Download PPT - 7 - Darwin`s `On the Origin of Species`

Darwin’s book
Darwin had been working for several years on his major
“species” book, but was still years away from finishing it.
On July 20, 1858, he decided to write an abstract of this
work for quicker publication, to be called An Abstract of
an Essay on the Origin of Species and Varieties Through
Natural Selection, a title the publisher (John Murray)
rightly considered too clunky.
The full title of the book in its first edition was On the
Origin of Species by Means of Natural Selection, or the
Preservation of Favoured Races in the Struggle for Life,
but by the sixth edition this had been shortened to just The
Origin of Species.
Title Page of the First
Edition of On the Origin of
Species.
“Races” in the long subtitle
(typical of Victorian books)
had the meaning of
“varieties” and not the
modern meaning. The first
use of “races” in the book is
a reference to “races of
cabbage.”
Darwin is identified as the
author of his journal about
the voyage of the Beagle.
Review of On the Origin of Species
On December 26, 1859, a very favorable review of Darwin’s
book appeared in the Times of London. The Times was a
very conventional, very orthodox newspaper (satirized by
Anthony Trollope as “Jupiter Olympus” – the voice of the
gods), which ran only one or two book reviews a month, and
it made no sense that if they reviewed On the Origin of
Species they would not severely attack it. Darwin was
greatly surprised, and wondered who had written it. The
review sounded to him as though it had been written by
Huxley, but it was inconceivable, given Huxley’s reputation,
that they would ever ask him to write for them.
Darwin wrote to Huxley about the review ….
“The author is a literary man & German scholar. – He has read
my book attentively; but what is very remarkable, it seems that
he is a profound naturalist. He knows my Barnacle book, &
appreciates it too highly. – Lastly he writes & thinks with
uncommon force & clearness; & what is even still rarer his
writing is seasoned with most pleasant wit… Who can it be?
Certainly I should have said that there was only one man in
England who could have written this essay & that you were the
man. But I suppose that I am wrong, & that there is some hidden
genius of great calibre. For how could you influence Jupiter
Olympus & make him give 3 ½ columns to pure science. The old
Fogies will think the world will come to an end. Well whoever
the man is, he has done great service to the cause, far more than
by a dozen reviews in common periodicals. … If you should
happen to be acquainted with the author for Heaven-sake tell me
who he is.”
5 Separate Theses of Darwin’s Views of Evolution
These are the major theses of what has often been
called “Darwinism.”
(1) Evolution as such. This is the thesis that the world
is not constant or recently created nor perpetually
cycling but rather is steadily changing and that
organisms are transformed in time.
(2) Common descent
(3) Multiplication of species
(4) Gradualism
(5) Natural selection
Examples:
A common ancestor of chimpanzees and bonobos
(formerly “pygmy chimpanzees” or “hippie chimpanzees)
split into these two lines several million years ago.
A common ancestor of chimpanzees, bonobos, and
humans split about 5 to 7 million years ago. Many splits
seem to have occurred on the hominid line, with only one
species remaining today, Homo sapiens.
The apple maggot (Rhagoletis pomonella) originally fed
on hawthorn apples, but in North America has split since
1800 into two species, one feeding on hawthorns and one
on apples, which the maggot also found palatable.
(1) Evolution as such
(2) Common descent. This is the thesis that every
group of organisms descended from a common
ancestor and that all groups of organisms, including
animals, plants, and microorganisms, ultimately go
back to a single origin of life on earth.
(3) Multiplication of species
(4) Gradualism
(5) Natural selection
The Fundamental Tree of Life
(1) Evolution as such
(2) Common descent
(3) Multiplication of species. This thesis explains the
origin of the enormous organic diversity. It postulates
that species multiply, either by ‘splitting’ into
daughter species or by ‘budding,’ that is, by the
establishment of geographically isolated founder
populations that evolve to new species.
(4) Gradualism
(5) Natural selection
Humans and their nearest relatives
(1) Evolution as such
(2) Common descent
(3) Multiplication of species
(4) Gradualism. According to this thesis, evolutionary
change takes place through the gradual change of
populations and not by the sudden (saltational)
production of new individuals that represent a new
type. Many of the older “Darwinians,” including
Huxley, preferred saltation; for one thing, it was
thought that the Earth was not old enough for such
gradual processes to have occurred.
(5) Natural selection
(1) Evolution as such
(2) Common descent
(3) Multiplication of species
(4) Gradualism
(5) Natural selection. According to this thesis,
evolutionary change comes about through the abundant
production of genetic variation in every generation.
The relatively few individuals who survive, owing to a
particularly well-adapted combination of inheritable
characters, give rise to the next generation.
The Natural Selection Argument
Fact 1. Every population has such high fertility that its
size would increase exponentially if not constrained.
This was pointed out by both William Paley and Thomas
Malthus, and is clear from our own observations of plants
and animals. Think of a single tomato plant, and all the
tomatoes it produces, and all the seeds in each tomato.
Think of a pine tree, and all the pine cones it produces
each year, all the seeds on each cone, and all the new pine
tree seedlings that can result therefrom – or think of a
redbud tree, or a maple tree, or almost any kind of weed.
Think of a queen honeybee and all the bees it can give
birth to each year. Think of all the children a human
female could give birth to in her lifetime.
Fact 2. The size of populations, except for temporary
annual fluctuations, remains stable over time
(observed steady-state stability). This we see from our
own observations again. A gardener who plants a few
tomato plants one year doesn’t necessarily find thousands
of tomato plants in the garden the next year. A
homeowner with a prolific crabapple tree doesn’t find
tens of thousands of crabapple seedlings on his property
the next year (or at least, will get rid of most of them!).
Rabbits breed quickly don’t usually multiply as wildly as
they could, in principle.
Fact 3. The resources available to every species are
limited. This point was made by Malthus, but is evident
from our own observations, whether we are talking about
plants or animals.
Inference 1. There is intense competition (struggle for
existence) among the members of a species. This point
was made by Malthus with regard to human populations,
de Candolle with regard to all organisms. It is most
evident for animals, but is also true for plants and
bacteria.
Fact 4. No two individuals of a population are exactly
the same (population thinking). (This was evident to
animal breeders, who deliberately chose the best
specimens to propagate, to horticulturists, for the same
reason, and, in general, to all taxonomists – but not
generally appreciated by non-scientists.)
Inference 2. Individuals of a population differ from
each other in the probability of survival (i.e., natural
selection). This inference was made by Darwin, for
populations of all types of organisms, but had previously
been noted by others, usually with respect to a particular
species or variety.
Fact 5. Many of the differences among the individuals
of a population are, at least in part, heritable. This
information came from animal breeders (breeders of
pigeons, livestock, etc.)
Inference 3. Natural selection, continued over many
generations, results in evolution. This was Darwin’s
conclusion from the five facts and two inferences listed
above.
This line of reasoning makes natural selection pretty
unavoidable and thus pretty certain. What is not clear is
how important it is in nature and at what rate it occurs.
Also, there could be other types of selection, and Darwin
identified one: sexual selection (to be discussed later).
Chapter Titles in On the Origin of Species
1. Variation under Domestication
2. Variation under Nature
3. Struggle for Existence
4. Natural Selection
5. Laws of Variation
6. Difficulties on Theory
7. Instinct
8. Hybridisation
9. On the Imperfection of the Geological Record
10. On the Geological Succession of Organic Beings
11. Geographical Distribution
12. Geographical Distribution continued
13. Mutual Affinities of Organic Beings: Morphology: Embryology:
Rudimentary Organs
14. Recapitulation and Conclusion
Chapter 1: Variation under Domestication
Darwin’s first chapter discusses “artificial selection” – the
breeding of better horses, pigeons, plants, etc. by subjecting
them to new conditions and deliberately choosing to reproduce
the best stock or the individuals with the desired characteristics.
This was a familiar topic to the British.
Darwin argues at length that it is not possible to distinguish
between new breeds produced by artificial human selection and
the new species produced in nature by natural selection.
Individuals of a domestic variety differ more from one another
than do individuals in a wild species.
More species occur in populous genera than in the less
populous – an unusual fact of no significance if one assumes
special creation, but understandable under the theory of natural
selection.
Pigeons
“Believing that it is always best to study some special
group, I have, after deliberation, taken up domestic
pigeons. I have kept every breed which I could purchase
or obtain, and have been most kindly favoured with skins
from several quarters of the world, more especially by the
Honourable W. Elliot from India, and by the Honourable
C. Murray from Persia. Many treatises in different
languages have been published on pigeons, and some of
them are very important, as being of considerable
antiquity. I have associated with several eminent fanciers,
and have been permitted to join two of the London Pigeon
Clubs. …
“The diversity of the breeds is something astonishing.
Compare the English carrier and the short-faced tumbler,
and see the wonderful difference in their beaks, entailing
corresponding differences in their skulls. The carrier,
more especially the male bird, is also remarkable from the
wonderful development of the carunculated skin about the
head, and this is accompanied by greatly elongated
eyelids, very large external orifices to the nostrils, and a
wide gape of mouth. The short-faced tumbler has a beak
in outline almost like that of a finch; and the common
tumbler has the singular and strictly inherited habit of
flying at a great height in a compact flock, and tumbling in
the air head over heels. …
“The runt is a bird of great size, with long, massive beak
and large feet; some of the sub-breeds of runts have very
long necks, others very long wings and tails, others
singularly short tails. The barb is allied to the carrier,
but, instead of a very long beak, has a very short and very
broad one. The pouter has a much elongated body, wings,
and legs; and its enormously developed crop, which it
glories in inflating, may well excite astonishment and even
laughter. The turbit has a very short and conical beak,
with a line of reversed feathers down the breast; and it has
the habit of continually expanding slightly the upper part
of the oesophagus. …
“The Jacobin has the feathers so much reversed along
the back of the neck that they form a hood, and it has,
proportionally to its size, much elongated wing and tail
feathers. The trumpeter and laugher, as their names
express, utter a very different coo from the other
breeds. The fantail has thirty or even forty tailfeathers, instead of twelve or fourteen, the normal
number in all members of the great pigeon family; and
these feathers are kept expanded, and are carried so
erect that in good birds the head and tail touch; the
oil-gland is quite aborted. Several other less distinct
breeds might have been specified.”
Variability of pigeons: A carrier pigeon and a pouter pigeon
Chapter 1 continued
Darwin, who had discussed
pigeons with many pigeonbreeders, described their
immense variability, but
concluded that “Great as
are the differences between
the breeds of the pigeon, I
am fully convinced that all
are descended from the
rock-pigeon Columba
livia.” Pigeon breeders,
who thought they were just
perfecting each breed, did
not believe Darwin.
Columba livia
The wild rock-pigeon
or rock-dove.
Variations
in domestic
fowl:
Hamburg
fowl (upper
left);
Spanish fowl
(upper
right);
Polish fowl
(bottom)
Chapter 2: Variation under Nature
In Chapter 2, Darwin showed that animal and plant populations
in the wild exhibit considerable variation. Some variations (like
monstrosities and variations due to the environment) are not
inheritable, but others are, and slight variations can build up into
large differences.
Darwin stated that variations could exist between varieties in a
species and between species, but the distinction between species
and varieties is difficult to ascertain and probably not
meaningful. This was proved by the fact that botanists and
zoologists often disagreed in classifying different organisms as
members of different species or members of different varieties:
“few well-marked and well-known varieties can be named which
have not been ranked as species by at least some competent
judges.”
Chapter 2 continued …
Darwin regarded the line between species and varieties as
somewhat arbitrary, although variation between species
would have to be greater than variation between varieties.
But larger samples of organisms of a species exhibited
greater variation, so species or genera that extended over
greater geographical areas would exhibit greater variation
(have more species or subspecies) than those confined to a
smaller area.
“I look at individual differences, though of small interest to
the systematist, as of high importance for us, as being the
first step towards such slight varieties as are barely thought
worth recording in works on natural history. And I look at
varieties which are in any degree more distinct and
permanent, as steps leading to more strongly marked and
more permanent varieties; and at these latter, as leading to
sub-species, and to species. … I attribute the passage of a
variety, from a state in which it differs very slightly from its
parent to one in which it differs more, to the action of
natural selection in accumulating (as will hereafter be more
fully explained) differences of structure in certain definite
directions. Hence I believe a well-marked variety may be
justly called an incipient species …”
Chapter 3: Struggle for Existence
This chapter describes the constant struggle for existence in
nature. Darwin mentions Malthus as an inspiration for this
insight. Darwin describes the constant struggle for existence
among organisms, mainly between individuals of the same
species but also to some extent between individuals of different
species. Nature itself also puts pressure on individuals through
natural disasters, epidemics, changes in climate, etc.
Individuals with better characteristics predominate, leading to
wonderful adaptations found in nature: the woodpecker’s beak
that allows it to catch insects to eat, the structure of a parasite
allowing it to attach to its host and feed on it, the ability of a
beetle to dive underwater for food, seeds that can be carried far
away by the wind, etc. Species with larger populations are
more likely to be able to adapt and survive by leaving more
progeny.
Darwin points out that the struggle for existence is easy
to overlook or forget about, despite its overwhelming
importance:
“We behold the face of nature bright with gladness, we
often see superabundance of food; we do not see, or we
forget, that the birds which are idly singing round us
mostly live on insects or seeds, and are thus constantly
destroying life; or we forget how largely these
songsters, or their eggs, or their nestlings, are
destroyed by birds and beasts of prey; we do not always
bear in mind, that though food may be now
superabundant, it is not so at all seasons of each
recurring year.”
“Owing to this struggle for life, any variation, however
slight and from whatever cause proceeding, if it be in
any degree profitable to an individual of any species, in
its infinitely complex relations to other organic beings
and to external nature, will tend to the preservation of
that individual, and will generally be inherited by its
offspring. The offspring, also, will thus have a better
chance of surviving, for, of the many individuals of any
species which are periodically born, but a small number
can survive. I have called this principle, by which each
slight variation, if useful, is preserved, by the term of
Natural Selection, in order to mark its relation to man's
power of selection.”
Chapter 4: Natural Selection
Chapter 4 fleshes out the operation of natural selection, a
metaphorical term to suggest that nature can act on
organisms in a way that results in changes like those of an
animal breeder. Darwin’s argument is based on the facts
we discussed earlier: (1) Organisms produce many more
offspring than necessary just to replace the parent
organisms; (2) resources to support life are limited, leading
to competition (the “struggle for existence”); (3) individual
organisms have different traits, which influence their
survival and reproduction rates; (4) organisms with the best
such traits leave the most progeny, and over many
generations these traits come to predominate and may lead
to new species: i.e., evolution.
Chapter 4 contains the only diagram in On the Origin of
Species, a “tree of life” diagram, reproduced below.
Chapter 5: Laws of Variation
This is the least successful chapter in On the Origin of Species,
and does not contain much of value. Darwin really doesn’t
know what the laws of variation are, and he knows he doesn’t,
because he says, “Our ignorance of the laws of variation is
profound. Not in one case out of a hundred can we pretend to
assign any reason why this or that part differs, more or less,
from the same part in the parents.” The chapter ends:
“Whatever the cause may be of each slight difference in the
offspring from their parents – and a cause for each must exist –
it is the steady accumulation, through natural selection, of such
differences, when beneficial to the individual, that gives rise to
all the more important modifications of structure, by which the
innumerable beings on the face of this earth are enabled to
struggle with each other, and the best adapted to survive.”
Chapter 6: Difficulties on Theory
Darwin wanted to answer objections to his theory before the
critics could make them. He addresses two in this chapter.
“Firstly, why, if species have descended from other species by
insensibly fine gradations, do we not everywhere see
innumerable transitional forms? Why is not all nature in
confusion instead of the species being, as we see them, well
defined?”
Darwin points out that as organisms become better adapted and
win in the struggle for existence, the transitional forms must lose
and disappear. This doesn’t happen instantaneously, of course,
so organisms are usually not perfectly adapted to their way of
life. The existence of organisms that are not perfectly adapted is
not compatible with the concept of a creator who created
perfectly adapted organisms.
“Secondly, is it possible that an animal having, for
instance, the structure and habits of a bat, could have been
formed by the modification of some animal with wholly
different habits? Can we believe that natural selection
could produce, on the one hand, organs of trifling
importance, such as the tail of a giraffe, which serves as a
fly-flapper, and, on the other hand, organs of such
wonderful structure, as the eye, of which we hardly as yet
fully understand the inimitable perfection?”
Darwin admitted he could not give complete answers to
any specific questions of this sort, but that with greater
knowledge of present and former organisms the evolution
of such traits and organs would be understood.
“To suppose that the eye, with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and
chromatic aberration, could have been formed by natural
selection, seems, I freely confess, absurd in the highest possible
degree. Yet reason tells me, that if numerous gradations from a
perfect and complex eye to one very imperfect and simple, each
grade being useful to its possessor, can be shown to exist; if
further, the eye does vary ever so slightly, and the variations be
inherited, which is certainly the case; and if any variation or
modification in the organ be ever useful to an animal under
changing conditions of life, then the difficulty of believing that a
perfect and complex eye could be formed by natural selection,
though insuperable by our imagination, can hardly be
considered real.”
Darwin went on to point out numerous example of things
that look like steps in the evolution of the eye in a variety
of different animals, very close to our current
understanding of how, in fact, the eye evolved. He adds,
“If it could be demonstrated that any complex organ
existed, which could not possibly have been formed by
numerous, successive, slight modifications, my theory
would absolutely break down. But I can find out no such
case.”
In recent years “intelligent design” advocates have listed
such complex organs, but evolutionary biologists have
been successful in determining how they evolved, often
from organs that originally served other purposes.
“We should be extremely cautious in concluding that an
organ could not have been formed by transitional
gradations of some kind. Numerous cases could be given
amongst the lower animals of the same organ performing
at the same time wholly distinct functions.”
Darwin cites, as an example, the swimbladder of fish,
whose purpose was to permit flotation at different depths
in the ocean, but which clearly evolved into the lung of
the higher vertebrates, which is used for respiration.
“I can, indeed, hardly doubt that all vertebrate animals
having true lungs have descended by ordinary generation
from an ancient prototype, of which we know nothing,
furnished with a floating apparatus or swimbladder.”
Chapter 7: Instinct
This chapter deals with another difficulty Darwin had
stated at the beginning of Chapter 6: “Thirdly, can
instincts be acquired and modified through natural
selection? What shall we say to so marvellous an instinct
as that which leads the bee to make cells, which have
practically anticipated the discoveries of profound
mathematicians?”
Darwin was writing about the hexagonal shape of the wax
cells made by the honeybee. He also mentions birds that
lay their eggs in other birds’ nests (avoiding having to care
for them) and certain ants that act as slaves to other ants,
and other examples.
Darwin indicated that instinct is difficult to define, but that
it related to habitual activities that were not learned. He
was sure that instinct was also the result of natural
selection, which then acted on behavior as well as physical
characteristics. Instinctive behavior would have developed
very slowly, over long periods of time.
Many chapters in On the Origin of Species were greatly
abridged compared to what they would have been had
Darwin written his “big species book” and not been rushed
into writing On the Origin of Species. He returned to
instinct in other books, notably The Expression of the
Emotions in Man and Animals, since he regarded emotions
as instincts that evolved through natural selection.
Chapter 8: Hybridisation
In Chapter 8 Darwin discussed hybridization, the crossing of
two species (i.e., one parent from each species). It was
thought at the time that if two individuals were of the same
species, their offspring would be fertile, but if they were of
different species, their offspring would be sterile.
The hybrids are usually sterile, and when they are fertile the
offspring of two hybrids are usually sterile. Darwin argued
that this may just be because of dissimilarities in their
reproductive organs, not because the individuals were or were
not of the same species or same variety. Often, however, the
reasons for sterility were not known.
Darwin believed their was no good way to distinguish
between species and varieties.
Chapter 9: Imperfection of the Geological Record
The theory of natural selection indicates that there were
intermediate forms between existing species and their
ancestral species, which normally would be extinct. These
might occur as fossils in the geological record but few were
known in Darwin’s time. He argued that their absence is
not a disproof of the theory. Gaps in the fossil record are to
be expected because the intermediate fossils may have been
destroyed or just not yet discovered.
What about forms intermediate between two existing
related species? They may never actually have existed,
according to the theory of evolution through natural
selection, only forms intermediate between each of them
and their common ancestor.
In many cases, intermediate forms were never very numerous or
widespread, as natural selection was constantly operating on
these forms.
Darwin accepted Lyell’s theory of uniformitarianism, that
geological processes had been slowly but constantly acting on
the earth’s surface, in many cases probably removing some of
the intermediate forms. Indeed, millions of years of strata have
disappeared completely in many parts of the world.
“[I]t is highly important for us to gain some notion, however
imperfect, of the lapse of years. During each of these years, over
the whole world, the land and the water has been peopled by
hosts of living forms. What an infinite number of generations,
which the mind cannot grasp, must have succeeded each other
in the long roll of years! Now turn to our richest geological
museums, and what a paltry display we behold!”
Modern geology was still a young science when On the
Origin of Species was published barely 75 years after
James Hutton’s first published work. Since its time there
has been much more work carried out and many new
fossils discovered. Many intermediate forms have been
discovered, but, clearly, not all. Many parts of the world
that were not explored in Darwin’s time have been since.
Most remarkably, think of all the paleoanthropological
studies that have been carried out on the African continent
and that have begun to fill in the “tree of life” beginning
with the common ancestor of chimpanzees and humans.
This particular “difficulty” cited by Darwin is not
presently regarded as a difficulty.
Chapter 10: On the Geological Succession of
Organic Beings
Darwin referred back to his “tree of life” in chapter 4 to
describe his picture of a “succession of types” as new
species constantly appear on earth, evolve and flourish,
and finally decline and become extinct, perhaps leaving
living descendants, but perhaps bringing their line to an
end.
Darwin believed that the geological record of the
succession of organic beings better agreed with the slow
and gradual modification of organic beings, through
“descent with modification” that with the immutability of
species, which the record clearly contradicts.
Chapters 11 and 12: Geographical Distribution
and Geographical Distribution continued
In these chapters Darwin discusses a variety of topics relating
evolution and speciation to biogeography:
1. Similar climates in different parts of the world may have
similar or dissimilar species – there is no hard and fast rule.
2. Large land masses separated from each other, and islands,
and separated bodies of water, all have their own generally
very different species.
3. When species can migrate to another land or water area,
they may evolve into different species: migration and
natural selection both affect the nature of the species found.
4. Darwin believed species originated in one place and
then migrated to other places, rather than appearing
(being created) differently in different places.
5. Some species migrate easily (birds and plants whose
seeds can migrate) while others do not (such as most
land animals).
6. Geological processes caused land and water levels to
fluctuate, affording opportunities for species to evolve
due to the appearance and disappearance of water
barriers. Darwin discussed the effect of the ice ages in
leading to new and different species and the extinction
of some.
Chapter 12 of The Origin of Species continues the
discussion of the effect of geographical distribution and
separation by focusing on island life. The Galápagos
Islands are used as the prime example, because Darwin had
visited them on the voyage of the HMS Beagle and written
about their species of animals, which exhibited the effects
of natural selection.
“I have carefully searched the oldest voyages, but have not
finished my search; as yet I have not found a single
instance, free from doubt, of a terrestrial mammal
(excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from a
continent or great continental island; and many islands
situated at a much less distance are equally barren.”
Chapter 13: Mutual Affinities of Organic Beings:
Morphology: Embryology: Rudimentary Organs.
In Chapter 13 Darwin shows that his theory explains many
different observations about organisms: their similarities
(internal, not necessarily external, because unrelated
species may have developed similar adaptations due to
similar environments), their morphology (structure),
embrology, and their rudimentary and atrophied structures
(relics of once-useful organs). The naturalists’ grouping of
organisms into various taxa (species, genera, etc.) is the
result of a relationship due to descent with modification
(“evolution” in modern language) from common ancestors.
Darwin insists that assuming the existence of a divine plan
adds nothing to our understanding of natural relationships.
The evolutionary relationships are evident in homologous
structures:
“What can be more curious than that the hand of a man,
formed for grasping, that of a mole for digging, the leg of
the horse, the paddle of the porpoise, and the wing of the
bat, should all be constructed on the same pattern, and
should include the same bones, in the same relative
positions?”
“The real affinities of all organic beings are due to
inheritance or community of descent. The natural system
is a genealogical arrangement, in which we have to
discover the lines of descent by the most permanent
characters, however slight their vital importance may
be.”
Chapter 14: Recapitulation and Conclusion
In his concluding chapter, Darwin summarized
the implications of his argument, confidently adding
that “young and rising naturalists” would share his
vision and reject the prejudices that led many
biologists to cling to older ideas. This chapter includes
Darwin’s only use of the word evolved (or any
variations thereof) – it’s the last word of the book.
Remember that Darwin never spoke of
“evolution,” rather of “descent with modification.”
“Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to
convince experienced naturalists whose minds are stocked with a
multitude of facts all viewed, during a long course of years, from a
point of view directly opposite to mine. It is so easy to hide our
ignorance under such expressions as the ‘plan of creation,’ ‘unity of
design,’ etc., and to think that we give an explanation when we only
restate a fact. Any one whose disposition leads him to attach more
weight to unexplained difficulties than to the explanation of a certain
number of facts will certainly reject my theory. A few naturalists,
endowed with much flexibility of mind, and who have already begun
to doubt on the immutability of species, may be influenced by this
volume; but I look with confidence to the future, to young and rising
naturalists, who will be able to view both sides of the question with
impartiality. Whoever is led to believe that species are mutable will
do good service by conscientiously expressing his conviction; for
only thus can the load of prejudice by which this subject is
overwhelmed be removed.”
Chapter 14 continued
Darwin’s last chapter refers to the whole book as “one long
argument.” He summarizes his theory and its difficulties, and
states that he believes it will lead to a “revolution in natural
history.”
The closing paragraph of On the Origin of Species:
“It is interesting to contemplate an entangled bank, clothed
with many plants of many kinds, with birds singing on the
bushes, with various insects flitting about, and with worms
crawling through the damp earth, and to reflect that these
elaborately constructed forms, so different from each other,
and dependent on each other in so complex a manner, have all
been produced by laws acting around us.”
Closing paragraph, continued …
“These laws, taken in the largest sense, being Growth
with Reproduction; Inheritance which is almost
implied by reproduction; Variability from the indirect
and direct action of the external conditions of life, and
from use and disuse; a Ratio of Increase so high as to
lead to a Struggle for Life, and as a consequence to
Natural Selection, entailing Divergence of Character
and the Extinction of less-improved forms. Thus, from
the war of nature, from famine and death, the most
exalted object which we are capable of conceiving,
namely, the production of the higher animals, directly
follows.”
Closing paragraph, concluded …
“There is grandeur in this view of life, with its several
powers, having been originally breathed into a few forms
or into one; and that, whilst this planet has gone cycling on
according to the fixed law of gravity, from so simple a
beginning endless forms most beautiful and most wonderful
have been, and are being, evolved.”
The last word of The Origin of Species is the only example
in the book of the word “evolved” or any words with the
same root. Presumably he tried to avoid using the word
because Lamarck and Chambers had used it and been
criticized.
Evidence for a Common Ancestor of All Life on Earth
1869: Nucleic acids were first isolated by Friedrich Miescher.
1944: Avery et al. identify DNA was identified as the genetic
material of all life – conceivably there could have been many
different genetic materials, or possibly each species could have had
a different genetic material. (Maybe currently unknown species
might have genetic materials different from DNA.) However, all
known life uses the same polymer, polynucleotide (DNA or RNA),
for storing species specific information. All known organisms base
replication on the duplication of this molecule.
1953: Crick and Watson discover the molecular structure of DNA –
a double helix. The DNA used by living organisms is synthesized
using only four nucleosides (deoxyadenosine, deoxythymidine,
deoxycytidine, and deoxyguanosine) out of the dozens known (at
least 102 occur naturally).
LUCA: Last Universal Common Ancestor
Working backwards in time, all life apparently had at least
one common ancestor, perhaps more.
Perhaps there were several early life forms – maybe some
with different genetic material, not the DNA or RNA we
are familiar with. But if so, all but one disappeared
(unless there are other life forms on Earth, with different
genetic material, that we have not yet discovered).
Of the common ancestors of all known life on Earth, the
last one – before the first – is the Last Universal Common
Ancestor, or LUCA.
In 1864 Darwin complained that Wallace always spoke
of “Darwin’s theory” without giving himself credit, and
Wallace wrote back:
“As to the theory of Natural Selection I shall always
maintain it to be yours and yours only. You had worked
it out in details I had never thought of, years before I
had a ray of light on the subject, and my paper would
never have convinced anybody or been noticed as more
than an ingenious speculation, whereas your book has
revolutionized the study of natural history, and carried
away captive the best men of the present age.”
William Irvine, in Apes, Angels, and Victorians, called
Wallace “so retiring, so reassuring, so generous.”
Darwin’s only reference to humans – apart from occasional
mentions like their hand bones – in the first edition of On
the Origin of Species was this: “Light will be thrown on
the origin of man and his history.” In later editions this
became “Much light will be thrown …”
He apparently felt there was enough in his book already to
disturb many readers without discussing humans – but this
was the subject most readers were interested in.
It was not long, however, before Huxley and Lyell wrote
books on this subject: Huxley’s Evidence as to Man's Place
in Nature and Lyell’s Geological Evidences of the Antiquity
of Man, both appearing in 1863.
Darwin eventually dealt with the topic of humans and their
history in The Descent of Man, published in 1871.