A Mini-Lecture on The Origin of Species and The Descent of Man
[This page is under construction but is far enough along for students to read]
Thanks to Dr. Mark Haller and Dr. Patricia Hansell, who have helped me. I have used elements of their work in the following presentation.
I have three goals in this mini-lecture
1. Introduce the class to basic concepts of evolution
2. Discuss Darwin’s achievements in The Origin of Species (1859)
3. Introduce the class to The Descent of Man, especially chapter 21
1. The Evolution of Evolution
Evolution is the key concept for which Darwin
is remembered. What led up the moment in 1859 when he first published
his ideas?
a. Geological study of ancient objects
b. Darwin’s experience: the voyage of the Beagle
c. Intellectual currents of the times: Malthus on population
a. Geological study of ancient objects
By mid the middle of the 19th century the scientific world was alive with geological discovery. Scientists had noticed the stratification of the earth’s crust—the layering of one type of material on top of another; they’d noticed that the order of the strata was consistent in different regions, and that different strata contained different kinds of fossilized plants and animals.
In 1785 the scientific principle of uniformitarianism was first formulated, stating that processes of geological change are unchanging. Since these processes are usually quite slow, this principle constituted a challenge to the Biblical age of the earth (about 6000 years by some estimates).
One effort to explain the existence of different fossil species in different layers (visible in stone quarries and in caves) was that of Cuvier, who suggested that there had been a series of "worldwide catastrophes" (Hansell) after each of which life was created anew, in slightly more complex form. This doctrine was called catastrophism and it explained the evidence by requiring huge episodes of destruction for each stratum. That sounds, to us, kind of improbable: but that is because noone was really ready for the alternative, EVOLUTION, which Darwin was to propose in 1859.
Why not? Basically because the belief in the
fixity
of species and of characteristics was widespread. It was hard to
believe that one animal might actually, over the years, change its shape.
Much of what learned people read—the Bible, or Aristotle—implied or declared
that forms were final and could not be changed. Even multiple
disasters of the sort Cuvier portrayed were easier to handle.
1859 was not only the year of the publication of Darwin’s Origin but of the discovery of man-made axes along with the bones of extinct mammals "at great depths from the present-day surface" of the earth (Hansell): this was the first evidence that man had lived at the same time as these mammals.
Another attempt before Darwin to make sense
of the fossil record was that of Lamarck, who argued that "acquired" traits
are inherited. An "acquired" trait might be a change in appearance
during a lifetime could be passed on to offspring: If a giraffe,
confronting the problem of unavailable leaves to eat, stretches its neck,
this acquired trait would be inherited by baby giraffes, bit by bit over
the generations. Note that this was NOT what Darwin was to
argue, and is contradicted by the work of modern geneticists.
century preceding Darwin.
A third important figure, and one who, unlike
Lamarck and Cuvier, influenced Darwin, was the geologist Lyell.
As a Uniformitarian,
Lyell argued that only long-term geological change could have produced
the mountains and valleys of our day. He understood, but could not
explain, why species become extinct. Darwin read Lyell after his
return on the Beagle in 1836.
Sidebar: this acknowledgment of facts that defy explanation is common in the history of science. In IH 51, we studied Galileo’s revolution, which depended in part on Galileo’s fellow-astronomer Kepler rejecting the "Platonic" model of the heavens with which he’d begun his career, once he realized that it was not matched by the scientific evidence. Kepler himself argued that planetary orbits were elliptical, which Galileo himself could not believe and which defied understanding until Newton developed the theory of gravity. Facts that resist understanding are crucial contributors to progress—once they’ve stimulated new sorts of understanding.
2. Darwin's Experience: The Voyage of the
Beagle
Darwin went through a series of educational pursuits, "changing his major" and working both in studies of nature and theology, before setting out for a five year voyage on the British ship the Beagle in 1831. This was to be an epochal voyage: by the time Darwin returned, he was relying far less on the Bible he took with him than on the natural world he encountered.
And what did he encounter? Here are some examples.
On the Coasts of South America, Darwin found both small tree sloths and the bones of a huge sloth. He began to doubt the fixity of species that religious and traditionalist groups had asserted: species could change.
He also found bones of ancient horses, another surprise since everyone "knew" that the Spaniards had only brought this animal to the New World in the 1500s.
And he noticed that that anatomy of the South American fox differed from that of the English fox?
Clearly, something was going on: clearly, there was some sort of connection between a species and its environment.
Now the Beagle went westward, landing on the Galapagos Islands 600 miles off of South America. Now Darwin collected all sorts of animals, and on his return to England noticed that the finches—small birds—he’d brought back had interesting traits. He’d brought back specimens from different islands, and each island had its own breed.
(We now now that there are in fact 13 species of finches on the islands, known as Darwin finches).
And there was more!—
He noticed that finches differed especially
in the SIZE AND SHAPE OF THEIR
BEAK.
And decided that each beak was APPROPRIATE TO EACH BIRD’S ENVIRONMENTAL NICHE, such as the seeds on particular plants.
Note that Darwin spent 23 years studying and thinking before publishing his great work, THE ORIGIN OF SPECIES in 1859. He might have taken longer but realized that another scientist, Wallace, was reaching similar conclusions. In the early editions of the ORIGIN he did not refer to the evolution of man—a touchy topic then and now—but argued for the occurrence of other sorts of change.
Here are the highlights of that book:
Organisms have descended with modification: evolution has occurred.
The evolution is governed by natural selection.
The environment shaped the physical features of animal species.
The evidence of the finches was that all finches were descended from to a common ancestor (from the mainland).
One of the problems he faced was how to explain these changes. He came to a conclusion after reading Thomas Malthus' ESSAY ON POPULATION in 1838. Malthus explained that infant mortality kept adult population groups stable. The LIGHT BULB for Darwin was that certain young animals were also dying, and that the ONES THAT DIED WERE THE ONES THAT WERE LESS WELL-ADAPTED TO THEIR EINVIRONMENT.
In other words, if animals in a specied had
slight variations, the variations that supported survival—finding and eating
seeds—would be SELECTED, and those animals would survive to maturity and
(natch!) REPRODUCE IN GREATER NUMBERS. So the right sort of beak
would become, over the generations, typical in a certain habitat.
This is "natural selection" or (in some formulations) "survival of the
fittest." It is in essence a sort of ADAPTATION TO THE ENVIRONMENT.
(It is not the only sort of adaptation, for sure.)
Darwin did not explain exactly how variations were transmitted across generations. That was left to the biologist MENDEL.
Here is some other evidence for evolution across generations.
Some further points about ORIGIN OF THE SPECIES:
Though he knew it would be incendiary in any case, Darwin presented his material in an extremely careful way, beginning in the first chapter with DOMESTIC ANIMALS that Englishmen knew how to breed: horse, sheep, pigeons.
[Try this exercise for fun: go to Chapter One and quickly search for references to "dog." You’ll get a quick sense of how Darwin argues his case.
http://www.literature.org/authors/darwin-charles/the-origin-of-species/chapter-01.html
With this groundwork, variation among species in the NATURAL WORLD is far easier to stomach.
If selection in the domestic world of England
is by owners of animals, in the world of nature it is far fiercer.
Population pressure, harsh climates, and other forces (see Malthus above)
favor animals who are best adapted for their environment. The "struggle
for existence" is less a struggle among animals than a struggle to find
food and shelter, and Nature is a more efficient and consistent selector
(look at dog breeds again: some are bred in ways that guarantee an early
death through bad hips or other problems: nature would correct all this
quickly).
THE DESCENT OF MAN
It was not until 1871 that Darwin decided to
extend his argument to mankind, with the work that we are reading.
Here Darwin says the same pressures that affected all other animals affected
humans. But in his day there were no related fossils to use as evidence,
so he compared us to other mammals. In working on this chapter, note
that Darwin does not deny that mankind has the ability to make MORAL decisions.
Website links:
http://www.literature.org/authors/darwin-charles/
http://www.talkorigins.org/origins/other-links.html
http://web.clas.ufl.edu/users/rhatch/pages/02-TeachingResources/readingwriting/darwin/05-DARWIN-PAGE.html