useful to any organic being ever do occur, assuredly individuals
thus characterised 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 characterised. 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 advance in organisation.
Nevertheless, low and simple forms will long endure if well fitted for
their simple conditions of life.
Natural selection, on the principle of qualities being inherited
at corresponding ages, can modify the egg, seed, or young, as easily
as the adult. 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. 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.
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. Natural selection also 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
naturalised in foreign lands. 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. 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.
We have seen that 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. Natural selection, as has just been remarked, leads
to divergence of character and to much extinction of the less improved
and intermediate forms of life. 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 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 connection 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 everbranching and beautiful ramifications.
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