is here excessively 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 more resemble the ass than the horse; but that the
prepotency runs more strongly in the male-ass than in the female, so that
the mule, which is the 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
mongrel animals alone are born closely like one of their parents; but it
can be shown that this does sometimes occur with hybrids; yet I grant much
less frequently with hybrids 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 monstrous 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 by selection.
Consequently, sudden reversions to the perfect character of either parent
would be more likely to occur with mongrels, which are descended from
varieties often suddenly produced and semi-monstrous in character, than
with hybrids, which are descended from species slowly and naturally
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 much or little from each other,
namely in the union of individuals of the same variety, or of different
varieties, or of distinct species.
Laying aside 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
harmonises 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 two most careful experimentalists who have ever lived, have
come to diametrically opposite conclusions in ranking forms by this test.
The sterility is innately variable in individuals of the same species, and
is eminently susceptible of favourable and unfavourable conditions. The
degree of sterility does not strictly follow systematic affinity, but is
governed 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
hybrid produced from this cross.
In the same manner as in grafting trees, the capacity of one species or
variety to take on another, is incidental on generally unknown differences
in their vegetative systems, so in crossing, the greater or less facility
of one species to unite with another, is incidental on unknown differences
in their reproductive systems. There is no more reason to think that
species have been specially endowed with various degrees of sterility to
prevent them 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 them becoming
inarched in our forests.
The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several circumstances; in
some cases largely on the early death of the embryo. The sterility of
hybrids, which have their reproductive systems imperfect, and which have
had this system and their whole organisation disturbed by being compounded
of two distinct species, seems closely allied to that sterility which so
frequently affects pure species, when their natural conditions of life have
been disturbed. This view is supported by a parallelism of another
kind;--namely, that the crossing of forms only slightly different is
favourable to the vigour and fertility of their offspring; and that slight
changes in the conditions of life are apparently favourable to the vigour
and fertility of all organic beings. It is not surprising that the degree
of difficulty in uniting two species, and the degree of sterility of their
hybrid-offspring should generally correspond, though due to distinct
causes; for both depend on the amount of difference of some kind between
the species which are crossed. Nor is it surprising that the facility of
effecting a first cross, the fertility of the hybrids produced, and the
capacity of being grafted together--though this latter capacity evidently
depends on widely different circumstances--should all run, to a certain
extent, parallel with the systematic affinity of the forms which are
subjected to experiment; for systematic affinity attempts to express all
kinds of resemblance between all species.
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 quite universally, fertile. Nor is this nearly general
and perfect fertility surprising, when we remember 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 not of
differences in the reproductive system. In all other respects, excluding
fertility, there is a close general resemblance between hybrids and
mongrels. Finally, then, the facts briefly given in this chapter do not
seem to me opposed to, but even rather to support the view, that there is
no fundamental distinction between species and varieties.
Chapter IX
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 vast
lapse of time, as inferred from the rate of deposition and of denudation --
On the poorness of our palaeontological collections -- On the intermittence
of geological formations -- 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.
In the six
th chapter I enumerated the chief objections 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
favourable for their presence, namely on an extensive and continuous area
with graduated physical conditions. I endeavoured to show, that the life
of each species depends 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 endeavoured, 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 improvement. 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 exterminate their
parent-forms. But just in proportion as this process of extermination has
acted on an enormous scale, so must the number of intermediate varieties,
which have formerly existed on the earth, be truly enormous. Why then is
not every geological formation and every stratum full of such intermediate
links? Geology assuredly does not reveal any such finely graduated organic
chain; and this, perhaps, is the most obvious and gravest objection which
can be urged against my 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
intermediate forms must, on my 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 have 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 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, whether
they had descended from this species or from some other allied species,
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 ever existed
directly intermediate between them, but between each and an unknown common
parent. The common parent will have had in its whole organisation 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 recognise the parent-form of any two or more species, even if we closely
compared the structure of the parent with that of its modified descendants,
unless at the same time we had a nearly perfect chain of the intermediate
links.
It is just possible by my 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, whilst its descendants had undergone a vast amount of change;
and the principle of competition between organism and organism, between
child and parent, will render this a very rare event; for in all cases the
new and improved forms of life will 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 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 species; and so on backwards, 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 this earth.
On the lapse of Time. -- Independently of our not finding fossil remains of
such infinitely numerous connecting links, it may be objected, that time
will not have sufficed for so great an amount of organic change, all
changes having been effected very slowly through natural selection. It is
hardly possible for me even to recall to the reader, who may not be 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, which the future historian will recognise as having
produced a revolution in natural science, yet does not admit how
incomprehensibly 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 each stratum. A man must for years
examine for himself great piles of superimposed strata, and watch the sea
at work grinding down old rocks and making fresh sediment, before he can
hope to comprehend anything of the lapse of time, the monuments of which we
see around us.
It is good to wander along lines of sea-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 reason
to believe that pure water can effect little or 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
reduced in size they can be rolled about by the waves, and then are more
quickly ground into pebbles, sand, or mud. But how often do we see along
the bases of
retreating cliffs rounded boulders, all thickly clothed by
marine productions, showing how little they are abraded and how seldom they
are rolled about! Moreover, 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 cliffs 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.
He who most closely studies the action of the sea on our shores, will, I
believe, be most deeply impressed with the slowness with which rocky coasts
are worn away. The observations on this head by Hugh Miller, and by that
excellent observer Mr. Smith of Jordan Hill, are most impressive. With the
mind thus impressed, let any one examine beds of conglomerate many thousand
feet in thickness, which, though probably formed at a quicker rate than
many other deposits, yet, from being formed of worn and rounded pebbles,
each of which bears the stamp of time, are good to show how slowly the mass
has been accumulated. Let him remember Lyell's profound remark, that the
thickness and extent of sedimentary formations are the result and measure
of the degradation which the earth's crust has elsewhere suffered. And
what an amount of degradation is implied by the sedimentary deposits of
many countries! Professor Ramsay has given me the maximum thickness, in
most cases from actual measurement, in a few cases from estimate, of each
formation in different parts of Great Britain; and this is the result:-
Feet
Palaeozoic strata (not including igneous beds)..57,154
Secondary strata................................13,190
Tertiary strata..................................2,240
--making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. Some of these 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, enormously long blank periods. So that the
lofty pile of sedimentary rocks in Britain, gives but an inadequate idea of
the time which has elapsed during their accumulation; yet what time this
must have consumed! Good observers have estimated that sediment is
deposited by the great Mississippi river at the rate of only 600 feet in a
hundred thousand years. This estimate may be quite erroneous; yet,
considering over what wide spaces very fine sediment is transported by the
currents of the sea, the process of accumulation in any one area must be
extremely slow.
But the amount of denudation which the strata have in many places suffered,
independently of the rate of accumulation of the degraded matter, probably
offers the best evidence of the lapse of time. I remember having been much
struck with the evidence of denudation, 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 still more plainly told 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, the
surface of the land has been so completely planed down by the action of the
sea, that no trace of these vast dislocations is externally visible.
The Craven fault, for instance, extends for upwards of 30 miles, and along
this line the vertical displacement of the strata has varied from 600 to