CHAPTER XI - ON THE GEOLOGICAL SUCCESSION
OF ORGANIC BEINGS
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 regions, 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 unmistakable
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, occur in the same order 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 European and North American tertiary
deposits. Even if the few fossil species which are common to the Old
and New Worlds were kept wholly out of view, the general parallelism
in the successive forms of life, in the palaeozoic and tertiary
stages, would still be manifest, and the several formations could be
easily correlated.
These observations, however, relate to the marine inhabitants of the
world: we have not sufficient data to judge whether the productions of
the land and of fresh water at distant points change 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
co-existed with seashells all still living; but as these anomalous
monsters co-existed with the mastodon and horse, it might at least
have been inferred that they had lived during one of the later
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 year, or to the same century, or even
that it has a very strict geological sense; for if all the marine
animals now living in Europe, and all those that lived in Europe
during the pleistocene period (a very remote period as measured by
years, including the whole glacial epoch) were compared with those now
existing in South America or in Australia, the most skilful naturalist
would hardly be able to say whether the present or the pleistocene
inhabitants of Europe resembled most closely those of the southern
hemisphere. So, again, several highly competent observers maintain
that the existing productions of the United States are more closely
related to those which lived in Europe during certain late tertiary
stages, than to the present inhabitants of Europe; and if this be
so, it is evident that fossiliferous beds now deposited 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 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 found only 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
these admirable observers, MM. de Verneuil and d'Archiae. 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.
This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural
selection. New species are formed by having some advantage over
older forms; and the forms, which are already dominant, or have some
advantage over the other forms in their own country, give birth to the
greatest number of new varieties or incipient species. We have
distinct evidence on this head, in the plants which are dominant, that
is, which are commonest and most widely diffused, producing the
greatest number of new varieties. It is also natural that the
dominant, varying, and far-spreading species, which have already
invaded to a certain extent the territories of other species, should
be those which would have the best chance of spreading still
further, and of giving rise in new countries to other new varieties
and species. The process of diffusion would often be very slow,
depending on climatal and geographical changes, on strange
accidents, and on the gradual acclimatisation of new species to the
various climates through which they might have to pass, but in the
course of time the dominant forms would generally succeed in spreading
and would ultimately prevail. The diffusion would, it is probable,
be slower with the terrestrial inhabitants of distinct continents than
with the marine inhabitants of the continuous sea. We might
therefore expect to find, as we do find, a less strict degree of
parallelism in the succession of the productions of the land than with
those of the sea.
Thus, as it seems to me, the parallel, and, taken in a large
sense, simultaneous, succession of the same forms of life throughout
the world, accords well with the principle of new species having
been formed by dominant species spreading widely and varying; the
new species thus produced being themselves dominant, owing to their
having had some advantage over their already dominant parents, as well
as over other species, and again spreading, varying, and producing new
forms. The old forms which are beaten and which yield their places
to the new and victorious forms, will generally be allied in groups,
from inheriting some inferiority in common; and therefore, as new
and improved groups spread throughout the world, old groups
disappear from the world; and the succession of forms everywhere tends
to correspond both in their first appearance and final disappearance.
There is one other remark connected with this subject worth
making. I have given my reasons for believing that most of our great
formations, rich in fossils, were deposited during periods of
subsidence; and that blank intervals of vast duration, as far as
fossils are concerned, occurred during the periods when the bed of the
sea was either stationary or rising, and likewise when sediment was
not thrown down quickly enough to embed and preserve organic
remains. During these long and blank intervals I suppose that the
inhabitants of each region underwent a considerable amount of
modification and extinction, and that there was much migration from
other parts of the world. As we have reason to believe that large
areas are affected by the same movement, it is probable that
strictly contemporaneous formations have often been accumulated over
very wide spaces in the same quarter of the world; but we are very far
from having any right to conclude that this has invariably been the
case, and that large areas have invariably been affected by the same
movements. When two formations have been deposited in two regions
during nearly, but not exactly, the same period, we should find in
both, from the causes explained in the foregoing paragraphs, the
same general succession in the forms of life; but the species would
not exactly correspond; for there will have been a little more time in
the one region than in the other for modification, extinction, and
immigration.
I suspect that cases of this nature occur in Europe. Mr.
Prestwich, in his admirable Memoirs on the eocene deposits of
England and France, is able to draw a close general parallelism
between the successive stages in the two countries; but when he
compares certain stages in England with those in France, although he
finds in both a curious accordance in the numbers of the species
belonging to the same genera, yet the species themselves differ in a
very, difficult to account for, considering the proximity of the two
areas,- unless, indeed, it be assumed that an isthmus separated two
seas inhabited by distinct, but contemporaneous, faunas. Lyell has
made similar observations on some of the later tertiary formations.
Barrande, also, shows that there is a striking general parallelism
in the successive Silurian deposits of Bohemia and Scandinavia;
nevertheless he finds a surprising amount of difference in the
species. If the several formations in these regions have not been
deposited during the same exact periods,- a formation in one region
often corresponding with a blank interval in the other,- and if in
both regions the species have gone on slowly changing during the
accumulation of the several formations and during the long intervals
of time between them; in this case the several formations in the two
regions could be arranged in the same order, in accordance with the
general succession of the forms of life, and the order would falsely
appear to be strictly parallel; nevertheless the species would not
be all the same in the apparently corresponding stages in the two
regions.
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