for we well know that many groups, formerly most extensively developed,
have now become extinct. Looking still more remotely to the future, we may
predict that, owing to the continued and steady increase of the larger
groups, a multitude of smaller groups will become utterly extinct, and
leave no modified descendants; and consequently that of the species living
at any one period, extremely few will transmit descendants to a remote
futurity. I shall have to return to this subject in the chapter on
Classification, but I may add that on this view of extremely few of the
more ancient species having transmitted descendants, and on the view of all
the descendants of the same species making a class, we can understand how
it is that there exist but very few classes in each main division of the
animal and vegetable kingdoms. Although extremely few of the most ancient
species may now have living and modified descendants, yet at the most
remote geological period, the earth may have been as well peopled with many
species of many genera, families, orders, and classes, as at the present
day.
Summary of Chapter -- If during the long course of ages and under varying
conditions of life, organic beings vary at all in the several parts of
their organisation, and I think this cannot be disputed; if there be, owing
to the high geometrical powers of increase of each species, at some age,
season, or year, a severe struggle for life, 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 existence, causing
an infinite diversity in structure, constitution, and habits, to be
advantageous to them, I think it would be a most extraordinary fact if no
variation ever had occurred useful to each being's own welfare, in the same
way as so many variations have occurred useful to man. But if variations
useful to any organic being 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 they will tend to
produce offspring similarly characterised. This principle of preservation,
I have called, for the sake of brevity, Natural Selection. 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 give 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 with other males.
Whether natural selection has really thus acted in nature, in modifying and
adapting the various forms of life to their several conditions and
stations, must be judged of by the general tenour and balance of evidence
given in the following chapters. But we already see 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 more living beings can be supported on the same area the
more they diverge in structure, habits, and constitution, of which we see
proof by looking at the inhabitants of any small spot or at naturalised
productions. 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 these descendants become, the better will
be their chance of succeeding in the battle of life. Thus the small
differences distinguishing varieties of the same species, will steadily
tend to increase till they come to 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, which vary most; and these will
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, I
believe, the nature of the affinities of all organic beings 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 group subordinate to
group, in the manner which we everywhere behold--namely, varieties of the
closely and unequally related together, 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 rather to be clustered round points, and these round
other points, and so on in almost endless cycles. On the view that each
species has been independently created, I can see no explanation of this
great fact in the classification of all organic beings; but, to the best of
my judgment, 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 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 all the other branches; so with the species which lived
during long-past geological periods, very few now have living and modified
descendants. From the first growth of the tree, many a limb and branch has
decayed and dropped off; and these lost 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 having been found 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 st
ill 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 external conditions -- Use and disuse, combined with natural
selection; organs of flight and of vision -- Acclimatisation -- Correlation
of growth -- Compensation and economy of growth -- False correlations --
Multiple, rudimentary, and lowly organised structures variable -- Parts
developed in an unusual manner are highly variable: specific characters
more variable than generic: secondary sexual characters variable --
Species of the same genus vary in an analogous manner -- Reversions to long
lost characters -- Summary.
I have hitherto sometimes spoken as if the variations--so common and
multiform in organic beings under domestication, and in a lesser degree in
those in a state of nature--had been due to chance. This, of course, is a
wholly incorrect expression, but it serves to acknowledge plainly our
ignorance of the cause of each particular variation. Some authors believe
it to be as much the function of the reproductive system to produce
individual differences, or very slight deviations of structure, as to make
the child like its parents. But the much greater variability, as well as
the greater frequency of monstrosities, under domestication or cultivation,
than under nature, leads me to believe that deviations of structure are in
and their more remote ancestors have been exposed during several
generations. I have remarked in the first chapter--but a long catalogue of
facts which cannot be here given would be necessary to show the truth of
the remark--that the reproductive system is eminently susceptible to
changes in the conditions of life; and to this system being functionally
disturbed in the parents, I chiefly attribute the varying or plastic
condition of the offspring. The male and female sexual elements seem to be
affected before that union takes place which is to form a new being. In
the case of 'sporting' plants, the bud, which in its earliest condition
does not apparently differ essentially from an ovule, is alone affected.
But why, because the reproductive system is disturbed, this or that part
should vary more or less, we are profoundly ignorant. Nevertheless, we can
here and there dimly catch a faint ray of light, and we may feel sure that
there must be some cause for each deviation of structure, however slight.
How much direct effect difference of climate, food, &c., produces on any
being is extremely doubtful. My impression is, that the effect is
extremely small in the case of animals, but perhaps rather more in that of
plants. We may, at least, safely conclude that such influences cannot have
produced the many striking and complex co-adaptations of structure between
one organic being and another, which we see everywhere throughout nature.
Some little influence may be attributed to climate, food, &c.: thus, E.
Forbes speaks confidently that shells at their southern limit, and when
living in shallow water, are more brightly coloured than those of the same
species further north or from greater depths. Gould believes that birds of
the same species are more brightly coloured under a clear atmosphere, than
when living on islands or near the coast. So with insects, Wollaston is
convinced that residence near the sea affects their colours. Moquin-Tandon
gives a list of plants which when growing near the sea-shore have their
leaves in some degree fleshy, though not elsewhere fleshy. Several other
such cases could be given.
The fact of varieties of one species, when they range into the zone of
habitation of other species, often acquiring in a very slight degree some
of the characters of such species, accords with our view that species of
all kinds are only well-marked and permanent varieties. Thus the species
of shells which are confined to tropical and shallow seas are generally
brighter-coloured than those confined to cold and deeper seas. The birds
which are confined to continents are, according to Mr. Gould,
brighter-coloured than those of islands. The insect-species confined to
sea-coasts, as every collector knows, are often brassy or lurid. Plants
which live exclusively on the sea-side are very apt to have fleshy leaves.
He who believes in the creation of each species, will have to say that this
shell, for instance, was created with bright colours for a warm sea; but
that this other shell became bright-coloured by variation when it ranged
into warmer or shallower waters.
When a variation is of the slightest use to a being, we cannot tell how
much of it to attribute to the accumulative action of natural selection,
and how much to the conditions of life. Thus, it is well known to furriers
that animals of the same species have thicker and better fur the more
severe the climate is under which they have lived; but who can tell how
much of this difference may be due to the warmest-clad individuals having
been favoured and preserved during many generations, and how much to the
direct action of the severe climate? for it would appear that climate has
some direct action on the hair of our domestic quadrupeds.
Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the other
hand, of different varieties being produced from the same species under the
same conditions. Such facts show how indirectly the conditions of life
must act. Again, innumerable instances are known to every naturalist of
species keeping true, or not varying at all, although living under the most
opposite climates. Such considerations as these incline me to lay very
little weight on the direct action of the conditions of life. Indirectly,
as already remarked, they seem to play an important part in affecting the
reproductive system, and in thus inducing variability; and natural
selection will then accumulate all profitable variations, however slight,
until they become plainly developed and appreciable by us.
Effects of Use and Disuse. -- From the facts alluded to in the first
chapter, I think there can be little doubt that use in our domestic animals
strengthens and enlarges certain parts, and disuse diminishes them; and
that such modifications are inherited. Under free nature, we can have no
standard of comparison, by which to judge of the effects of long-continued
use or disuse, for we know not the parent-forms; but many animals have
&nb
sp; structures which can be explained by the effects of disuse. As Professor
Owen has remarked, there is no greater anomaly in nature than a bird that
cannot fly; yet there are several in this state. The logger-headed duck of
South America can only flap along the surface of the water, and has its
wings in nearly the same condition as the domestic Aylesbury duck. As the
larger ground-feeding birds seldom take flight except to escape danger, I
believe that the nearly wingless condition of several birds, which now
inhabit or have lately inhabited several oceanic islands, tenanted by no
beast of prey, has been caused by disuse. The ostrich indeed inhabits
continents and is exposed to danger from which it cannot escape by flight,
but by kicking it can defend itself from enemies, as well as any of the
smaller quadrupeds. We may imagine that the early progenitor of the
ostrich had habits like those of a bustard, and that as natural selection
increased in successive generations the size and weight of its body, its
legs were used more, and its wings less, until they became incapable of
flight.
Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are very often broken
off; he examined seventeen specimens in his own collection, and not one had
even a relic left. In the Onites apelles the tarsi are so habitually lost,
that the insect has been described as not having them. In some other
genera they are present, but in a rudimentary condition. In the Ateuchus
or sacred beetle of the Egyptians, they are totally deficient. There is
not sufficient evidence to induce us to believe that mutilations are ever
inherited; and I should prefer explaining the entire absence of the
anterior tarsi in Ateuchus, and their rudimentary condition in some other
genera, by the long-continued effects of disuse in their progenitors; for
as the tarsi are almost always lost in many dung-feeding beetles, they must
be lost early in life, and therefore cannot be much used by these insects.
In some cases we might easily put down to disuse modifications of structure
which are wholly, or mainly, due to natural selection. Mr. Wollaston has
discovered the remarkable fact that 200 beetles, out of the 550 species
inhabiting Madeira, are so far deficient in wings that they cannot fly; and
that of the twenty-nine endemic genera, no less than twenty-three genera
have all their species in this condition! Several facts, namely, that
beetles in many parts of the world are very frequently blown to sea and
perish; that the beetles in Madeira, as observed by Mr. Wollaston, lie much
concealed, until the wind lulls and the sun shines; that the proportion of
wingless beetles is larger on the exposed Dezertas than in Madeira itself;
and especially the extraordinary fact, so strongly insisted on by Mr.
Wollaston, of the almost entire absence of certain large groups of beetles,
elsewhere excessively numerous, and which groups have habits of life almost
necessitating frequent flight;--these several considerations have made me
believe that the wingless condition of so many Madeira beetles is mainly
due to the action of natural selection, but combined probably with disuse.
For during thousands of successive generations each individual beetle which
flew least, either from its wings having been ever so little less perfectly
developed or from indolent habit, will have had the best chance of
surviving from not being blown out to sea; and, on the other hand, those
beetles which most readily took to flight will oftenest have been blown to
sea and thus have been destroyed.
The insects in Madeira which are not ground-feeders, and which, as the
flower-feeding coleoptera and lepidoptera, must habitually use their wings
to gain their subsistence, have, as Mr. Wollaston suspects, their wings not
at all reduced, but even enlarged. This is quite compatible with the