Evolution as Fact and Theory
by Stephen Jay Gould
Mather, who died last year at age ninety, was a pillar of both science and Christian
religion in America and one of my dearest friends. The difference of a half-century
in our ages evaporated before our common interests. The most curious thing we
shared was a battle we each fought at the same age. For Kirtley had gone to
Tennessee with Clarence Darrow to testify for evolution at the Scopes trial of
1925. When I think that we are enmeshed again in the same struggle for one of the
best documented, most compelling and exciting concepts in all of science, I don't
know whether to laugh or cry.
According to idealized principles of scientific discourse,
the arousal of dormant issues should reflect fresh data that give renewed life to
abandoned notions. Those outside the current debate may therefore be excused for
suspecting that creationists have come up with something new, or that
evolutionists have generated some serious internal trouble. But nothing has
changed; the creationists have presented not a single new fact or argument.
Darrow and Bryan were at least more entertaining than we lesser antagonists today.
The rise of creationism is politics, pure and simple; it represents one issue (and
by no means the major concern) of the resurgent evangelical right. Arguments that
seemed kooky just a decade ago have reentered the mainstream.
The basic attack of modern creationists falls apart on two
general counts before we even reach the supposed factual details of their assault
against evolution. First, they play upon a vernacular misunderstanding of the
word "theory" to convey the false impression that we evolutionists are covering
up the rotten core of our edifice. Second, they misuse a popular philosophy of
science to argue that they are behaving scientifically in attacking evolution.
Yet the same philosophy demonstrates that their own belief is not science, and
that "scientific creationism" is a meaningless and self-contradictory phrase,
an example of what Orwell called "newspeak."
In the American vernacular, "theory" often means "imperfect
fact"part of a hierarchy of confidence running downhill from fact to theory
to hypothesis to guess. Thus creationists can (and do) argue: evolution is "only"
a theory, and intense debate now rages about many aspects of the theory. If
evolution is less than a fact, and scientists can't even make up their minds
about the theory, then what confidence can we have in it? Indeed, President
Reagan echoed this argument before an evangelical group in Dallas when he said
(in what I devoutly hope was campaign rhetoric): "Well, it is a theory. It is a
scientific theory only, and it has in recent years been challenged in the world
of sciencethat is, not believed in the scientific community to be as
infallible as it once was."
Well, evolution is a theory. It is also a fact. And
facts and theories are different things, not rungs in a hierarchy of increasing
certainty. Facts are the world's data. Theories are structures of ideas that
explain and interpret facts. Facts do not go away when scientists debate rival
theories to explain them. Einstein's theory of gravitation replaced Newton's, but
apples did not suspend themselves in mid-air, pending the outcome. And humans
evolved from apelike ancestors whether they did so by Darwin's proposed mechanism
or by some other, yet to be discovered.
Moreover, "fact" does not mean "absolute certainty." The final
proofs of logic and mathematics flow deductively from stated premises and achieve
certainty only because they are not about the empirical world.
Evolutionists make no claim for perpetual truth, though creationists often do (and
then attack us for a style of argument that they themselves favor). In science,
"fact" can only mean "confirmed to such a degree that it would be perverse to
withhold provisional assent." I suppose that apples might start to rise tomorrow,
but the possibility does not merit equal time in physics classrooms.
Evolutionists have been clear about this distinction between
fact and theory from the very beginning, if only because we have always
acknowledged how far we are from completely understanding the mechanisms (theory)
by which evolution (fact) occurred. Darwin continually emphasized the difference
between his two great and separate accomplishments: establishing the fact of
evolution, and proposing a theorynatural selectionto explain the
mechanism of evolution. He wrote in The Descent of
Man: "I had two distinct objects in view; firstly, to show that species
had not been separately created, and secondly, that natural selection had been
the chief agent of change. . . . Hence if I have erred in . . . having
exaggerated its [natural selection's] power . . . I have at least, as I hope,
done good service in aiding to overthrow the dogma of separate creations."
Thus Darwin acknowledged the provisional nature of natural
selection while affirming the fact of evolution. The fruitful theoretical debate
that Darwin initiated has never ceased. From the 1940s through the 1960s,
Darwin's own theory of natural selection did achieve a temporary hegemony that
it never enjoyed in his lifetime. But renewed debate characterizes our decade,
and, while no biologists questions the importance of natural selection, many
doubt its ubiquity. In particular, many evolutionists argue that substantial
amounts of genetic change may not be subject to natural selection and may
spread through the populations at random. Others are challenging Darwin's
linking of natural selection with gradual, imperceptible change through all
intermediary degrees; they are arguing that most evolutionary events may occur
far more rapidly than Darwin envisioned.
Scientists regard debates on fundamental issues of theory as
a sign of intellectual health and a source of excitement. Science isand how
else can I say it?most fun when it plays with interesting ideas, examines
their implications, and recognizes that old information might be explained in
surprisingly new ways. Evolutionary theory is now enjoying this uncommon vigor.
Yet amidst all this turmoil no biologist has been lead to doubt the fact that
evolution occurred; we are debating how it happened. We are all
trying to explain the same thing: the tree of evolutionary descent linking all
organisms by ties of genealogy. Creationists pervert and caricature this debate
by conveniently neglecting the common conviction that underlies it, and by
falsely suggesting that evolutionists now doubt the very phenomenon we are
struggling to understand.
Secondly, creationists claim that "the dogma of separate
creations," as Darwin characterized it a century ago, is a scientific theory
meriting equal time with evolution in high school biology curricula. But a
popular viewpoint among philosophers of science belies this creationist
Popper has argued for decades that the primary criterion of science is the
falsifiability of its theories. We can never prove absolutely, but we can falsify.
A set of ideas that cannot, in principle, be falsified is not science.
The entire creationist program includes little more than a
rhetorical attempt to falsify evolution by presenting supposed contradictions
among its supporters. Their brand of creationism, they claim, is "scientific"
because it follows the Popperian model in trying to demolish evolution. Yet
Popper's argument must apply in both directions. One does not become a scientist
by the simple act of trying to falsify a rival and truly scientific system; one
has to present an alternative system that also meets Popper's criterion
it too must be falsifiable in principle.
"Scientific creationism" is a self-contradictory, nonsense
phrase precisely because it cannot be falsified. I can envision observations and
experiments that would disprove any evolutionary theory I know, but I cannot
imagine what potential data could lead creationists to abandon their beliefs.
Unbeatable systems are dogma, not science. Lest I seem harsh or rhetorical, I
quote creationism's leading intellectual, Duane Gish, Ph.D. from his recent
(1978) book, Evolution? The Fossils Say No!
"By creation we mean the bringing into being by a supernatural Creator of the
basic kinds of plants and animals by the process of sudden, or fiat, creation.
We do not know how the Creator created, what process He used, for
He used processes which are not
now operating anywhere in the natural
universe [Gish's italics]. This is why we refer to creation as special
creation. We cannot discover by scientific investigations anything about the
creative processes used by the Creator." Pray tell, Dr. Gish, in the light of
your last sentence, what then is scientific creationism?
Our confidence that evolution occurred centers upon three
general arguments. First, we have abundant, direct, observational evidence of
evolution in action, from both the field and laboratory. This evidence ranges
from countless experiments on change in nearly everything about fruit flies
subjected to artificial selection in the laboratory to the famous populations
of British moths that became black when industrial soot darkened the trees upon
which the moths rest. (Moths gain protection from sharp-sighted bird predators
by blending into the background.) Creationists do not deny these observations;
how could they? Creationists have tightened their act. They now argue that God
only created "basic kinds," and allowed for limited evolutionary meandering
within them. Thus toy poodles and Great Danes come from the dog kind and moths
can change color, but nature cannot convert a dog to a cat or a monkey to a
The second and third arguments for evolutionthe case
for major changesdo not involve direct observation of evolution in
action. They rest upon inference, but are no less secure for that reason. Major
evolutionary change requires too much time for direct observation on the scale
of recorded human history. All historical sciences rest upon inference, and
evolution is no different from geology, cosmology, or human history in this
respect. In principle, we cannot observe processes that operated in the past.
We must infer them from results that still surround us: living and fossil
organisms for evolution, documents and artifacts for human history, strata and
topography for geology.
The second argumentthat the imperfection of nature
reveals evolutionstrikes many people as ironic, for they feel that
evolution should be most elegantly displayed in the nearly perfect adaptation
expressed by some organismsthe camber of a gull's wing, or butterflies
that cannot be seen in ground litter because they mimic leaves so precisely.
But perfection could be imposed by a wise creator or evolved by natural
selection. Perfection covers the tracks of past history. And past
historythe evidence of descentis the mark of evolution.
Evolution lies exposed in the imperfections that
record a history of descent. Why should a rat run, a bat fly, a porpoise swim,
and I type this essay with structures built of the same bones unless we all
inherited them from a common ancestor? An engineer, starting from scratch,
could design better limbs in each case. Why should all the large native
mammals of Australia be marsupials, unless they descended from a common
ancestor isolated on this island continent? Marsupials are not "better," or
ideally suited for Australia; many have been wiped out by placental mammals
imported by man from other continents. This principle of imperfection extends
to all historical sciences. When we recognize the etymology of September,
October, November, and December (seventh, eighth, ninth, and tenth), we know
that the year once started in March, or that two additional months must have
been added to an original calendar of ten months.
The third argument is more direct: transitions are often
found in the fossil record. Preserved transitions are not commonand
should not be, according to our understanding of evolution (see next section)
but they are not entirely wanting, as creationists often claim. The lower jaw
of reptiles contains several bones, that of mammals only one. The
non-mammalian jawbones are reduced, step by step, in mammalian ancestors until
they become tiny nubbins located at the back of the jaw. The "hammer" and
"anvil" bones of the mammalian ear are descendants of these nubbins. How could
such a transition be accomplished? the creationists ask. Surely a bone is
either entirely in the jaw or in the ear. Yet paleontologists have discovered
two transitional lineages of therapsids (the so-called mammal-like reptiles)
with a double jaw jointone composed of the old quadrate and articular
bones (soon to become the hammer and anvil), the other of the squamosal and
dentary bones (as in modern mammals). For that matter, what better transitional
form could we expect to find than the oldest human, Australopithecus
afarensis, with its apelike palate, its human upright stance, and a
cranial capacity larger than any ape’s of the same body size but a full 1,000
cubic centimeters below ours? If God made each of the half-dozen human species
discovered in ancient rocks, why did he create in an unbroken temporal sequence
of progressively more modern featuresincreasing cranial capacity, reduced
face and teeth, larder body size? Did he create to mimic evolution and test
our faith thereby?
Faced with these facts of evolution and the philosophical
bankruptcy of their own position, creationists rely upon distortion and
innuendo to buttress their rhetorical claim. If I sound sharp or bitter,
indeed I amfor I have become a major target of these practices.
I count myself among the evolutionists who argue for a
jerky, or episodic, rather than a smoothly gradual, pace of change. In 1972
my colleague Niles Eldredge and I developed the theory of punctuated
equilibrium. We argued that two outstanding facts of the fossil
recordgeologically "sudden" origin of new species and failure to change
thereafter (stasis)reflect the predictions of evolutionary theory, not
the imperfections of the fossil record. In most theories, small isolated
populations are the source of new species, and the process of speciation
takes thousands or tens of thousands of years. This amount of time, so long
when measured against our lives, is a geological microsecond. It represents
much less than 1 per cent of the average life-span for a fossil invertebrate
speciesmore than ten million years. Large, widespread, and well
established species, on the other hand, are not expected to change very much.
We believe that the inertia of large populations explains the stasis of most
fossil species over millions of years.
We proposed the theory of punctuated equilibrium largely
to provide a different explanation for pervasive trends in the fossil record.
Trends, we argued, cannot be attributed to gradual transformation within
lineages, but must arise from the different success of certain kinds of
species. A trend, we argued, is more like climbing a flight of stairs
(punctuated and stasis) than rolling up an inclined plane.
Since we proposed punctuated equilibria to explain trends,
it is infuriating to be quoted again and again by creationistswhether
through design or stupidity, I do not knowas admitting that the fossil
record includes no transitional forms. Transitional forms are generally
lacking at the species level, but they are abundant between larger groups.
Yet a pamphlet entitled "Harvard Scientists Agree Evolution Is a Hoax" states:
"The facts of punctuated equilibrium which Gould and Eldredge
forcing Darwinists to swallow fit the picture that Bryan insisted on, and
which God has revealed to us in the Bible."
Continuing the distortion, several creationists have
equated the theory of punctuated equilibrium with a caricature of the beliefs
of Richard Goldschmidt, a
great early geneticist. Goldschmidt argued, in a famous book published in 1940,
that new groups can arise all at once through major mutations. He referred to
these suddenly transformed creatures as "hopeful monsters." (I am attracted
to some aspects of the non-caricatured version, but Goldschmidt's theory
still has nothing to do with punctuated equilibriumsee essays in section
3 and my explicit essay on Goldschmidt in The Pandas
Thumb.) Creationist Luther Sunderland talks of the "punctuated
equilibrium hopeful monster theory" and tells his hopeful readers that "it
amounts to tacit admission that anti-evolutionists are correct in asserting
there is no fossil evidence supporting the theory that all life is connected
to a common ancestor." Duane Gish writes, "According to Goldschmidt, and now
apparently according to Gould, a reptile laid an egg from which the first
bird, feathers and all, was produced." Any evolutionists who believed such
nonsense would rightly be laughed off the intellectual stage; yet the only
theory that could ever envision such a scenario for the origin of birds is
creationismwith God acting in the egg.
I am both angry at and amused by the creationists; but
mostly I am deeply sad. Sad for many reasons. Sad because so many people who
respond to creationist appeals are troubled for the right reason, but venting
their anger at the wrong target. It is true that scientists have often been
dogmatic and elitist. It is true that we have often allowed the white-coated,
advertising image to represent us"Scientists say that Brand X cures
bunions ten times faster than
" We have not fought it adequately because
we derive benefits from appearing as a new priesthood. It is also true that
faceless and bureaucratic state power intrudes more and more into our lives
and removes choices that should belong to individuals and communities. I can
understand that school curricula, imposed from above and without local input,
might be seen as one more insult on all these grounds. But the culprit is not,
and cannot be, evolution or any other fact of the natural world. Identify and
fight our legitimate enemies by all means, but we are not among them.
I am sad because the practical result of this brouhaha will
not be expanded coverage to include creationism (that would also make me sad),
but the reduction or excision of evolution from high school curricula.
Evolution is one of the half dozen "great ideas" developed by science. It
speaks to the profound issues of genealogy that fascinate all of usthe
"roots" phenomenon writ large. Where did we come from? Where did life arise?
How did it develop? How are organisms related? It forces us to think, ponder,
and wonder. Shall we deprive millions of this knowledge and once again teach
biology as a set of dull and unconnected facts, without the thread that weaves
diverse material into a supple unity?
But most of all I am saddened by a trend I am just beginning
to discern among my colleagues. I sense that some now wish to mute the healthy
debate about theory that has brought new life to evolutionary biology. It
provides grist for creationist mills, they say, even if only by distortion.
Perhaps we should lie low and rally around the flag of strict Darwinism, at
least for the momenta kind of old-time religion on our part.
But we should borrow another metaphor and recognize that
we too have to tread a straight and narrow path, surrounded by roads to
perdition. For if we ever begin to suppress our search to understand nature, to
quench our own intellectual excitement in a misguided effort to present a united
front where it does not and should not exist, then we are truly lost.
[ Stephen Jay Gould, "Evolution as Fact and Theory," May 1981;
from Hen's Teeth
and Horse's Toes, New York: W. W. Norton & Company, 1994, pp. 253-262. ]
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