distinct progenitors. For instance, it is just possible, if our
fantail-pigeons were all destroyed, that fanciers, by striving during long
ages for the same object, might make a new breed hardly distinguishable
from our present fantail; but if the parent rock-pigeon were also
destroyed, and in nature we have every reason to believe that the
parent-form will generally be supplanted and exterminated by its improved
offspring, it is quite incredible that a fantail, identical with the
existing breed, could be raised from any other species of pigeon, or even
from the other well-established races of the domestic pigeon, for the
newly-formed fantail would be almost sure to inherit from its new
progenitor some slight characteristic differences.
Groups of species, that is, genera and families, follow the same general
rules in their appearance and disappearance as do single species, changing
more or less quickly, and in a greater or lesser degree. A group does not
reappear after it has once disappeared; or its existence, as long as it
lasts, is continuous. I am aware that there are some apparent exceptions
to this rule, but the exceptions are surprisingly few, so few, that E.
Forbes, Pictet, and Woodward (though all strongly opposed to such views as
I maintain) admit its truth; and the rule strictly accords with my theory.
For as all the species of the same group have descended from some one
species, it is clear that as long as any species of the group have appeared
in the long succession of ages, so long must its members have continuously
existed, in order to have generated either new and modified or the same old
and unmodified forms. Species of the genus Lingula, for instance, must
have continuously existed by an unbroken succession of generations, from
the lowest Silurian stratum to the present day.
We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give an
explanation of this fact, which if true would have been fatal to my views.
But such cases are certainly exceptional; the general rule being a gradual
increase in number, till the group reaches its maximum, and then, sooner or
later, it gradually decreases. If the number of the species of a genus, or
the number of the genera of a family, be represented by a vertical line of
varying thickness, crossing the successive geological formations in which
the species are found, the line will sometimes falsely appear to begin at
its lower end, not in a sharp point, but abruptly; it then gradually
thickens upwards, sometimes keeping for a space of equal thickness, and
ultimately thins out in the upper beds, marking the decrease and final
extinction of the species. This gradual increase in number of the species
of a group is strictly conformable with my theory; as the species of the
same genus, and the genera of the same family, can increase only slowly and
progressively; for the process of modification and the production of a
number of allied forms must be slow and gradual,--one species giving rise
first to two or three varieties, these being slowly converted into species,
which in their turn produce by equally slow steps other species, and so on,
like the branching of a great tree from a single stem, till the group
becomes large.
On Extinction. -- We have as yet spoken only incidentally of the
disappearance of species and of groups of species. On the theory of
natural selection the extinction of old forms and the production of new and
improved forms are intimately connected together. The old notion of all
the inhabitants of the earth having been swept away at successive periods
by catastrophes, is very generally given up, even by those geologists, as
Elie de Beaumont, Murchison, Barrande, &c., whose general views would
naturally lead them to this conclusion. On the contrary, we have every
reason to believe, from the study of the tertiary formations, that species
and groups of species gradually disappear, one after another, first from
one spot, then from another, and finally from the world. Both single
species and whole groups of species last for very unequal periods; some
groups, as we have seen, having endured from the earliest known dawn of
life to the present day; some having disappeared before the close of the
palaeozoic period. No fixed law seems to determine the length of time
during which any single species or any single genus endures. There is
reason to believe that the complete extinction of the species of a group is
generally a slower process than their production: if the appearance and
vertical line of varying thickness, the line is found to taper more
gradually at its upper end, which marks the progress of extermination, than
at its lower end, which marks the first appearance and increase in numbers
of the species. In some cases, however, the extermination of whole groups
of beings, as of ammonites towards the close of the secondary period, has
been wonderfully sudden.
The whole subject of the extinction of species has been involved in the
most gratuitous mystery. Some authors have even supposed that as the
individual has a definite length of life, so have species a definite
duration. No one I think can have marvelled more at the extinction of
species, than I have done. When I found in La Plata the tooth of a horse
embedded with the remains of Mastodon, Megatherium, Toxodon, and other
extinct monsters, which all co-existed with still living shells at a very
late geological period, I was filled with astonishment; for seeing that the
horse, since its introduction by the Spaniards into South America, has run
wild over the whole country and has increased in numbers at an unparalleled
rate, I asked myself what could so recently have exterminated the former
horse under conditions of life apparently so favourable. But how utterly
groundless was my astonishment! Professor Owen soon perceived that the
tooth, though so like that of the existing horse, belonged to an extinct
species. Had this horse been still living, but in some degree rare, no
naturalist would have felt the least surprise at its rarity; for rarity is
the attribute of a vast number of species of all classes, in all countries.
If we ask ourselves why this or that species is rare, we answer that
something is unfavourable in its conditions of life; but what that
something is, we can hardly ever tell. On the supposition of the fossil
horse still existing as a rare species, we might have felt certain from the
analogy of all other mammals, even of the slow-breeding elephant, and from
the history of the naturalisation of the domestic horse in South America,
that under more favourable conditions it would in a very few years have
stocked the whole continent. But we could not have told what the
unfavourable conditions were which checked its increase, whether some one
or several contingencies, and at what period of the horse's life, and in
what degree, they severally acted. If the conditions had gone on, however
slowly, becoming less and less favourable, we assuredly should not have
perceived the fact, yet the fossil horse would certainly have become rarer
and rarer, and finally extinct;--its place being seized on by some more
successful competitor.
It is most difficult always to remember that the increase of every living
being is constantly being checked by unperceived injurious agencies; and
that these same unperceived agencies are amply sufficient to cause rarity,
and finally extinction. We see in many cases in the more recent tertiary
formations, that rarity precedes extinction; and we know that this has been
the progress of events with those animals which have been exterminated,
either locally or wholly, through man's agency. I may repeat what I
published in 1845, namely, that to admit that species generally become rare
before they become extinct--to feel no surprise at the rarity of a species,
and yet to marvel greatly when it ceases to exist, is much the same as to
admit that sickness in the individual is the forerunner of death--to feel
no surprise at sickness, but when the sick man dies, to wonder and to
suspect that he died by some unknown deed of violence.
The theory of natural selection is grounded on the belief that each new
variety, and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition; and
the consequent extinction of less-favoured forms almost inevitably follows.
It is the same with our domestic productions: when a new and slightly
improved variety has been raised, it at first supplants the less improved
varieties in the same neighbourhood; when much improved it is transported
far and near, like our short-horn cattle, and takes the place of other
breeds in other countries. Thus the appearance of new forms and the
disappearance of old forms, both natural and artificial, are bound
together. In certain flourishing groups, the number of new specific forms
which have been produced within a given time is probably greater than that
of species has not gone on indefinitely increasing, at least during the
later geological periods, so that looking to later times we may believe
that the production of new forms has caused the extinction of about the
same number of old forms.
The competition will generally be most severe, as formerly explained and
illustrated by examples, between the forms which are most like each other
in all respects. Hence the improved and modified descendants of a species
will generally cause the extermination of the parent-species; and if many
new forms have been developed from any one species, the nearest allies of
that species, i.e. the species of the same genus, will be the most liable
to extermination. Thus, as I believe, a number of new species descended
from one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that a new
species belonging to some one group will have seized on the place occupied
by a species belonging to a distinct group, and thus caused its
extermination; and if many allied forms be developed from the successful
intruder, many will have to yield their places; and it will generally be
allied forms, which will suffer from some inherited inferiority in common.
But whether it be species belonging to the same or to a distinct class,
which yield their places to other species which have been modified and
improved, a few of the sufferers may often long be preserved, from being
fitted to some peculiar line of life, or from inhabiting some distant and
isolated station, where they have escaped severe competition. For
instance, a single species of Trigonia, a great genus of shells in the
secondary formations, survives in the Australian seas; and a few members of
the great and almost extinct group of Ganoid fishes still inhabit our fresh
waters. Therefore the utter extinction of a group is generally, as we have
seen, a slower process than its production.
With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the palaeozoic period and of
Ammonites at the close of the secondary period, we must remember what has
been already said on the probable wide intervals of time between our
consecutive formations; and in these intervals there may have been much
slow extermination. Moreover, when by sudden immigration or by unusually
rapid development, many species of a new group have taken possession of a
new area, they will have exterminated in a correspondingly rapid manner
many of the old inhabitants; and the forms which thus yield their places
will commonly be allied, for they will partake of some inferiority in
common.
Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it be
at our presumption in imagining for a moment that we understand the many
complex contingencies, on which the existence of each species depends. If
we forget for an instant, that each species tends to increase inordinately,
and that some check is always in action, yet seldom perceived by us, the
whole economy of nature will be utterly obscured. Whenever we can
precisely say why this species is more abundant in individuals than that;
why this species and not another can be naturalised in a given country;
then, and not till then, we may justly feel surprise why we cannot account
for the extinction of this particular species or group of species.
On the Forms of Life changing almost simultaneously throughout the World. -
- Scarcely any palaeontological discovery is more striking than the fact,
that the forms of life change almost simultaneously throughout the world.
Thus our European Chalk formation can be recognised in many distant parts
of the world, under the most different climates, where not a fragment of
the mineral chalk itself can be found; namely, in North America, in
equatorial South America, in Tierra del Fuego, at the Cape of Good Hope,
and in the peninsula of India. For at these distant points, the organic
remains in certain beds present an unmistakeable degree of resemblance to
those of the Chalk. It is not that the same species are met with; for in
some cases not one species is identically the same, but they belong to the
same families, genera, and sections of genera, and sometimes are similarly
characterised in such trifling points as mere superficial sculpture.
Moreover other forms, which are not found in the Chalk of Europe, but which
occur in the formations either above or below, are similarly absent at
these distant points of the world. In the several successive palaeozoic
formations of Russia, Western Europe and North America, a similar
parallelism in the forms of life has been observed by several authors: so
it is, according to Lyell, with the several European and North American
tertia
ry deposits. Even if the few fossil species which are common to the
Old and New Worlds be kept wholly out of view, the general parallelism in
the successive forms of life, in the stages of the widely separated
palaeozoic and tertiary periods, would still be manifest, and the several
formations could be easily correlated.
These observations, however, relate to the marine inhabitants of distant
parts of the world: we have not sufficient data to judge whether the
productions of the land and of fresh water change at distant points in the
same parallel manner. We may doubt whether they have thus changed: if the
Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to Europe
from La Plata, without any information in regard to their geological
position, no one would have suspected that they had coexisted with still
living sea-shells; but as these anomalous monsters coexisted with the
Mastodon and Horse, it might at least have been inferred that they had
lived during one of the latter tertiary stages.
When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same thousandth or hundred-thousandth year, or
even that it has a very strict geological sense; for if all the marine
animals which live at the present day in Europe, and all those that lived
in Europe during the pleistocene period (an enormously remote period as
measured by years, including the whole glacial epoch), were to be compared
with those now living in South America or in Australia, the most skilful
naturalist would hardly be able to say whether the existing or the
pleistocene inhabitants of Europe resembled most closely those of the
southern hemisphere. So, again, several highly competent observers believe
that the existing productions of the United States are more closely related
to those which lived in Europe during certain later tertiary stages, than
to those which now live here; and if this be so, it is evident that
fossiliferous beds deposited at the present day on the shores of North
America would hereafter be liable to be classed with somewhat older
European beds. Nevertheless, looking to a remotely future epoch, there
can, I think, be little doubt that all the more modern marine formations,
namely, the upper pliocene, the pleistocene and strictly modern beds, of
Europe, North and South America, and Australia, from containing fossil
remains in some degree allied, and from not including those forms which are
only found in the older underlying deposits, would be correctly ranked as
simultaneous in a geological sense.
The fact of the forms of life changing simultaneously, in the above large
sense, at distant parts of the world, has greatly struck those admirable
observers, MM. de Verneuil and d'Archiac. After referring to the
parallelism of the palaeozoic forms of life in various parts of Europe,
they add, 'If struck by this strange sequence, we turn our attention to
North America, and there discover a series of analogous phenomena, it will
appear certain that all these modifications of species, their extinction,
and the introduction of new ones, cannot be owing to mere changes in marine
currents or other causes more or less local and temporary, but depend on
general laws which govern the whole animal kingdom.' M. Barrande has made
forcible remarks to precisely the same effect. It is, indeed, quite futile
to look to changes of currents, climate, or other physical conditions, as
the cause of these great mutations in the forms of life throughout the
world, under the most different climates. We must, as Barrande has
remarked, look to some special law. We shall see this more clearly when we
treat of the present distribution of organic beings, and find how slight is
the relation between the physical conditions of various countries, and the
nature of their inhabitants.