do not at any one period present an inextricable chaos of varying
and intermediate links: first, because new varieties are very slowly
formed, for variation is a slow process, and natural selection can do
nothing until favorable individual differences or variations occur,
and until a place in the natural polity of the country can be better
filled by some modification of some one or more of its inhabitants.
And such new places will depend on slow changes of climate, or on the
occasional immigration of new inhabitants, and, probably, in a still
more important degree, on some of the old inhabitants becoming slowly
modified, with the new forms thus produced and the old ones acting and
reacting on each other. So that, in any one region and at any one time,
we ought to see only a few species presenting slight modifications of
structure in some degree permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within the
recent period as isolated portions, in which many forms, more
especially among the classes which unite for each birth and wander
much, may have separately been rendered sufficiently distinct to
rank as representative species. In this case, intermediate varieties
between the several representative species and their common parent must
formerly have existed within each isolated portion of the land, but
these links during the process of natural selection will have been
supplanted and exterminated, so that they will no longer be found in a
living state.
Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will, it
is probable, at first have been formed in the intermediate zones, but
they will generally have had a short duration. For these intermediate
varieties will, from reasons already assigned (namely, from what we
know of the actual distribution of closely-allied or representative
species, and likewise of acknowledged varieties), exist in the
intermediate zones in lesser numbers than the varieties which they tend
to connect. From this cause alone the intermediate varieties will be
liable to accidental extermination; and, during the process of further
modification through natural selection, they will almost certainly
be beaten and supplanted by the forms which they connect; for these
from existing in greater numbers will, in the aggregate, present more
varieties and thus be further improved through natural selection and
gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory be
true, numberless intermediate varieties, linking closely together all
the species of the same group, must assuredly have existed; but the
very process of natural selection constantly tends, as has been so
often remarked, to exterminate the parent-forms and the intermediate
links. Consequently evidence of their former existence could be found
only among fossil remains, which are preserved, as we shall attempt to
show in a future chapter, in an extremely imperfect and intermittent
record.
* * * * *
[Page 283.]
Professor Pictet, in commenting on early transitional forms, and taking
birds as an illustration, can not see how the successive modifications
of the anterior limbs of a supposed prototype could possibly have been
of any advantage. But look at the penguins of the Southern Ocean; have
not these birds their front limbs in this precise intermediate state
of “neither true arms nor true wings”? Yet these birds hold their
place victoriously in the battle for life; for they exist in infinite
numbers and of many kinds. I do not suppose that we here see the real
transitional grades through which the wings of birds have passed; but
what special difficulty is there in believing that it might profit
the modified descendants of the penguin, first to become enabled to
flap along the surface of the sea like the logger-headed duck, and
ultimately to rise from its surface and glide through the air?
* * * * *
[Page 289.]
The several difficulties here discussed, namely--that, though we find
in our geological formations many links between the species which now
exist and which formerly existed, we do not find infinitely numerous
fine transitional forms closely joining them all together; the
sudden manner in which several groups of species first appear in our
European formations--the almost entire absence, as at present known,
of formations rich in fossils beneath the Cambrian strata--are all
undoubtedly of the most serious nature. We see this in the fact that
the most eminent paleontologists, namely, Cuvier, Agassiz, Barrande,
Pictet, Falconer, E. Forbes, etc., and all our greatest geologists, as
Lyell, Murchison, Sedgwick, etc., have unanimously, often vehemently,
maintained the immutability of species. But Sir Charles Lyell now gives
the support of his high authority to the opposite side; and most
geologists and paleontologists are much shaken in their former belief.
Those who believe that the geological record is in any degree perfect
will undoubtedly at once reject the theory. For my part, following
out Lyell’s metaphor, I look at the geological record as a history
of the world imperfectly kept, and written in a changing dialect; of
this history we possess the last volume alone, relating only to two or
three countries. Of this volume, only here and there a short chapter
has been preserved; and of each page, only here and there a few lines.
Each word of the slowly-changing language, more or less different in
the successive chapters, may represent the forms of life which are
entombed in our consecutive formations, and which falsely appear to us
to have been abruptly introduced. On this view, the difficulties above
discussed are greatly diminished, or even disappear.
HOW COULD THE TRANSITIONAL FORM HAVE SUBSISTED?
[Page 138.]
It has been asked by the opponents of such views as I hold, how, for
instance, could a land carnivorous animal have been converted into
one with aquatic habits; for how could the animal in its transitional
state have subsisted? It would be easy to show that there now exist
carnivorous animals presenting close intermediate grades from strictly
terrestrial to aquatic habits; and, as each exists by a struggle for
life, it is clear that each must be well adapted to its place in
nature. Look at the _Mustela vison_ of North America, which has webbed
feet, and which resembles an otter in its fur, short legs, and form of
tail. During the summer this animal dives for and preys on fish, but
during the long winter it leaves the frozen waters, and preys, like
other polecats, on mice and land animals. If a different case had
been taken, and it had been asked how an insectivorous quadruped could
possibly have been converted into a flying bat, the question would have
been far more difficult to answer. Yet I think such difficulties have
little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for,
out of the many striking cases which I have collected, I can give only
one or two instances of transitional habits and structures in allied
species; and of diversified habits, either constant or occasional, in
the same species. And it seems to me that nothing less than a long list
of such cases is sufficient to lessen the difficulty in any particular
case like that of the bat.
WHY NATURE TAKES NO SUDDEN LEAPS.
[Origin of
Species,
page 156.]
Finally, then, although in many cases it is most difficult even to
conjecture by what transitions organs have arrived at their present
state, yet, considering how small the proportion of living and known
forms is to the extinct and unknown, I have been astonished how rarely
an organ can be named, toward which no transitional grade is known to
lead. It certainly is true that new organs, appearing as if created for
some special purpose, rarely or never appear in any being--as indeed
is shown by that old but somewhat exaggerated canon in natural history
of “Natura non facit saltum.” We meet with this admission in the
writings of almost every experienced naturalist; or as Milne-Edwards
has well expressed it, Nature is prodigal in variety, but niggard in
innovation. Why, on the theory of Creation, should there be so much
variety and so little real novelty? Why should all the parts and organs
of many independent beings, each supposed to have been separately
created for its proper place in nature, be so commonly linked together
by graduated steps? Why should not Nature take a sudden leap from
structure to structure? On the theory of natural selection, we can
clearly understand why she should not; for natural selection acts only
by taking advantage of slight successive variations; she can never take
a great and sudden leap, but must advance by short and sure though slow
steps.
IMPERFECT CONTRIVANCES OF NATURE ACCOUNTED FOR.
[Page 163.]
If our reason leads us to admire with enthusiasm a multitude of
inimitable contrivances in nature, this same reason tells us, though
we may easily err on both sides, that some other contrivances are less
perfect. Can we consider the sting of the bee as perfect, which, when
used against many kinds of enemies, can not be withdrawn, owing to the
backward serratures, and thus inevitably causes the death of the insect
by tearing out its viscera?
If we look at the sting of the bee, as having existed in a remote
progenitor as a boring and serrated instrument like that in so many
members of the same great order, and that it has since been modified
but not perfected for its present purpose with the poison originally
adapted for some other object, such as to produce galls, since
intensified, we can perhaps understand how it is that the use of the
sting should so often cause the insect’s own death: for, if on the
whole the power of stinging be useful to the social community, it
will fulfill all the requirements of natural selection, though it may
cause the death of some few members. If we admire the truly wonderful
power of scent by which the males of many insects find their females,
can we admire the production for this single purpose of thousands
of drones, which are utterly useless to the community for any other
purpose, and which are ultimately slaughtered by their industrious and
sterile sisters? It may be difficult, but we ought to admire the savage
instinctive hatred of the queen-bee, which urges her to destroy the
young queens, her daughters, as soon as they are born, or to perish
herself in the combat; for undoubtedly this is for the good of the
community; and maternal love or maternal hatred, though the latter
fortunately is most rare, is all the same to the inexorable principle
of natural selection. If we admire the several ingenious contrivances
by which orchids and many other plants are fertilized through insect
agency, can we consider as equally perfect the elaboration of dense
clouds of pollen by our fir-trees, so that a few granules may be wafted
by chance on to the ovules?
INSTINCTS AS A DIFFICULTY.
[Origin of
Species,
page 205.]
Many instincts are so wonderful that their development will probably
appear to the reader a difficulty sufficient to overthrow my whole
theory. I may here premise that I have nothing to do with the origin of
the mental powers, any more than I have with that of life itself. We
are concerned only with the diversities of instinct and of the other
mental faculties in animals of the same class.
I will not attempt any definition of instinct. It would be easy to
show that several distinct mental actions are commonly embraced by
this term; but every one understands what is meant when it is said
that instinct impels the cuckoo to migrate and to lay her eggs in
other birds’ nests. An action, which we ourselves require experience
to enable us to perform, when performed by an animal, more especially
by a very young one, without experience, and when performed by many
individuals in the same way, without their knowing for what purpose it
is performed, is usually said to be instinctive. But I could show that
none of these characters are universal. A little dose of judgment or
reason, as Pierre Huber expresses it, often comes into play, even with
animals low in the scale of nature.
* * * * *
[Page 206.]
If we suppose any habitual action to become inherited--and it can be
shown that this does sometimes happen--then the resemblance between
what originally was a habit and an instinct becomes so close as not
to be distinguished. If Mozart, instead of playing the piano-forte
at three years old with wonderfully little practice, had played a
tune with no practice at all, he might truly be said to have done
so instinctively. But it would be a serious error to suppose that
the greater number of instincts have been acquired by habit in
one generation, and then transmitted by inheritance to succeeding
generations. It can be clearly shown that the most wonderful instincts
with which we are acquainted, namely, those of the hive-bee and of many
ants, could not possibly have been acquired by habit.
* * * * *
[Page 208.]
Why, it has been asked, if instinct be variable, has it not granted
to the bee “the ability to use some other material when wax was
deficient”? But what other natural material could bees use? They will
work, as I have seen, with wax hardened with vermilion or softened with
lard. Andrew Knight observed that his bees, instead of laboriously
collecting propolis, used a cement of wax and turpentine, with which he
had covered decorticated trees. It has lately been shown that bees,
instead of searching for pollen, will gladly use a very different
substance, namely, oatmeal. Fear of any particular enemy is certainly
an instinctive quality, as may be seen in nestling birds, though it
is strengthened by experience, and by the sight of fear of the same
enemy in other animals. The fear of man is slowly acquired, as I
have elsewhere shown, by the various animals which inhabit desert
islands; and we see an instance of this even in England, in the greater
wildness of all our large birds in comparison with our small birds;
for the large birds have been most persecuted by man. We may safely
attribute the greater wildness of our large birds to this cause; for in
uninhabited islands large birds are not more fearful than small; and
the magpie, so wary in England, is tame in Norway, as is the hooded
crow in Egypt.
SOME INSTINCTS ACQUIRED AND SOME LOST.
[Page 210.]
It may be doubted whether any one would have thought of training a
dog to point, had not some one dog naturally shown a tendency in this
line; and this is known occasionally to happen, as I once saw, in a
pure terrier: the act of pointing is probably, as many have thought,
only the exaggerated pause of an animal preparing to spring on its
prey. When the first tendency to point was once displayed, methodical
selection and the inherited effects of compulsory training in each
successive generation would soon complete the work; and unconscious
selection is still in progress, as each man tries to procure, without
intending to improve the breed, dogs which stand and hunt best. On the
other hand, habit alone in some cases has sufficed; hardly any animal
is more difficult to tame than the young of the wild rabbit; scarcely
any animal is tamer than the young of the tame rabbit; but I can hardly
suppose that domestic rabbits have often been selected for tameness
alone; so that we must attribute at least the greater part of the
inherited change from extreme wildness to extreme tameness to habit and
long-continued close confinement.
Natural instincts are lost under domestication: a remarkable instance
of this is seen in those breeds of fowls which very rarely or never
become “broody,” that is, never wish to sit on their eggs. Familiarity
alone prevents our seeing how largely and how permanently the minds
of our domestic animals have been modified. It is scarcely possible
to doubt that the love of man has become instinctive in the dog. All
wolves, foxes, jackals, and species of the cat genus, when kept tame,
are most eager to attack poultry, sheep, and pigs; and this tendency
has been found incurable in dogs which have been brought home as
puppies from countries such as Tierra del Fuego and Australia, where
the savages do not keep these domestic animals. How rarely, on the
other hand, do our civilized dogs, even when quite young, require
to be taught not to attack poultry, sheep, and pigs! No doubt they
occasionally do make an attack, and are then beaten; and, if not cured,
they are destroyed; so that habit and some degree of selection have
probably concurred in civilizing by inheritance our dogs. On the other
hand, young chickens have lost, wholly by habit, that fear of the dog
and cat which no doubt was originally instinctive in them; for I am
informed by Captain Hutton that the young chickens of the parent-stock,
the _Gallus bankiva_, when reared in India under a hen, are at first
excessively wild. So it is with young pheasants reared in England under
a hen. It is not that chickens have lost all fear, but fear only of
dogs and cats, for if the hen gives the danger-chuckle, they will run
(more especially young turkeys) from under her, and conceal themselves
in the surrounding grass or thickets; and this is evidently done for
the instinctive purpose of allowing, as we see in wild ground-birds,
their mother to fly away. But this instinct retained by our chickens
has become useless under domestication, for the mother-hen has almost
lost by disuse the power of flight.
Hence, we may conclude that, under domestication, instincts have been
acquired, and natural instincts have been lost, partly by habit,
and partly by man selecting and accumulating, during successive
generations, peculiar mental habits and actions, which at first
appeared from what we must in our ignorance call an accident.
INNUMERABLE LINKS NECESSARILY LOST.
[Origin of
Species,
page 264.]
The main cause 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 must the number of
intermediate varieties, which have formerly existed, 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 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
intermediate 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 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_.
* * * * *
[Page 265.]
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 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 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.
PLENTY OF TIME FOR THE NECESSARY GRADATIONS.
[Page 266.]
Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected that time can not 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,” 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.
* * * * *
[Page 269.]
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 subaërial denudation, Mr. Croll
shows, by calculating the known amount of sediment annually brought
down by certain rivers, relatively to their areas of drainage, that
one thousand 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 even 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 hundred years implies, represented as it is by a measure utterly
insignificant in a hall of the above dimensions. Several eminent
breeders, during a single lifetime, have so largely modified some of
the higher animals, which propagate their kind much more slowly than
most of the lower animals, that they have formed what well deserves
to be called a new sub-breed. Few men have attended with due care to
any one strain for more than half a century, so that a hundred years
represents the work of two breeders in succession.
* * * * *
[Page 270.]