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 new varieties arising, which have some advantage over older
forms; and those forms, which are already dominant, or have some advantage
over the other forms in their own country, would naturally oftenest give
rise to new varieties or incipient species; for these latter must be
victorious in a still higher degree in order to be preserved and to
survive. We have distinct evidence on this head, in the plants which are
dominant, that is, which are commonest in their own homes, and are most
widely diffused, having produced the greatest number of new varieties. It
is also natural that the dominant, varying, and far-spreading species,
which already have 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 new varieties and
species. The process of diffusion may often be very slow, being dependent
on climatal and geographical changes, or on strange accidents, but in the
long run the dominant forms will generally succeed in spreading. 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 apparently do find, a less
strict degree of parallel succession in the productions of the land than of
the sea.
Dominant species spreading from any region might encounter still more
dominant species, and then their triumphant course, or even their
existence, would cease. We know not at all precisely what are all the
conditions most favourable for the multiplication of new and dominant
species; but we can, I think, clearly see that a number of individuals,
from giving a better chance of the appearance of favourable variations, and
that severe competition with many already existing forms, would be highly
favourable, as would be the power of spreading into new territories. A
certain amount of isolation, recurring at long intervals of time, would
probably be also favourable, as before explained. One quarter of the world
may have been most favourable for the production of new and dominant
species on the land, and another for those in the waters of the sea. If
two great regions had been for a long period favourably circumstanced in an
equal degree, whenever their inhabitants met, the battle would be prolonged
and severe; and some from one birthplace and some from the other might be
victorious. But in the course of time, the forms dominant in the highest
degree, wherever produced, would tend everywhere to prevail. As they
prevailed, they would cause the extinction of other and inferior forms; and
as these inferior forms would be allied in groups by inheritance, whole
groups would tend slowly to disappear; though here and there a single
member might long be enabled to survive.
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 inheritance, and to having
already had some advantage over their parents or over other species; these
again spreading, varying, and producing new species. The 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 will disappear from the world; and the succession of forms in both
ways will everywhere tend to correspond.
There is one other remark connected with this subject worth making. I have
given my reasons for believing that all our greater fossiliferous
formations were deposited during periods of subsidence; and that blank
intervals of vast duration 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 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 have occurred 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 manner 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 form of life, and the order
would falsely appear to be strictly parallel; nevertheless the species
would not all be the same in the apparently corresponding stages in the two
regions.
On the Affinities of extinct Species to each other, and to living forms. --
Let us now look to the mutual affinities of extinct and living species.
They all fall
into one grand natural system; and this fact is at once
explained on the principle of descent. The more ancient any form is, the
more, as a general rule, it differs from living forms. But, as Buckland
long ago remarked, all fossils can be classed either in still existing
groups, or between them. That the extinct forms of life help to fill up
the wide intervals between existing genera, families, and orders, cannot be
disputed. For if we confine our attention either to the living or to the
extinct alone, the series is far less perfect than if we combine both into
one general system. With respect to the Vertebrata, whole pages could be
filled with striking illustrations from our great palaeontologist, Owen,
showing how extinct animals fall in between existing groups. Cuvier ranked
the Ruminants and Pachyderms, as the two most distinct orders of mammals;
but Owen has discovered so many fossil links, that he has had to alter the
whole classification of these two orders; and has placed certain pachyderms
in the same sub-order with ruminants: for example, he dissolves by fine
gradations the apparently wide difference between the pig and the camel.
In regard to the Invertebrata, Barrande, and a higher authority could not
be named, asserts that he is every day taught that palaeozoic animals,
though belonging to the same orders, families, or genera with those living
at the present day, were not at this early epoch limited in such distinct
groups as they now are.
Some writers have objected to any extinct species or group of species being
considered as intermediate between living species or groups. If by this
term it is meant that an extinct form is directly intermediate in all its
characters between two living forms, the objection is probably valid. But
I apprehend that in a perfectly natural classification many fossil species
would have to stand between living species, and some extinct genera between
living genera, even between genera belonging to distinct families. The
most common case, especially with respect to very distinct groups, such as
fish and reptiles, seems to be, that supposing them to be distinguished at
the present day from each other by a dozen characters, the ancient members
of the same two groups would be distinguished by a somewhat lesser number
of characters, so that the two groups, though formerly quite distinct, at
that period made some small approach to each other.
It is a common belief that the more ancient a form is, by so much the more
it tends to connect by some of its characters groups now widely separated
from each other. This remark no doubt must be restricted to those groups
which have undergone much change in the course of geological ages; and it
would be difficult to prove the truth of the proposition, for every now and
then even a living animal, as the Lepidosiren, is discovered having
affinities directed towards very distinct groups. Yet if we compare the
older Reptiles and Batrachians, the older Fish, the older Cephalopods, and
the eocene Mammals, with the more recent members of the same classes, we
must admit that there is some truth in the remark.
Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex, I
must request the reader to turn to the diagram in the fourth chapter. We
may suppose that the numbered letters represent genera, and the dotted
lines diverging from them the species in each genus. The diagram is much
too simple, too few genera and too few species being given, but this is
unimportant for us. The horizontal lines may represent successive
geological formations, and all the forms beneath the uppermost line may be
considered as extinct. The three existing genera, a14, q14, p14, will form
a small family; b14 and f14 a closely allied family or sub-family; and o14,
e14, m14, a third family. These three families, together with the many
extinct genera on the several lines of descent diverging from the
parent-form A, will form an order; for all will have inherited something in
common from their ancient and common progenitor. On the principle of the
continued tendency to divergence of character, which was formerly
illustrated by this diagram, the more recent any form is, the more it will
generally differ from its ancient progenitor. Hence we can understand the
rule that the most ancient fossils differ most from existing forms. We
must not, however, assume that divergence of character is a necessary
contingency; it depends solely on the descendants from a species being thus
enabled to seize on many and different places in the economy of nature.
Therefore it is quite possible, as we have seen in the case of some
Silurian forms, that a species might go on being slightly modified in
relation to its slightly altered conditions of life, and yet retain
throughout a vast period the same general characteristics. This is
represented in the diagram by the letter F14.
All the many forms, extinct and recent, descended from A, make, as before
remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.
By looking at the diagram we can see that if many of the extinct forms,
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for
instance, the genera a1, a5, a10, f8, m3, m6, m9 were disinterred, these
three families would be so closely linked together that they probably would
have to be united into one great family, in nearly the same manner as has
occurred with ruminants and pachyderms. Yet he who objected to call the
extinct genera, which thus linked the living genera of three families
together, intermediate in character, would be justified, as they are
intermediate, not directly, but only by a long and circuitous course
through many widely different forms. If many extinct forms were to be
discovered above one of the middle horizontal lines or geological
formations--for instance, above No. VI.--but none from beneath this line,
then only the two families on the left hand (namely, a14, &c., and b14,
&c.) would have to be united into one family; and the two other families
(namely, a14 to f14 now including five genera, and o14 to m14) would yet
remain distinct. These two families, however, would be less distinct from
instance, we suppose the existing genera of the two families to differ from
each other by a dozen characters, in this case the genera, at the early
period marked VI., would differ by a lesser number of characters; for at
this early stage of descent they have not diverged in character from the
common progenitor of the order, nearly so much as they subsequently
diverged. Thus it comes that ancient and ex
tinct genera are often in some
slight degree intermediate in character between their modified descendants,
or between their collateral relations.
In nature the case will be far more complicated than is represented in the
diagram; for the groups will have been more numerous, they will have
endured for extremely unequal lengths of time, and will have been modified
in various degrees. As we possess only the last volume of the geological
record, and that in a very broken condition, we have no right to expect,
except in very rare cases, to fill up wide intervals in the natural system,
and thus unite distinct families or orders. All that we have a right to
expect, is that those groups, which have within known geological periods
undergone much modification, should in the older formations make some
slight approach to each other; so that the older members should differ less
from each other in some of their characters than do the existing members of
the same groups; and this by the concurrent evidence of our best
palaeontologists seems frequently to be the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each other
and to living forms, seem to me explained in a satisfactory manner. And
they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great period in
the earth's history will be intermediate in general character between that
which preceded and that which succeeded it. Thus, the species which lived
at the sixth great stage of descent in the diagram are the modified
offspring of those which lived at the fifth stage, and are the parents of
those which became still more modified at the seventh stage; hence they
could hardly fail to be nearly intermediate in character between the forms
of life above and below. We must, however, allow for the entire extinction
of some preceding forms, and for the coming in of quite new forms by
immigration, and for a large amount of modification, during the long and
blank intervals between the successive formations. Subject to these
allowances, the fauna of each geological period undoubtedly is intermediate
in character, between the preceding and succeeding faunas. I need give
only one instance, namely, the manner in which the fossils of the Devonian
system, when this system was first discovered, were at once recognised by
palaeontologists as intermediate in character between those of the
overlying carboniferous, and underlying Silurian system. But each fauna is
not necessarily exactly intermediate, as unequal intervals of time have
elapsed between consecutive formations.
It is no real objection to the truth of the statement, that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions to
the rule. For instance, mastodons and elephants, when arranged by Dr.
Falconer in two series, first according to their mutual affinities and then
according to their periods of existence, do not accord in arrangement. The
species extreme in character are not the oldest, or the most recent; nor
are those which are intermediate in character, intermediate in age. But
supposing for an instant, in this and other such cases, that the record of
the first appearance and disappearance of the species was perfect, we have
no reason to believe that forms successively produced necessarily endure
for corresponding lengths of time: a very ancient form might occasionally
last much longer than a form elsewhere subsequently produced, especially in
the case of terrestrial productions inhabiting separated districts. To
compare small things with great: if the principal living and extinct races
of the domestic pigeon were arranged as well as they could be in serial
affinity, this arrangement would not closely accord with the order in time