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vated plants. Gartner further states that when any two species, although
most closely allied to each other, are crossed with a third species, the hybrids
are widely different from each other; whereas if two very distinct varieties
of one species are crossed with another species, the hybrids do not differ
much. But this conclusion, as far as I can make out, is founded on a single
experiment, and seems directly opposed to the results of several experiments
made by Kolreuter.

Such alone are the unimportant differences which Gartner is able to point
out between hybrid and mongrel plants. On the other hand, the degrees and
kinds of resemblance in mongrels and in hybrids to their respective parents,
more especially in hybrids produced from nearly related species, follow, ac-
cording to Gartner, the same laws. When two species are crossed, one has
sometimes a prepotent power of impressing its likeness on the hybrid. So I
believe it to be with varieties of plants; and with animals, one variety cer-
tainly often has this prepotent power over another variety. Hybrid plants
produced from a reciprocal cross generally resemble each other closely, and
so it is with mongrel plants from a reciprocal cross. Both hybrids and mon-
grels can be reduced to either pure parent form by repeated crosses in suc-
cessive generations with either parent.

These several remarks are apparently applicable to animals, but the sub-
ject is here much complicated, partly owing to the existence of secondary
sexual characters, but more especially owing to prepotency in transmitting
likeness running more strongly in one sex than in the other, both when one
species is crossed with another and when one variety is crossed with another
variety. For instance, I think those authors are right who maintain that the
ass has a prepotent power over the horse, so that both the mule and the hinny
resemble more closely the ass than the horse; but that the prepotency runs
more strongly in the male than in the female ass, so that the mule, which is
an offspring of the male ass and mare, is more like an ass than is the hinny,
which is the offspring of the female ass and stallion.

Much stress has been laid by some authors on the supposed fact, that it is
only with mongrels that the offspring are not intermediate in character, but
closely resemble one of their parents: but this does sometimes occur with
hybrids, yet I grant much less frequently than with mongrels. Looking to the
cases which I have collected of cross-bred animals closely resembling one
parent, the resemblances seem chiefly confined to characters almost mon-
strous in their nature, and which have suddenly appeared — such as albinism,
melanism, deficiency of tail or horns, or additional fingers and toes; and do
not relate to characters which have been slowly acquired through selection.
A tendency to sudden reversions to the perfect character of either parent
would, also, be much more likely to occur with mongrels, which are de-
scended from varieties often suddenly produced and semi-monstrous in char-



218 THE ORIGIN OF SPECIES

acter, than with hybrids, which are descended from species slowly and nat-
urally produced. On the whole, I entirely agree with Dr. Prosper Lucas,
who, after arranging an enormous body of facts with respect to animals,
comes to the conclusion that the laws of resemblance of the child to its
parents are the same, whether the two parents differ little or much from
each other, namely, in the union of individuals of the same variety, or of
different varieties, or of distinct species.

Independently of the question of fertility and sterility, in all other respects
there seems to be a general and close similarity in the offspring of crossed
species, and of crossed varieties. If we look at species as having been specially
created, and at varieties as having been produced by secondary laws, this
similarity would be an astonishing fact. But it harmonizes perfectly with the
view that there is no essential distinction between species and varieties.

SUMMARY OF CHAPTER

First crosses between forms, sufficiently distinct to be ranked as species, ,
and their hybrids, are very generally, but not universally, sterile. The sterility
is of all degrees, and is often so slight that the most careful experimentalists ;
have arrived at diametrically opposite conclusions in ranking forms by this
test. The sterility is innately variable in individuals of the same species, and
is eminently susceptible to action of favorable and unfavorable conditions.
The degree of sterility does not strictly follow systematic affinity, but is gov-
erned by several curious and complex laws. It is generally different, and
sometimes widely different, in reciprocal crosses between the same two
species. It is not always equal in degree in a first cross and in the hybrids pro-
duced from this cross.

In the same manner as in grafting trees, the capacity in one species or
variety to take on another, is incidental on differences, generally of an un-
known nature, in their vegetative systems, so in crossing, the greater or less
facility of one species to unite with another is incidental on unknown differ-
ences in their reproductive systems. There is no more reason to think that
species have been specially endowed with various degrees of sterility to pre-
vent their crossing and blending in nature, than to think that trees have
been specially endowed with various and somewhat analogous degrees of
difficulty in being grafted together in order to prevent their inarching in our
forests.

The sterility of first crosses and of their hybrid progeny has not been ac- i
quired through natural selection. In the case of first crosses it seems to de- '
pend on several circumstances; in some instances, in chief part on the early
death of the embryo. In the case of hybrids, it apparently depends on their i[
whole organization having been disturbed by being compounded from two^
distinct forms, the sterility being closely allied to that which so frequently^;
affects pure species, when exposed to new and unnatural conditions of life.
He who will explain these latter cases will be able to explain the sterility of '
hybrids. This view is strongly supported by a parallelism of another kind;



HYBRIDISM 219

namely, that, firstly, slight changes in the conditions of life add to the vigor
and fertility of all organic beings; and secondly, that the crossing of forms
which have been exposed to slightly different conditions of life, or which
have varied, favors the size, vigor, and fertility of their offspring. The facts
given on the sterility of the illegitimate unions of dimorphic and trimorphic
plants and of their illegitimate progeny, perhaps render it probable that some
unknown bond in all cases connects the degree of fertility of first unions with
that of their offspring. The consideration of these facts on dimorphism, as
well as of the results of reciprocal crosses, clearly leads to the conclusion
that the primal cause of the sterility of crossed species is confined to differ-
ences in their sexual elements. But why, in the case of distinct species, the
sexual elements should so generally have become more or less modified, lead-
ing to their mutual infertility, we do not know; but it seems to stand in some
close relation to species having been exposed for long periods of time to
nearly uniform conditions of life.

It is not surprising that the difficulty in crossing any two species, and the
sterility of their hybrid offspring, should in most cases correspond, even if
due to distinct causes ; for both depend on the amount of difference between
the species which are crossed. Nor is it surprising that the facility of effect-
ing a first cross, and the fertility of the hybrids thus produced, and the ca-
pacity of being grafted together — though this latter capacity evidently de-
pends on widely different circumstances — should all run, to a certain extent,
parallel with the systematic affinity of the forms subjected to experiment;
for systematic affinity includes resemblances of all kinds.

First crosses between forms known to be varieties, or sufficiently alike to
be considered as varieties, and their mongrel offspring, are very generally —
but not, as is so often stated, invariably — ^fertile. Nor is this almost universal
and perfect fertility surprising, when it is remembered how liable we are to
argue in a circle with respect to varieties in a state of nature; and when we
remember that the greater number of varieties have been produced under
domestication by the selection of mere external differences, and that they
have not been long exposed to uniform conditions of life. It should also be
especially kept in mind, that long-continued domestication tends to eliminate
sterility, and is therefore little likely to induce this same quality. Independ-
ently of the question of fertility, in all other respects there is the closest
general resemblance between hybrids and mongrels, in their variability,
in their power of absorbing each other by repeated crosses, and in their
inheritance of characters from both parent-forms. Finally, then, although we
are as ignorant of the precise cause of the sterility of first crosses and of
hybrids as we are why animals and plants removed from their natural condi-
tions become sterile, yet the facts given in this chapter do not seem to me
opposed to the belief that species aboriginally existed as varieties.



CHAPTER X
On the Imperfection of the Geological Record

On the Absence of Intermediate Varieties at the Present Day — On the Nature of
Extinct Intermediate Varieties; on their Number — On the Lapse of Time, as in-
ferred from the Rate of Denudation and of Deposition — On the Lapse of Time
as estimated by Years — On the Poorness of our Palaeontological Collections — On
the Intermittence of Geological Formations — On the Denudation of Granitic
Areas — On the Absence of Intermediate Varieties in any one Formation — On the
Sudden Appearance of Groups of Species — On their Sudden Appearance in the
lowest known Fossiliferous Strata — Antiquity of the Habitable Earth.

In the sixth chapter I enumerated the chief objection^ which might be justly-
urged against the views maintained in this volume. Most of them have now
been discussed. One, namely, the distinctness of specific forms and their not
being blended together by innumerable transitional links, is a very obvious
difficulty. I assigned reasons why such links do not commonly occur at the
present day under the circumstances apparently most favorable for their
presence, namely, on an extensive and continuous area with graduated
physical conditions. I endeavored to show that the life of each species de-
pends in a more important manner on the presence of other already defined
organic forms, than on climate, and, therefore, that the really governing
conditions of life do not graduate away quite insensibly like heat or moisture.
I endeavored, also, to show that intermediate varieties, from existing in
lesser numbers than the forms which they connect, will generally be beaten
out and exterminated during the course of further modification and improve-
ment. The main cause, however, of innumerable intermediate links not now
occurring everywhere throughout nature, depends on the very process of
natural selection, through which new varieties continually take the places
of and supplant their parent-forms. But just in proportion as this process of
extermination has acted on an enormous scale, so much the number of in-
termediate varieties, which have formerly existed, be truly enormous. Why
then is not every geological formation and every stratum full of such inter-
mediate links? Geology assuredly does not reveal any such finely-graduated
organic chain; and this, perhaps, is the most obvious and serious objection
which can be urged against the theory. The explanation lies, as I believe, in
the extreme imperfection of the geological record.

In the first place, it should always be borne in mind what sort of inter-
mediate forms must, on the theory, have formerly existed. I have found it
difficult, when looking at any two species, to avoid picturing to myself forms
directly intermediate between them. But this is a wholly false view; we should
always look for forms intermediate between each species and a common but
unknown progenitor; and the progenitor will generally have differed in some
respects from all its modified descendants. To give a simple illustration: the
fantail and pouter pigeons are both descended from the rock-pigeon; if we
possessed all the intermediate varieties which have ever existed, we should
have an extremely close series between both and the rock-pigeon; but we

220



THE IMPERFECTION OF THE GEOLOGICAL RECORD 221

should have no varieties directly intermediate between the fantail and
pouter; none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds. These two
breeds, moreover, have become so much modified, that, if we had no historical
or indirect evidence regarding their origin, it would not have been possible
to have determined, from a mere comparison of their structure with that of
the rock-pigeon, C. livia, whether they had descended from this species or
from some other allied form, such as C. oenas.

So, with natural species, if we look to forms very distinct, for instance to
the horse and tapir, we have no reason to suppose that links directly inter-
mediate between them ever existed, but between each and an unknown
common parent. The common parent will have had in its whole organization
much general resemblance to the tapir and to the horse; but in some points
of structure may have differed considerably from both, even perhaps more
than they differ from each other. Hence, in all such cases, we should be
unable to recognize the parent form of any two or more species, even if we
closely compared the structure of the parent with that of its modified de-
scendants, unless at the same time we had a nearly perfect chain of the inter-
mediate links.

It is just possible, by the theory, that one of two living forms might have
descended from the other; for instance, a horse from a tapir; and in this case
direct intermediate links will have existed between them. But such a case
would imply that one form had remained for a very long period unaltered,
while its descendants had undergone a vast amount of change ; and the prin-
ciple of competition between organism and organism, between child and
parent, will render this a very rare event; for in all cases the new and im-
proved forms of life tend to supplant the old and unimproved forms.

By the theory of natural selection all living species have been connected
with the parent-species of each genus, by differences not greater than we see
between the natural and domestic varieties of the same species at the present
day; and these parent species, now generally extinct, have in their turn been
similarly connected with more ancient forms; and so on backward, always
converging to the common ancestor of each great class. So that the number
of intermediate and transitional links, between all living and extinct species,
must have been inconceivably great. But assuredly, if this theory be true^
such have lived upon the earth.

ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF
DEPOSITION AND EXTENT OF DENUDATION

Independently of our not finding fossil remains of such infinitely numerous
connecting links, it may be objected that time cannot have sufficed for so
great an amount of organic change, all changes having been effected slowly.
It is hardly possible for me to recall to the reader who is not a practical
geologist, the facts leading the mind feebly to comprehend the lapse of time.
He who can read Sir Charles Lyell's grand work on the Principles of Geology,



222 THE ORIGIN OF SPECIES

which the future historian will recognize as having produced a revolution in
natural science, and yet does not admit how vast have been the past periods
of time, may at once close this volume. Not that it suffices to study the
Principles of Geology, or to read special treatises by different observers on
separate formations, and to mark how each author attempts to give an
inadequate idea of the duration of each formation, or even of each stratum.
We can best gain some idea of past time by knowing the agencies at work,
and learning how deeply the surface of the land has been denuded, and how
much sediment has been deposited. As Lyell has well remarked, the extent
and thickness of our sedimentary formations are the result and the measure
of the denudation which the earth's crust has elsewhere undergone. There-
fore a man should examine for himself the great piles of superimposed strata,
and watch the rivulets bringing down mud, and the waves wearing away the
sea cliffs, in order to comprehend something about the duration of past time,
the monuments of which we see all around us.

It is good to wander along the coast, when formed of moderately hard
rocks, and mark the process of degradation. The tides in most cases reach
the cliffs only for a short time twice a day, and the waves eat into them only
when they are charged with sand or pebbles; for there is good evidence that
pure water effects nothing in wearing away rock. At last the base of the cliff
is undermined, huge fragments fall down, and these, remaining fixed, have
to be worn away atom by atom, until after being reduced in size they can be
rolled about by the waves, and then they are more quickly ground into
pebbles, sand, or mud. But how often do we see along the bases of retreating
cHffs rounded bowlders, all thickly clothed by marine productions, showing
how little they are abraded, and how seldom they are rolled about! More-
over, if we follow for a few miles any line of rocky cliff, which is undergoing
degradation, we find that it is only here and there, along a short length or
round a promontory, that the clifTs are at the present time suffering. The
appearance of the surface and the vegetation show that elsewhere years have
elapsed since the waters washed their base.

We have, however, recently learned from the observations of Ramsay, in
the van of many excellent observers — of Jukes, Geikie, Croll, and others,
that subaerial degradation is a much more important agency than coast-
action, or the power of the waves. The whole surface of the land is exposed to
the chemical action of the air and of the rain-water, with its dissolved
carbonic acid, and in colder countries to frost; the disintegrated matter is
carried down even gentle slopes during heavy rain, and to a greater extent
than might be supposed, especially in arid districts, by the wind; it is then
transported by the streams and rivers, which, when rapid, deepen their
channels, and triturate the fragments. On a rainy day, even in a gently
undulating country, we see the effects of subaerial degradation in the muddy
rills which flow down every slope. Messrs. Ramsay and Whitaker have shown,
and the observation is a most striking one, that the great lines of escarpment
in the Wealden district and those ranging across England, which formerly
were looked at as ancient seacoasts, cannot have been thus formed, for each



THE IMPERFECTION OF THE GEOLOGICAL RECORD 223

line is composed of one and the same formation, while our sea cliffs are
everywhere formed by the intersection of various formations. This being the
case, we are compelled to admit that the escarpments owe their origin in
chief part to the rocks of which they are composed, having resisted subaerial
denudation better than the surrounding surface; this surface consequently
has been gradually lowered, with the lines of harder rock left projecting.
Nothing impresses the mind with the vast duration of time, according to our
ideas of time, more forcibly than the conviction thus gained that subaerial
agencies, which apparently have so little power, and which seem to work so
slowly, have produced great results.

When thus impressed with the slow rate at which the land is worn away
through subaerial and littoral action, it is good, in order to appreciate the
past duration of time, to consider, on the one hand, the masses of rock which
have been removed over many extensive areas, and on the other hand the
thickness of our sedimentary formations. I remember having been much
struck when viewing volcanic islands, which have been worn by the waves
and pared all round into perpendicular cliffs of one or two thousand feet in
height; for the gentle slope of the lava streams, due to their formerly liquid
state, showed at a glance how far the hard, rocky beds had once extended
into the open ocean. The same story is told still more plainly by faults —
those great cracks along which the strata have been upheaved on one side,
or thrown down on the other, to the height or depth of thousands of feet;
for since the crust cracked, and it makes no great difference whether the
upheaval was sudden, or, as most geologists now believe, was slow and
effected by many starts, the surface of the land has been so completely planed
down that no trace of these vast dislocations is externally visible. The Craven
fault, for instance, extends for upward of thirty miles, and along this line
the vertical displacement of the strata varies from 600 to 3,000 feet. Pro-
fessor Ramsay has published an account of a downthrow in Anglesea of
2,300 feet; and he informs me that he fully believes that there is one in
Merionethshire of 12,000 feet; yet in these cases there is nothing on the
surface of the land to show such prodigious movements; the pile of rocks on
either side of the crack having been smoothly swept away.

On the other hand, in all parts of the world the piles of sedimentary strata
are of wonderful thickness. In the Cordillera, I estimated one mass of
conglomerate at 10,000 feet; and although conglomerates have probably
been accumulated at a quicker rate than finer sediments, yet from being
formed of worn and rounded pebbles, each of which bears the stamp of time,
they are good to show how slowly the mass must have been heaped together.
Professor Ramsay has given me the maximum thickness, from actual
measurement in most cases, of the successive formations in different parts of
Great Britain; and this is the result:

Feet.

Palaeozoic strata (not including igneous beds) 57jI54

Secondary strata 13,190

Tertiary strata 2,240



224 THE ORIGIN OF SPECIES

— making altogether 72,584 feet; that is, very nearly thirteen and three-
quarters British miles. Some of the formations, which are represented in
England by thin beds, are thousands of feet in thickness on the Continent.
Moreover, between each successive formation we have, in the opinion of most
geologists, blank periods of enormous length. So that the lofty pile of sedi-

< mentary rocks in Britain gives but an inadequate idea of the time which has
elapsed during their accumulation. The consideration of these various facts
impresses the mind almost in the same manner as does the vain endeavor to
grapple with the idea of eternity.

Nevertheless this impression is partly false. Mr. Croll, in an interesting
paper, remarks that we do not err "in forming too great a conception of the
length of geological periods," but in estimating them by years. When
geologists look at large and complicated phenomena, and then at the figures
representing several million years, the two produce a totally different effect
on the mind, and the figures are at once pronounced too small. In regard
to subaerial denudation, Mr. Croll shows, by calculating the known amount
of sediment annually brought down by certain rivers, relatively to their areas
of drainage, that 1,000 feet of solid rock, as it became gradually disintegrated,
would thus be removed from the mean level of the whole area in the course
of six million years. This seems an astonishing result, and some considerations
lead to the suspicion that it may be too large, but if halved or quartered it
is still very surprising. Few of us, however, know what a million really means.
Mr. Croll gives the following illustration: Take a narrow strip of paper,
eighty-three feet four inches in length, and stretch it along the wall of a large
hall ; then mark off at one end the tenth of an inch. This tenth of an inch
will represent one hundred years, and the entire strip a million years. But
let it be borne in mind, in relation to the subject of this work, what a hun-
dred years implies, represented as it is by a measure utterly insignificant in a



Online LibraryCharles DarwinThe origin of species → online text (page 30 of 50)