DARWIN’S EVIDENCE #1
OLD EARTH
SUMMARY:
Charles Lyell wrote a book using the ideas of James Hutton that showed how the earth
had undergone slow, continuous physical change. The same forces such as wind, rain,
frost, erosion, uplift by the heat in the earth, and sedimentation were at work on the earth
many years ago just as they are today. This idea, called uniformitarianism, showed that
the earth could be at least millions of years old, with no sign of a beginning, no prospect
of an end. We cannot see most of the changes in the earth since they are slow and our
lives are very short.
QUOTE:
“It is hard for me to convince the reader who is not a geologist that the earth is very old.
We can get an idea of how old the earth is by knowing the agents at work. The eroded
material of the earth’s crust has been deposited elsewhere to make sedimentary rocks. If
we look at the thickness of these rocks we can tell how long it took to make them.”
(Darwin, 1858, p.153)
DARWIN’S EVIDENCE #2
FOSSILS
SUMMARY:
Even though there were large gaps in the fossil record, there were some fossil series
known that showed a gradual change in living things from one kind to another. There
were also fossils that were identical species to those that were alive today.
QUOTE:
“Every year tends to fill up the blanks in the fossil record and to make the gap between
the lost and existing forms less and less…If we compare any but the most closely related
rock layers, all the species will be found to have undergone some change.”
(Darwin, 1858, p.166)
These snails illustrate a smooth series of intermediate fossils from the oldest known
ancestors to present-day forms. The transitions along the way are gradual, often making
it difficult to divide the lineage into distinct species.
Fossils of Trilobites, which are extinct.
DARWIN’S EVIDENCE # 3
VESTIGIAL ORGANS
SUMMARY:
Living animals contained small remnants (leftovers) of organs that did not do anything.
Flightless insects, for instance, are often equipped with tiny functionless wings. Certain
snakes keep pieces of useless legs.
QUOTE:
“The eyes of moles and of some burrowing rodents are small in size, and in some cases
are covered up by skin and fur. Since frequent irritation of the eyes must injure the
animal, and animals that live underground can get along without eyes, reducing the size,
sealing together the eyelids and growing fur over them might be an advantage.”
“Organs or parts which seem useless are very common in nature. In many snakes there
are small bones of the pelvis and hind limbs. Unborn whales have teeth, but when grown
up they do not have a tooth in their head. There are many insects with wings so small
that they cannot fly, and sometimes the wings are covered with wing cases that are sealed
together so that the wings cannot be used.” (Darwin, 1858, p.428)
DARWIN’S EVIDENCE #4
HOMOLOGOUS STRUCTURES
SUMMARY:
Darwin was struck by the fact “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.” (Darwin, 1858, p.208)
QUOTE:
“We see the same great law in the construction of the mouths of insects. What can be
more different than the very long spiral trunk-like mouth of a sphinx-moth, the curious
folded one of a bee or bug, and the great jaws of a beetle? Yet all these mouths, serving
such different purposes, are formed of an upper lip, mandibles and two pair of maxillae.”
(Darwin, 1858, p.416)
whale
Lion
Human
Bat
Horse
DARWIN’S EVIDENCE #5
EMBRYOLOGY
SUMMARY:
Biologists saw that as embryos developed they went through stages that resembled the
adult forms of more primitive types. For example, human embryos have gill slits similar
to those of a fish. There are stages where it is almost impossible to tell the difference
between the embryos of reptiles, birds, and mammals.
QUOTE:
“Certain organs in the individual which in the full-grown animal are very different and do
different things, are exactly alike in the embryo…Agassiz, having forgotten to label the
embryo of some vertebrate animal, cannot tell now whether it is a mammal, bird or
reptile.” (Darwin, 1858, p.419)
DARWIN'S EVIDENCE #6
ANIMAL AND PLANT BREEDING
OR ARTIFICIAL SELECTION
SUMMARY:
Selective breeding of animals and plants is when
humans choose which plants or animals to breed.
This gives a great variety of different kinds. Many of
these differences are inherited from one generation
to the next.
QUOTES:
"No one supposes that our best domestic plants and animals have been
produced by a single difference from the original plant or animal...
We see an astonishing improvement in many florists' flowers when
the flowers of the present day are compared with drawings made only
twenty or thirty years ago. When a race of plants is once pretty well
established, the seed-raisers do not pick out the best plants, but merely
go over their seed-bed, and pull up the "rogues" as they call the plants
that are different from the original. With animals this kind of selection
is, in fact, likewise followed; for hardly anyone is so careless as to
breed from his worst animals." (Darwin, 1858, p.49)
"No one would ever expect to get a first-rate heartsease or dalia
(garden flowers) from the seed of a wild plant. The final result
happens when people cultivate the best-known variety, sow its
seeds, and, when a slightly better variety chanced to appear,
selecting it, and so on. Slowly accumulating a large amount of
change explains the well-known fact that in a number of cases we
cannot recognize the wild parent-stocks (versions) of the plants
which we grow in our flower gardens." (Darwin, 1858, p.52-53)
By a similar process of selection, and by careful training, English
racehorses have gotten faster and bigger than the parent Arab ... Lord
Spencer and others have shown how the cattle of England have
increased in weight and mature earlier when compared with the stock
that they used to keep in this country ... By comparing the historical
accounts and current descriptions of the carrier and tumbler pigeons in
Britain, India and Persia, we can trace the stages through which they
have ... passed, and come to differ so greatly from the rock-pigeon.
(Darwin, 1858, p.51)
Fancy pigeon breeds of one of the London pigeon clubs to which Darwin belonged.
DARWIN'S EVIDENCE #7
THE SUCCESSION OF TYPES
SUMMARY:
In South America, Darwin found fossils of an extinct
armadillo that was very similar to living armadillos. He
also found the fossils of the extinct ground sloth to be
similar to the living sloth. The extinct forms were
much larger, but Darwin thought that their resemblance
was too close to be accidental.
EXTINCT GROUND SLOTH
SLOTH (PRESENT
DAY)
EXTINCT GIANT GLYPTODON
Armadillo (Present Day)
QUOTE:
"Mr. Clift many years ago showed that the fossil
mammals from the Australian coves were closely related
to the living marsupials of that continent. In South
America a similar relationship is seen in the fossils of
the gigantic pieces of armor of the glyptodont which are
like those of the living armadillo. Most of the fossil
mammals buried in South America are related to living
animals. In the caves of Brazil there are many extinct
species closely related to the species still living in South
America. I was so much impressed with these facts on
this wonderful relationship on the same continent
between the dead and the living.” Darwin, 1858, p.174