PPT - 7 - Darwin's 'on the Origin of Species'

8/2/2019 PPT - 7 - Darwin's 'on the Origin of Species'

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Darwins 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 calledAn 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 thePreservation of Favoured Races in the Struggle for Life,

but by the sixth edition this had been shortened to just The

Origin of Species.


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Title Page of the First

Edition ofOn the Origin of

Species.

Races in the long subtitle

(typical of Victorian books)

had the meaning of

varieties and not themodern 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 theBeagle.


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Review ofOn the Origin of Species

On December 26, 1859, a very favorable review of Darwins

book appeared in the Times of London. The Times was avery 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 byHuxley, but it was inconceivable, given Huxleys reputation,

that they would ever ask him to write for them.

Darwin wrote to Huxley about the review .


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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 withuncommon 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 theman. 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 columns to pure science. The old

Fogies will think the world will come to an end. Well whoeverthe 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.


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5 Separate Theses of Darwins Views of Evolution

These are the major theses of what has often beencalled Darwinism.

(1) Evolution as such. This is the thesis that the world

is not constant or recently created nor perpetuallycycling but rather is steadily changing and that

organisms are transformed in time.

(2) Common descent(3) Multiplication of species

(4) Gradualism

(5) Natural selection


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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 splitsseem to have occurred on the hominid line, with only one

species remaining today,Homo sapiens.

The apple maggot (Rhagoletis pomonella) originally fedon 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.


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(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


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The Fundamental Tree of Life


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(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 intodaughter species or by budding, that is, by the

establishment of geographically isolated founder

populations that evolve to new species.

(4) Gradualism

(5)Natural selection


10/64Humans and their nearest relatives


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(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 suchgradual processes to have occurred.

(5) Natural selection


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(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.


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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 thetomatoes 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 therefromor think of aredbud 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.


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Fact 2. The size of populations, except for temporary

annual fluctuations, remains stable over time

(observed steady-state stability). This we see from ourown observations again. A gardener who plants a few

tomato plants one year doesnt necessarily find thousands

of tomato plants in the garden the next year. A

homeowner with a prolific crabapple tree doesnt findtens of thousands of crabapple seedlings on his property

the next year (or at least, will get rid of most of them!).

Rabbits breed quickly dont usually multiply as wildly as

they could, in principle.


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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 aboutplants or animals.

Inference 1. There is intense competition (struggle for

existence) among the members of a species. This pointwas 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.


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Fact 4. No two individuals of a population are exactly

the same (population thinking). (This was evident to

animal breeders, who deliberately chose the bestspecimens to propagate, to horticulturists, for the same

reason, and, in general, to all taxonomistsbut 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 previouslybeen noted by others, usually with respect to a particular

species or variety.


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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 ofpigeons, livestock, etc.)

Inference 3. Natural selection, continued over many

generations, results in evolution. This was Darwins

conclusion from the five facts and two inferences listed

above.

This line of reasoning makes natural selection prettyunavoidable 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).


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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 Distribution12. Geographical Distribution continued

13. Mutual Affinities of Organic Beings: Morphology: Embryology:

Rudimentary Organs

14. Recapitulation and Conclusion


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Chapter 1: Variation under Domestication

Darwins first chapter discusses artificial selection the

breeding of better horses, pigeons, plants, etc. by subjectingthem 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 distinguishbetween 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

populousan unusual fact of no significance if one assumes

special creation, but understandable under the theory of natural

selection.


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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 theHonourable 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 considerableantiquity. I have associated with several eminent fanciers,

and have been permitted to join two of the London Pigeon

Clubs.


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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, entailingcorresponding differences in their skulls. Thecarrier,

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 elongatedeyelids, very large external orifices to the nostrils, and a

wide gape of mouth. Theshort-faced tumbler has a beak

in outline almost like that of a finch; and thecommon

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.


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Therunt 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, otherssingularly short tails. Thebarb is allied to the carrier,

but, instead of a very long beak, has a very short and very

broad one. Thepouter has a much elongated body, wings,

and legs; and its enormously developed crop, which itglories in inflating, may well excite astonishment and even

laughter. Theturbit 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 partof the oesophagus.


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TheJacobin 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. Thetrumpeter andlaugher, as their names

express, utter a very different coo from the other

breeds. Thefantailhas thirty or even forty tail-

feathers, 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.


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Variability of pigeons: A carrier pigeon and a pouter pigeon


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Chapter 1continued

Darwin, who had discussed

pigeons with many pigeon-breeders, described their

immense variability, but

concluded that Great as

are the differences betweenthe breeds of the pigeon, I

am fully convinced that all

are descended from the

rock-pigeon Columbalivia. Pigeon breeders,

who thought they were just

perfecting each breed, did

not believe Darwin.

Columba livia

The wild rock-pigeon

or rock-dove.


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Variations

in domestic

fowl:Hamburg

fowl (upper

left);

Spanish fowl(upper

right);

Polish fowl

(bottom)


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Chapter 2: Variation under Nature

In Chapter 2, Darwin showed that animal and plant populations

in the wild exhibit considerable variation. Some variations (likemonstrosities 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 aspecies 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 asmembers 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.


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Chapter 2 continued

Darwin regarded the line between species and varieties as

somewhat arbitrary, although variation between specieswould 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.


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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 atvarieties 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 morefully explained) differences of structure in certain definite

directions. Hence I believe a well-marked variety may be

justly called an incipient species


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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 woodpeckers 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.


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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 alwaysbear in mind, that though food may be now

superabundant, it is not so at all seasons of each

recurring year.


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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, inits 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 betterchance 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 ofNatural Selection, in order to mark its relation to man's

power of selection.


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Chapter 4:Natural Selection

Chapter 4 fleshes out the operation of natural selection, a

metaphorical term to suggest that nature can act onorganisms in a way that results in changes like those of an

animal breeder. Darwins argument is based on the facts

we discussed earlier: (1) Organisms produce many more

offspring than necessary just to replace the parentorganisms; (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 bestsuch traits leave the most progeny, and over many

generations these traits come to predominate and may lead

to new species: i.e., evolution.


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Chapter 4 contains the only diagram in On the Origin of

Species, a tree of life diagram, reproduced below.


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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 doesnt

know what the laws of variation are, and he knows he doesnt,

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 parentsand a cause for each must exist

it is the steady accumulation, through natural selection, of suchdifferences, 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.


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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, welldefined?

Darwin points out that as organisms become better adapted and

win in the struggle for existence, the transitional forms must lose

and disappear. This doesnt 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.


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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 suchwonderful 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.

T h h i h ll i i i i bl i f


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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 naturalselection, 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; iffurther, 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 aperfect and complex eye could be formed by natural selection,

though insuperable by our imagination, can hardly be

considered real.


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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 theorywould absolutely break down. But I can find out no such

case.

In recent years intelligent design advocates have listedsuch complex organs, but evolutionary biologists have

been successful in determining how they evolved, often

from organs that originally served other purposes.


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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 givenamongst 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 depthsin 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 animalshaving true lungs have descended by ordinary generation

from an ancient prototype, of which we know nothing,

furnished with a floating apparatus or swimbladder.


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Chapter 7: Instinct

This chapter deals with another difficulty Darwin had

stated at the beginning of Chapter 6: Thirdly, caninstincts 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 thatlay 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.


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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 naturalselection, 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 writingOn the Origin of Species.

He returned toinstinct in other books, notably The Expression of the

Emotions in Man and Animals, since he regarded emotions

as instincts that evolved through natural selection.

Ch t 8 H b idi ti


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Chapter 8:Hybridisation

In Chapter 8 Darwin discussed hybridization, the crossing of

two species (i.e., one parent from each species). It wasthought 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 theoffspring 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.


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Chapter 9:Imperfection of the Geological Record

The theory of natural selection indicates that there were

intermediate forms between existing species and theirancestral species, which normally would be extinct. These

might occur as fossils in the geological record but few were

known in Darwins 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 existingrelated 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.

I i di f


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In many cases, intermediate forms were never very numerous or

widespread, as natural selection was constantly operating on

these forms.

Darwin accepted Lyells theory of uniformitarianism, that

geological processes had been slowly but constantly acting on

the earths 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 byhosts 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!


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Modern geology was still a young science when On the

Origin of Species was published barely 75 years after

James Huttons 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 Darwins 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.


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Chapter 10: On the Geological Succession of

Organic Beings

Darwin referred back to his tree of life in chapter 4 todescribe 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 slowand gradual modification of organic beings, through

descent with modification that with the immutability of

species, which the record clearly contradicts.

Chapters 11 and 12: G hi l Di t ib ti


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Chapters 11 and 12: Geographical Distribution

and Geographical Distribution continued

In these chapters Darwin discusses a variety of topics relatingevolution and speciation to biogeography:

1. Similar climates in different parts of the world may have

similar or dissimilar speciesthere 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.


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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 inleading to new and different species and the extinction

of some.

Chapter 12 of The Origin of Species continues the


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Chapter 12 ofThe Origin of Species continues the

discussion of the effect of geographical distribution and

separation by focusing on island life. The Galpagos

Islands are used as the prime example, because Darwin hadvisited them on the voyage of the HMSBeagle 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: M t l Affi iti f O i B i


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Chapter 13:Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs.

In Chapter 13 Darwin shows that his theory explains manydifferent 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.


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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 relativepositions?

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.


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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 includesDarwins only use of the word evolved(or any

variations thereof) its 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


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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 certainnumber 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 risingnaturalists, 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.


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Chapter 14 continued

Darwins 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 ofOn 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 wormscrawling 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.


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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 tolead 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.


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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 wonderfulhave been, and are being, evolved.

The last word ofThe 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.

E id f C A t f All Lif E th


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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 geneticmaterial of all lifeconceivably 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, allknown 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 DNAa 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).


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LUCA L t U i l C A t


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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 formsmaybe 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 onebefore the firstis the Last Universal Common

Ancestor, or LUCA.


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In 1864 Darwin complained that Wallace always spoke

of Darwins 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 wouldnever 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.

Darwins only reference to humans apart from occasional


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y p

mentions like their hand bonesin the first edition ofOn

the Origin of Specieswas this: Light will be thrown on

the origin of man and his history. In later editions thisbecame Much light will be thrown

He apparently felt there was enough in his book already to

disturb many readers without discussing humansbut this

was the subject most readers were interested in.

It was not long, however, before Huxley and Lyell wrote

books on this subject: HuxleysEvidence as to Man's Place

in Nature and Lyells Geological Evidences of the Antiquity

of Man, both appearing in 1863.

Darwin eventually dealt with the topic of humans and their

PPT - 7 - Darwin's 'on the Origin of Species'

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