Science
in Christian PerspectiveScience
in Christian Perspective
Explanation, Testability,
and the Theory of Evolution Part II
T. H. LEITH
Department of Natural Science
Atkinson College
York University
Toronto, Ontario, Canada
From: JASA 32 (September 1980): 156-163.
[Part 1 of this two-part paper was published is the March 1980 issue,
pp. 1318.]
The Relationship of Explanation to Prediction
Some analyses of evolution, having argued that it does not explain in the
fashion of physical science, have concluded also that it cannot predict as we
expect the physical sciences to do. Others like Popper believe conversely that
evolutionary theory fails to predict and thus does not explain nature in the
manner which we expect of science. Both are, as I have tried to show, mistaken
in their respective initial assumption but each has raised an important
issue-the relationship between prediction and explanation. It is worthy of some
closer examination.
Let us look again at the deductive model of scientific inference: one or more
laws of some type L1 L2,. .. together with statements
about specific circumstances C1C2. . provides a
statement about an event E. In
prediction we begin with the laws and the circumstances obtaining and derive a
statement about the occurrence of an event. In explanation we begin with E (the explanandum)
and reason that it arises because of the laws and circumstances stated (the explanans).
We move with equal ease or difficulty in either direction because the logical
relationship is time-symmetrical.
(One comment in passing: I have used the term "prediction" but this
suggests that whether E is predicted or explained depends on whether E lies in
our future or in our past. In historical studies, such as parts of geology, we
must however include the postdiction that, given the uniform application of
nature's laws and the presence of the same relevant circumstances in the past,
an event which we would anticipate in the future is also as likely to have
occurred in the past.)
If there is a temporal symmetry in the logical relation of explanans
and explanandum
when explaining and predicting (or postdicting) there is on the other hand, no
necessity that there be a symmetry in a scientist's confidence when he does so.
Physics, when universal laws are employed as they commonly are,
provides its practitioners
with as much assurance in their predictions as in their explanations
but the evolutionist
is in a very different position. For one thing, even if he too could
employ universal
laws, which would not themselves be the source of uncertainty, the evolutionist
realizes that individual organisms or populations are found in conditions which
are quite unlikely to be as knowable for the future as they are, from
the stratigraphic
record, for the past. The explanans, as a result, is far less satisfactory in
one case than it is in the other.
This may seem to suggest that postdictions do not suffer from the
same difficulties
as pred ons in evolution. We must he careful here. Certainly, a scientist will
feel secure in stating that an event has occurred in the past, if he
has a record
of it, but unsure that it will happen in the future where he can have
no record.
However, if he used the same laws and the some uncertain circumstances that he
must use for prediction, as we noted above he could postdict a past event with
no more confidence than he could predict a similar event. Also,
knowing that the
event has occurred, provides him with no more assurance that he has explained
it in terms of some explanans than using the same explanans provides for postdieting the
event or predicting
a similar occurrence.
Where the stratigraphic record helps the evolutionist here is in presenting him
with a reasonably well-defined set of circumstances preceding a past
event. These,
together with appropriate laws may he employed to explain the event and, had he
used the same explanans, could have enabled him to predict it with
equal confidence
if he had been alive at an earlier time. Were the scientist today to
assume that
the same explanans is applicable in the future he could predict with the same
assurance. Were he to believe that it was applicable at some locale and time in
the past which is different from the above event he could also
postdict a similar
event just as confidently.
Because he often has a geological record the scientist is clearly
aided in arriving
at sets of conditions on different past occasions and at different places from
which he can postdict different events. (and likewise explain them).
For the future,
he can say only that, if the same sets of conditions prevail, he will forecast
similar events: these conditions though may be not only different but
quite uncertain.
Surely, then, if some of the scientists postdictions are corroborated by later
research and very few are falsified, he will be increasingly
confident about events
which he has postdicted but whose traces are not yet observed. His predictions
on the other hand can have been tested at best over only the short
period between
when they were offered and the present. Thus, even if some have been
corroborated,
his confidence in his forecasts over a longer time, and certainly over future
intervals of the order of those found within the fossil record, is unlikely to
he nearly as great.20
Explanation and Prediction in Darwin's Theory
We have now seen that evolutionary explanation, in spite of variant
descriptions
offered of it, has a style which is scientific by the standards of
understanding
within the physical sciences. It is, of course, often supplemented by narration
where brevity or inadequate information prevents a proper explanatory account.21
Concommitant with this sort of explanation is the presence of predictions and
postdiction, and therefore the possibility of testing.
However, these are general observations. Evolutionary concepts and
laws may have
characteristics differing from those usually found in the physical
sciences. Predictions
may suffer from limitations unfamiliar to the physical scientist. Then too, as
in the other sciences, explanations offered in evolution may vary in scope, in
conceptual content, in testability, and the specific predictions and
postdictions
accompanying each may differ.
Let us look here at Darwin's theory with these thoughts in mind. Pun
has recently
outlined the Darwinian schema,22 the principal theses of which may he
summarized
as follows (though Darwin knew that they were supported by evidence of varied
worth or arguments of differing character):
One may argue that selection and adaptation are testable implications of Darwinian evolution.
1) random variation wit/tin species in nature produces organisms
adapted to their
environment in varied degrees;
2) natural selection preserves preferentially those variants best fitted to the
environment;
3) species under natural selection are modified and eventually give rise to new
species;
4) the fossil sequence is a fragmented record of past modifications;
5) life originated chemically and by natural laws;
6) the interval from this origin to the present and even the period
recorded paleontalagically,
is very long;
7) variation is inherited by the process of pangenesis, a process
which entails
the inheritance of acquired characteristics.
Over the past century these theses have been evaluated in varied
ways. For example,
in 1837 Lyell commented that
in attempting to explain geological phenomena, the bias has always been on the wrong side ... a disposition to reason a priori on
the extraordinary
violence and suddenness of changes ... instead of attempting strenously to
frame theories in accordance with the ordinary operations of nature.21
There is no doubt that much of the early, and some of the later,
denials of Darwin's
theses are grounded in a similar prejudice.24 Others, like William
Thomnson (Lord
Kelvin) considered the generalizations developed by Darwin imprecise, incapable
of quantitative measurement or mathematical expression, and in
disagreement with
established theory. He also preferred to believe that life was as old
as the universe,
arriving on earth by
means of meteorites.25 Through similar judgments are
common and important in science, I shall not consider them to be
tests of Darwinism
in the spirit of my earlier comments.
Another sort of evaluation lay in an examination of the theses
individually. Thus,
for example, pangenesis, with its concepts of blended inheritance and
of gemmules,
was eventually found wanting. On the other hand, the idea that life
is of inorganic
origin and arose by natural processes is today widely accepted as feasible and
even probable." Likewise, the vastness of geological time recorded in the
fossil sequence (over 3 billion years) is now considered firmly
corroborated though
this was frequently denied before the discovery of radioactivity:
witness Kelvin's
1868 comment that "the existing state of things on earth, life
on the earth,
all geological history showing continuity of life, must he limited within some
such period of past time as one hundred million years."' -This
was considered
far too short by evolutionists, unless of course rates of extinction
and modification
were somehow more rapid in the past than at present; that in turn called for an
explanation which wasn't forthcoming.
Of course these theses are not on a par with (1) - (3) above which
formulate the
basic Darwinian theory. Theses (4) and (6) follow from the theory together with
certain other assumptions while (5) and (7) express additional postulates, the
one extrapolating the naturalness of biological evolution into the prebotic period
and the latter suggesting the nature of the inheritance of modification. What
then may he said about the testability of the remaining kernel of
Darwin's scheme?
Consider first the matter of variation. Williams25 notes that the
scheme predicts
transitional forms between an ancestral species and descendent
species which are
reproductively isolated. This, together with the assumption that the
causal agencies
for speciation which are found in the past are also present today, leads to the
conclusion that populations will he found at present which contain
all the predicted
stages of transition. This appears to be the case." Again, Lovtrupt30
argues that Darwins theory does not call for organisms to be
necessarily optimally
adapted to their environment and thus calls for extinction to he
expected on occasion.
This prediction is both testable and corroborated. Or again, Darwinism predicts
that organic forms will increase in variety and complexity with time: this too
appears to he testable.31
Earlier we mentioned Rose's critique of Popper who enunciated a
variety of common
claims against the testability of such ideas as speciation,
adaptation, and evolutionary
rates. In response to the claim that evolution does not predict the evolution
of variety, Ruse notes that in a world of varied ecological niches
(past and present)
evolutionists would he disturbed if no evidence of speciation were revealed.32
There is however considerable evidence that populations isolated from
the ancestral
group have evolved into new species under differing conditions; sorely that is
a fulfilled prediction.
Likewise, one may argue that selection and adaptation are testable implications
of Darwinian evolution. For example, Darwin predicted that competition between
species of the same genus and inhabiting the same environment would generally
be more severe than between species of different genera.33 That is
clearly either
true or false. Nor is adaptation devoid of empirical content (i.e.
merely an analytical
definition as Popper claimed): there is evidence that what is of adaptive value
in one situation is also of value in similar situations, that a characteristic
of an organism may he adaptive at one place and time and not in another, and
that adaptation isn't always equated with survival (except in terms
of group averages)
because less-well-adapted individuals often survive better than the
well-adapted."'
More specifically, claims as to the function of specific parts of organisms may
be tested and such tests are really attempts to demonstrate the
predicted adaptedness
of these parts.
As a final example we may note Darwin's prediction that evolutionary
change will
be gradual: "If it could he demonstrated that any complex organ existed,
which could not have been formed by numerous slight modifications, my
theory would
absolutely break down" and "Natural selection acts only by
taking advantage
of slight
successive variations: she can never take a great arid sudden
leap.35 Clearly
these were empirical claims, however difficult it has proven to
obtain agreement
on the tests involved as loan) of its know.
I must assume that, with these examples, it is understandable that biologists
and geologists were justified in treating Darwin's scheme as both explanatory
and predictive as most of them slid in the half century following
Darwin. Of course,
there were numerous issues in historical areas on which Darwin's theory was not
predictive (or postdictive): the specifics of biogenesis or the
specifies of the
expected interrelationships among man), groups and the modes by which
one changed
into another are examples. At best Darwin, or those accepting his
scheme, might
speculate on these matters in the absence of either data or
prediction. The evolutionary
theory, though it claimed universal application for its principles, lacked both
sufficient knowledge of environmental conditions and an adequate set
of postulates
for making many testable statements about biological development.
It is necessary to emphasize as well that Darwin's followers, like contemporary
workers in evolution, frequently did not focus upon predictive matters at all.
As Manser was seen to argue earlier, they treated evolution by natural selection
as a conceptual scheme whereby things are to he related in certain
ways and seen
as exhibiting certain processes. Kuhn 36 has called this activity
normal science,"
a process of articulating a thesis and elaborating it, a process of
solving puzzles
by applying it rather than potting it to test. The procedure is
familiar to students
in (say) physics laboratories: they are asked to perform experiments
hot one could
scarcely call what they do tests of physical theory. They are rather
illustrating
its application.
This is not the occasion to examine either Kuhn's description of normal science
for appropriateness or, if it is correct, such activity as proper policy among
researchers. We have also seers that conceptual schemes, if
scientific, are predictive
and that Darwin's scheme exhibits that property. Here I merely wish to point to
a facet of Darwinian (and later) evolution which looms large, for it explains
what is going on when critics claim that evolutionists argue in a
circular fashion,
that they believe nature provides evidence for evolution while
interpreting that
evidence as if evolution were already demonstrated.
Kitts illustrates what worries the critic.
We cannot fault paleontologists...for refuising to accept events out of history
as falsifications of the general laws to which they are
committed ... this is not a kind of dogmatism directed at shaping
the world according to some arbitrary preconceived scheme. It is a means for getting from the
present to the past.37
Again he says,
Thus the paleontologist can provide knowledge that cannot be
provided by biological
principles alone. Buit he cannot provide us with evolution. We can leave
the fossil record free of a theory of evolution. An eevolutionist,
however, cannot leave the fossil record free of the evolutionary hypothesis.
But the danger of circulatity...is present. For most biologists the strongest
reason for accepting the evolotionarv hiypothesis is their
acceptance of some theory that entails it.38
Let me clarify by pointing out that Darwin arrived at his theory by a creative
synthesis of suggestions inelucloig fossil sequences) 5 Such
sequences may often
he ordered proper!)' in time by stratigraphic (relative) techniques
or radioactive
(absolute) determinations. However in eases where these are not
usable the sequences
denveci elsewhere may then he applied to determine a proper time order on the
assuniption that sequences are invariant from place to place. This procedure is
neither circular nor need it presuppose phvlogenetic evolution but, as with I )arwin, the temporal sequences max' (they' need not, of course)
suggest all evolutionary
proems. Used in conjunction with other infnrrnathui the' may take their place
then among the foundations of a theory' of evolution. in torn, the theor' now
requires that, as long as it is held by paleontology, workers in that held will
employ evolutionary interpretations. There is no circularity' here
either as long
as the paleontologist does not confuse "illustrations" with
"evidence."
As an evolutionist he or she will read the fossil record as
illustrating evolution;
its evidential role in suggesting the theory of evolution is now past.
The evolutionist is in a position where his demonstrations of the workings of
enolulionary processes frequently go beyond observational or
empirical data.
Two comnieots most now he made. For one thing, the evolutionist (as we remarked
earlier) is in a position where his demonstrations of the workings of
evolutionary
processes frequently go beyond observational or empirical data. The
illustrations
are speculative; they merely show how the theory might be developed
to cover hiatuses
in our knowledge. Thus it can be said that "Darwin (lid not show
in the Origin
that species had originated by natural selection; he merely showed,
on the basis
of certain facts and assumptions, how this might have
happened...", a speculation
giving the impression that he had given cxaniples of trans formations
by natural
selection. Or, "It is a matter of faith on the part of the biologist that
biogenesis did occur and he can choose whatever method of biogenesis happens to
suit him personally'; the evidence for what (lid happen is not
available. "U
Kerkut's statement is rather overdrawn in the sense that methods of biogenesis
must operate (if they are to he understood naturally') within
numerous constraints
of physical and chemical sorts provided by geological and laboratory' evidence.
Nonetheless, it does point up the subjective element involved and the
flexibility'
which still remains. These mark any speculative enterprise and lead
to Ihonipsons
charge that demonstrations "can be modified without difficulty to fit any
conceivable case." l'2
My' second comment has to do with the evidential role of biological information
(we mentioned the fossil record,
SEPTEMBER 1980
for example) after the Darwinian theory' has been accepted as a
useful conceptual
scheme.13 I have argued that the theory remains testable, that it is possible
to corroborate or falsify' Darwinism without assuming what most he
shown. Specifically,
the fossil record might very' well turn out to provide information
quite different
from the ex
pectations of the theory'. Thus, only while one is specu-lating
within the unknown
is a charge like Thompson's
appropriate: if the unknown is reduced by new infor
osation, and though we may' continue to speculate within remaining
areas of ignorance,
we cannot modify our earlier speculations without difficulty iis lie
claims. This
is because the falsifying information calls for a new theory' and new modes of
illustrating its workings.
The Structure of Neo-Darwinism
It has riot been my purpose to more than mention incidentally' the
corroboration
or falsification that nature provided to the Darwinian theory. From what I have
said it should, however, be apparent that the theory was testable in specific
areas to very different degrees and that, as an explanatory device, it served
at times merely' as a guide to speculation. This has been, of course,
the common
lot of scientific theories through the years and, as it is also their
lot to suffer
alteration or replacement, so it was with Darwin's scheme. In time
its ancillary'
postulate, pangenesis, was replaced by an idea denying the
inheritance of acquired
characteristics and involving Menciehan principles. The new theory carried with
it new explanatory', as well as new testing, possibilities, but
mssore importantly'
for us it has drawn increasing attention as well to various features
which appear
to contrast with schemes employed in the physical sciences. Thus, in
what follows,
I shall discuss explanation arid testability' in terms of these
contrasts rather
than, as I did in the last section, focussing upon the way's in which
evolutionary'
theory' is respectably' similar to physical schemes.
The Character of Prediction. We have seen that evolutionary' theory' has often
been taken to be non-predictive. We have also noted, explicitly' or
implicitly',
several reasons for this judgment but the point deserves a little
further comment
in the light of N en-Darwinian thinking.
It is characteristic of the contemporary' scheme, even more fully'
than in Darwin,
that its analysis is in terms of the properties of groups. If variation has to
do with an individual's adaptation to its surroundings, evolution insolves the
accumulation of variations suited to a given environment and the elimination of
others. Speciation involves the transformation through time of one collection
into a different set of individuals. If we talk of selection we talk
of gene frequencies,
and if we talk of the survival of the fittest we mean that one
population expands
at the expense of another. Thus, because we analyze in this fashion,
we must predict
in the same way. The predictions of everything from mathematical genetic models
to the cirasving out of the intuitive implications of evolutionary postulates
are in terms of groups. Oddly, this is often forgotten.
Williams4' offers an explanation: because they see individuals and not things
like species, many critics tend to
159
T, H. LEITH
seek predictions about individual properties instead of recognizing that they
must predict in terms of group concepts. She makes an interesting contrast here
to the way they treat the predictions of the kinetic theory of gases. These are
of familiar things like temperature and pressure so the theory does not bother
the critics, whereas evolution is bothersome with its predictions in terms of
abstract concepts. If it could only be seen that evolution is
forcasting or postdicting
patterns, and not single events or individual properties as we usually perceive
them, perhaps it would be recognized that discomfort here is at
bottom psychological.
Falsification or corroboration then rests in whether nature is patterned as we
anticipate. It cannot rest in the truth or falsity of statements
about particular
things for these are derived from the actual predictions and assert
only probabilities,
e.g. from Neo-Darwinian theory we may deduce a group's survival but
only the probability
of a particular individual surviving.
There is, of course, the problem of characterizing these predictions
explicitly.
Properly this should he done in terms of the properties of patterns. In a gas,
for example, which is a group of molecules, we may predict the average squared
velocity of the molecules because this average is a property of the
gas. In taxonomy,
Mayr speaks of sorting "numerous specimens and species into
'natural groups'
by scanning their total gestalt, based on an evaluation of very many
characters,
most of which he does not analyze or record in detail."45 This gestalt is
the pattern which actually defines the taxon.
Were we, on the other hand, to try to predict the behavior of a single molecule
at some specified temperature we cannot Nor can we make a prediction about all
of the molecules in the gas at that temperature-they don't have any
relevant property
in common. Yet, in biology the taxonomist is often found defining a
taxon by certain
constant and readily-recognized characters which are neither
individually necessary
nor collectively sufficient rather than by the gestalt by which he identified
it. It is common in biology to characterize patterns in this way, in terms of
properties which are psychologically familiar. Predictions involving patterns
are therefore sought out improperly.
Let me quote Williams.
It is difficult to find (predictions) in the literature not because
they are rare
hot because they do not conform to our expectations of what
evolutionary predictions
would look like. They are not predictions about the human-sized phenomena which
we intuitively recognize as individual events; they are, rather,
predictions about
patterns in sets of these human-sized phenomena. It has been
extremely difficult
to perceive these patterns ... because our know]edge of thesis is obtained by
piecing together information obtained from literally hundreds of thousands of
human-sized individual events. To see these evolutionary events as individuals,
in a single gestalt, is difficult enough; to verbalize adequately this gestalt
is vastly more difficult, 16
The Character of Evolutionary Concepts. The remarks above have illustrated one
of the nuances which mark souse distinction between physical theory
and the theory
of evolution. A second shows up, I think, in the conceptual apparatus
of Neo-Darwinism,
for this contemporary
160
synthesis of genetics and Darwinian theses employs a variety of terms
which tend
to exhibit difficulties not cotnnsonly present in the physical sciences. Let me
show this first for the term "speciation."
As is well-known the processes covered by this concept are quite varied though
each involves a single basis: selective pressures which push apart genetically
groups of common ancestry. Workers in the field have identified a
common allopatric
form requiring a period of geographic isolation, a syinpatric form (fottnd in
the same geographic area but presumably involving gradual adaptation
to different
niches within the region), stasipatrie change involving chromosomal
rearrangement,
a parapatrie type of evolution along gradients in allele frequencies
(i.e. clines)
and also processes involving asexual reproduction.4
It has been a most ingenious piece of work, vet it is flawed by the
taxonomist's
present inability to determine their relative importance. He has
simply no available
method for distingtsishiug widely in nature among their products: he
has uncovered
what he takes to he truths about nature's ways without the means of
knowing most
of the time where the truths apply. It isn't a restriction toady any
easier either
by the fact that the geographic isolations involved in many cases of presutned
allopatric speciation are no more than possibilities that as yet lack
independent
confirmation.
Perhaps the physical scientist is not unfamiliar with similar problems in his
discipline, but they certainly loom
much less large than for the biologist. Nor does he commonly have to deal with
difficulties in applying his concepts to situations in the remote past. Let me
illustrate this in the ease of the term "adaptation.""
When a paleontologist uncovers a fossil exhibiting various structures
his problem
is at once one of determining their likely function. Usually he can do this at
best by analogy to present forms, a process which shows only that the
structures
may have fulfilled some particular role. If he now accepts this role as likely
he should then he interested in the adaptation which these forms express. Here
he has a problem for he will realize that a structure might well he adapted to
some function which our analogy' has missed: it may also suggest
adaptation which
was not in fact the case. As a result, the adaptedness of organisms
in paleontology
is recognized by him only in retrospect. In that manner what might otherwise he
taken as an insignificant variant may be seen, from the later fossil record, to
be a rudimentary new organ. In the same fashion what might not
otherwise be seen
as an early member of a new group may be seen to be such from that
record. Clearly
then, adaptation is a concept whose application to the past is rather different
from the way it is einployedin the current scene and whose utility in
paleontology
is heavily dependent on the fullness of later information.
Lastly, I wish to mention the term "complexity," so widely
used in evolutionary
literature. NeoDarwinisns predicts short-term increases in fitness
which may involve
increases or decreases in the complexity of the organisms involved. It does not
forecast increasing com
JOURNAL OF THE AMERICAN SCIENTIFIC AFFILIATIONTESTABILITY AND EVOLUTION
plexity over the long term except that its additional postulate of
the inorganic
origin of life suggests very simple beginnings and the record indicates later
changes toward more complex forms. Biology, however, lacks precise measures for
complexity-.',shieh presumably invoke numbers of parts, their
relative functions,
their interactions, and such-and it needs them if theory is to predict what is
observed. Also needed is a scheme that will predict both the
perpetuation of complex
phenotypes and their increase with time. n In any event, we have here
a situation
indicative of a young science: unlike much of contemporary physical science it
lacks a means of comprehending a major feature of nature lying within
its concern.
The Nature of Laws. Earlier we have argued that one of the commonest forms of
evolutionary writing, historical narration, requires the wide-spread
and at least
implicit use of what appears to be law-like generalizations. Later we
have claimed
that evolutionary theory makes predictions, particularly about the properties
of groups and hence about the probability of individual occurrences.
Such predictions
would, we'd expect, usually be thought of as being laws for that is
what theories
commonly predict. Yet it is frequently claimed that there are few
laws of biology
or of evolution. I suggest that the problem here, as I noted in
discussing prediction
earlier, is that laws in these disciplines do not meet our
expectations that arise
from observing much of the physical sciences. I make a few further comments on
the contrasts here.
Among the physical sciences it is usual to find process laws, laws which permit
us to infer any past or future states of a system given a knowledge
of the values
of the relevant variables on some particular occasion. While biology
might occasionally
exhibit such laws, in cases where the system involved is sufficiently closed to
hermit prediction and postdietion, ft is difficult to employ them in evolution
where climatic and geological conditions in the past and in the future may vary
in unknown ways from present conditions. At best we might then
predict or postdiet
possibilities. Should we know past conditions from
the historical record, or have acceptable ways of forecasting climate
and geological
change, these predictions or postdictions could of course be made with greater
confidence.
However, the initial problem in biology or evolutionary studies is to conic up
with laws at all. That task, as we have seen, calls for identifying
patterns and
groups which can be described with enough breadth to permit laws to
be formulated.
Part of the reason that these have not been developed in quantity empirically,
as many have in physical science, is because the proper identifications haven't
been made. Part of the reason they haven't come frequently enough from drawing
out the consequences of evolutionary theory lies in the same failing.
When such laws are formulated they are usually quite unlike the
process laws mentioned
above. Some permit only inferences regarding the past and others are
historical,
employing a knowledge of the past trends to extrapolate into the
future. Commonly
the may he causal in form (A causes B) or developmental (I) because fT because
B
SEPTEMBER 1980
because A).5° Laws of this causal sort reveal a further problem
in evolutionary
generalizations with which I shall close this section.
The major issue in discussions about causality involves a deter
niination of those
conditions which are necessary, and of those which are sufficient, to
bring about
sonic event which to he explained. The former usually proves to be far easier
than the latter so that it is fairly common to find evolutionary statements
such
as "If E then Ni, Ns, Ni ... preceded it." We are then
afforded a means
to postdieting something about the past of an event whenever we come upon it.
However, if the causes Ni, Ni, Ni
are not sufficient to explain the event, we are clearly not permitted
to predict
it simply because the agents are present. Should we, however, occasionally find
some cause sufficient for an event to occur but not necessary to it (i.e. the
event will occur if the causal agency is present but may occur in its absence)
we might then say "If S then E" but not "If E then S." Thus
in this situation prediction from the presence of the proper causal agent would
be possible but postdietioo from the presence of the event could
suggest at best
that the agent might have preceded it. For at least these reasons it should now
he apparent why in evolution inferences about the past are more
readily obtained
than inferences involving the future.
What Tempos and Modes are Explanatory and Predictive?
There is one further issue that I wish to discuss. There
are those who may accept evolution as a scientific theory
in almost the same terms as I have done to this point and vet
consider it unscientific
on one remaining criterion. That stunsblinghloek is the inability',
as they perceive
matters, of the theory to explain a major feature of the fossil
record; a record
which evolutionists take to be the historical trace of evolutionary change. The
object of their attention is the apparent absence of intermediate forms between
one phylum and another and also between the subsidiary high categories such as
classes and orders.
These gaps attracted the attention of Darwin in the Origin. Here they
were explained
if not by the incompleteness of our researches, then largely' as the
result either
of erosion, which removed the strata containing the transition forms,
or of conditions
which caused any sediments svhieh might have retained the missing record not to
have been deposited. In over a century, since then paleontologists have usually
become convinced that their information is now sufficiently' complete
to eliminate
erosion or non-deposition as an explanation of the missing forms in
numerous instances.
Something else must therefore account for their absence.
Those who accept Neo-Darwinism as more or less sufficient, together with what
we know of environmental situations, to explain the appearance of new species
have naturally attempted this task." Assuming that their usual
tools of mutation,
isolation mechanisms, selection, and adaptation apply to the appearance of the
higher categories as well they have concluded, as Simpson says, that'F. H. LEITH
It appears that both camps are reduced to using their theses in a
purely explanatory
fashion in the gap problem. Because of a situation like this the
critic may claim
that evolution has broken down as a scientific theory. I find this
judgment improper.
the distinguishing features related mainly to the scale arid the
adaptive relationships of the esohition of (these) categories, they
ins'oive certain
durations, intensities, and combinations of factors. There is no
reason to hc'ties
e that any different factors are involved than those seen in lower categories
or i. 11
n orico)evoiution.
As a result, the apparent lack of transitional forms between higher
taxortottiieal
categories is not sufficient to suggest that these structural plans persist as
long as life has existed. The missing forms are ttterely the
consequence of processes
which Neo-Darwinism is capable of comprehending and thus the plans
may be fitted
to an evolutionary scheme.
A minority of evolutionists, agreeing with the second part of this statement,
have disagreed with the first portion. For them Simpson's arguments
appear inadequate
to their task and his conclusions unwarranted. Instead, they seek an answer to
the missing forms in sudden changes of a sort not included within Neo-Darwinisns
as Simpson understood it.':' For example, Sehindewolf assumes the
sudden appearance
of non-adapted structures, which however eventually find a specialized adaptive
role, and denies that new adaptive categories can originate gradually.
The issue is now one of debating the extent to which paleontology should bow to
accepted NeoDarwinian evidences and principles. As yet contemporary laboratory
studies and evolutionary principles that work rather well in the field do not
provide a place for sudden changes of the sort the sssinority group proposes.
Indeed the ntinority might consider their occurrence to be unlikely in nature
within the foreseeable future and thus might accept Neo-Darwinisin,
like the majority
group, as an adequate theory for most purposes in contemporary studies. It is
on the propriety of extrapolating that theory across the fossil record that the
two groups differ. The majority claim that the gaps must he
interpreted in terms
of an explanatory scheme found worthy in most other matters. The minority feels
that the lack of transition forms requires its to supplement that scheme, when
it is used in interpreting the fossil record, by otherwise
tinsupported tnecbanisttts.
To their opponents this is an or! hoc procedure and a gesture of despair which
they find unnecessary .51
Unlikely as both groups moight take it to he, what would occur should forms be
found within the gaps? We noted earlier that fossil structures cannot be shown
not to he
162
adapted. Consequently we would he unable to use the forms to falsify
a Neo-Darwinian
thesis like Simpson's which takes all structures as adaptive. Of
course, we would
he unable to corroborate Schindewolf's sort of thesis, with its
non-adapted structures,
for the same reason. We are, therefore, left with is theory otherwise accepted
as reasonably well-supported elsewhere by both groups, which cannot
he critically
tested in the gaps as we know them: we are left too with a theory not otherwise
tested which cannot he corroborated within the gaps it was intended
to explain.
It appears that both camps are reduced to using their
theses in a purely explanatory fashion in the gapq prob-lem. As Crene and Kitts
point out, each interprets the facts here in a different way and thus
the objections
of the one to the arguments of the other are, from the alternative
point of view,
irrelevant.55 Because of a situation like this the critic may claim
that evolution
has broken down as a scientific theory. I find this judgment improper. We have
sinsply another case of a theory, or variants of a theory, being
employed as the
basis for speculation within a hiatus in our knowledge. One difference here is
that, where we usually speculate in line with our theory as dot's
Simpson, Sehindewolf
wishes to (even feels compelled to) speculate in terms of additional
concepts.
Scientists do not like to find thsctnsclvcs forced into this position
on important
issues. It leaves them, for example, drawing up very different
schemes for evolutionary
development, the phylogenetic Trees.,' It also leaves a variety of opinions in
the field on the matter of the appearance of the various phyla, early and more
or less together, around the end of the Precambrian. So it tnost he, but surely
it says as much about the recalcitrance of nature in giving up her secrets as
it does about failings in our evolutionary theories. These are the prodnet of
imagination, enlightened by observation and cxperitticnt, and tested
in the crucible
of experience. They are ittsperfcct and, until we find them to err, their range
of application is undetermined. That simply places them firtttlv
among other attempts
at scientific understanding.
REFERENCES
-"Compare A, Crtinhatrtri, "Explanation and
Prediction" in B. Baunirin
Philosophy of Sciertee. Vol. 1, tttterscis'nce Puhtislten's. Ness' lurk, 196:3,
pp. 51-S5; C. I tettittel. ''Esptartatisrn'' and Aspects. pp 369ff: I). Hall,
Philrtsoplttj; and SI. Scris'en, "Explanation and
Prediction," "Explanations."
and his "The 'temporal Asymmetry of Explantations and Predictions'' in B.
tianrttritti fed), Philosophy, pp. 9711)3, Scris'ert is criticized by
Crtinbattnt
arid is sliscnssed in Vt . Kane tO al, letters on "Cause and
Effect in Biology," Ss'ie,tce, Vol. 135, 16/3/ 62, pp. 972-981 and in K.
Stayr.
"Came & Effect
in Biology," Science, \'td, 134, 19/11/61, pp. 1501-1506.
'Set,' for esarttple A. Crttrtspttstr & P. Parker, "Evolution of the
Slartitnahan Slasticattrcv Apparatus," i'totc'rir'art Scientist. So) 66,
:3-4/78, pp 192-2111.
1' P. 3'. Port, ''A Critical Evaluation of Es srltttisstt,'' lotte. Arrtee. Sri.
Affltt.
Vol. 29, 6/77, pp. 84-91. Compare the theses with %lanser's statement of them
earlier in this patter.
2 C. l.vell, lift', Letters and journals, John Slnrrav, London, 1881, Vol. 2,
p. 3.
See B. K. 1). Clark, Darwin: Before and After, Paternoster, London, 11)66: P.
Zitrtrttt'rtnratr fed.), Darwin, Evolution, & Creation, (:orncttrdta Ptth.
Iloitse. .81. Louis, 1959: II. Morris. The Tivilight of Evolution, Baker Book
House, Grand Rapids. 1964 and Vt , Larttrtserts fed.). Why So) (:rt'atittttp.
l'resbvteriatt and llebtr,tted Prtlnl, Co., ,',tttlt'>'. N.J.,
JOURNAL OF THE AMERICAN SCIENTIFIC AFFILIATION
TESTABILITY AND EVOLUTION
1970 for current examples though each offers other criticisms as ~N
ell, The historical
account riots he found in SI'. irs inc. A;n's. Angcfs &
Victorians, Wejilenfeld
& Nicolsoti, London, 1956; C. ilnnnielf arT,, Darwin & the
Darwinian Revolution, i)onhledas, Garden Cite, 1959, iiii. 231-427; J - Greene, The Death
of Adam. Ness'
Amer. Library, New York, 1961; "Dam in Anniversary Issue," Victorian
Studies, 9/1959; and J - Moore. The Post-Darwoiani Con trot, r'rsies, Cambridge
Ciiiversitv Press, 1979.
"See II. Sharhn, ''On Being Scientif ir: .5 Critictoc )
Es'oliutionars Geology
and Biology in the Nineteenth Century.'' .Annals of Sri.. Vol. 29, No. 3, 1972,
pp. 271-266.
"See C. Pounaiojiernnia (ed.), Chemical Evolution of fit(, Lady
Prrr'nrubrinn.Aradernir
Press, New York. 1977;\f llnlteo, The Origin of /ift' h ?5,'atnrnl
Causes, Elses'iee
Pub. Co., New York, 197!: S. Fox & K. Dose, ;tlo!ecnlnr Ecol ntiou and the
Origin of fife, 55 II. Freeman, San Francisco, 11172; K. Ks-em olden
(vii.), Grocheinist
ry tool the Origin of Life, Wiley, N esc York, 11175; and S. Miller & I..
Orgel, The Origins of Life on Earth. Prentice-I tall, Engli'woonl Cliffs, N.J.,
1975.
9. Bowls field, Lord Kelvin mid the Age of the Earth, Science II story Pnhns.,
N.Y., 11175; II. Sliarlin, ''On Being Scientific'"; and J. Ilattiangadi.
"Alternatives and lnconitor'nsnrables, Philosophy of ,Scu'ni'i',
12/TI, pp.
51)2-507.
Ss''F'alsili.dale Predictions of Evolutionary Theory." tip. 516-521).
K. SLier. Populations, Spec'ies. and Erolntitni, I lars art! Univ.
Press. Canihridge,
1970, i. 260.
''S. Lovtrup, 'ariation, Selection, Isolation, [ns'ironnn'nt: An
Analysis of Darss'in's
Theory,'' Thi'oria, Vol. 43, 1977, liii. 65-72.
"See B. Cr;inn'r, "On Evolution & Its Relation to
Natural Selection," Dia' logan', \'ol, 16, 1977, pp. 71)8-714.
°"Karl Popper's Philosophy of Biology," pp. 643-646.
''C. D;irsi'in . On file Origin of Sperm's by Nation! Sr'lection, 6th edition,
John \lnrrav, London, 1885, i. 59.
Ruse, ''Karl Poppers Philosophy if l1ioiog'." ii. 6-16-649.
con i/n Origo, of Stitch's by Nat nra! Si'lection, p;,. 1.16, 156.
01' Kolni. The Strni-tnre of Scientific B,'iolufions. I 'my. of Chicago Press,
1962.
°1'hi Structure of Geology, i. 159.
"Ibid., p. 161.
"See the account ni Sir C. ne Br'i'r, C/in di's Daro'in:,S S,'ii'ntific B
nigea;ili y,
lNioldr'rlav & Garden City, 1965.
'l'lonnpson in his lntrorlnrtion to The Origin of S;),,(o's, Es-er5
man edition,
tile si, sit.
"G. Kerkni, Duplications of £ rolnoon, Pergaooni,New York, 1960, p.
150.
''Inlrodnrtion, Ii. si.
"Further details on Darwin's ronreptnal apparatus are available
in the )olloss'ing:
iii. It nsr', ''(',harles Darwin's The ory of Es olution,'' Jour. of the lust,
of Biology, Vol. 8, 1975, pp. 219241; 'I'. Dnh'ihansky, ''On Some
I" nnd,onental
Concepts of Darwinian Biology,'' In 'I . I )oliehan
sky et al (irIs.), Evolutionary Biology, Vol. 2, Meredith, New York ,
1968, pp. 1-54; 51. Willianis, "Deducing the Consequences of
Evolution,"
Jour. T/ieor. Biology, Vol. 29, 1970, pp. 343-355; and I. Lerner,
Concept of Natural
Selection," Proi'. Amer. ]'hit. Soc.. Vol. io:5, 1959. qpp. 173-182.
i Predictions of Evolutionary Theory,--- tilt.526-536.
1E. Ylayr. Principles of Systematic Zoology, McGraw-Hill, New York.
1969, p. 209.
'"F'alsifia)ile Predictions of Es'olufionary Theory," pp. .53,5-536.
Sue further: SI. Snoth, "The Status of Nr'o- Darss-inisn,"
in C. \Vadnliugton
led.), Towards a Theoretical Biology, Alclint' Pith. Cii,, Chicago, 1969,
pp. 82-89; 7,. Kochanski, ''Conditions & Limitation of Prediction-Staking
in Biology"; 'I'. ljohzhansks, Gi'nctit's of the Evolutionary N(,\% York,
1970; B, Fisher, The Genet
hid Theory of Natural St'lt','tioii, Dosc'r Publications, Ness' York, 1958;B.
l,r'ss'oniin, Tli r' Gt'uetin' Bnsis of Ecolntionary C/iangt', Colnniins Univ.
Press, Ness' York, 1974; 5. W right, Erolntiou ann the
Gt'nt'tii"s of Populations.
Univ. of Chicago Press. Chicago, 1969; I). I ,essontin, "Models,
XI allieniatic's
& Metaphors.'' Synthese. Vol. 15, 1963, pp.
SEPTEMBER 1980
222-244; and I). III"', Phil,oodiy of lliologi,'a! Si-it',, i'n', qpp. 59-6;).
After submitting this paper for publication I came upon a paper In's!ability,
Disri'pntahilitc and the Structure of the Modern Svittlictic 'l'Iu'orv of Evolution"
by A. Caplan in Erkenntnis, Nol. 43, 1978, lip. 261-78, ( apIs emphasizes the
distinction of es-olntionarv predictions and retronlichous front those commonly
expected among biologists but, unlike nit', argues that this calls
for a loosening
of the usual vir'ss' of theories ill science.
';See 1. Endler, Geographic- Variation, Special ii; ii, and Clines, Princeton I/isis - Press, Princeton, 1977; XI. White, Modes of
Spt't'iation, 55
- II. Freeman,
San Francisco, 1978,
could have used other examples: Compare II. Bradley, The Species f ont'i'pt in
Pah't,nfology, Systematic's Assoc-., London, 1953- 55 i' has-c also seen that
adaptation has been the subject of debate when applied to the
present: set' (nether
B, Slnnson, ''Biologic-al Adaptation," P/i)losi'ploj of Sewn, e, X ',,l.
38, 1971, pp. 200-215; XI. Chiselin, ''On Se
niantic' Pitfalls of Biological Adaptation,'' Philosophy of Sr'it'nn'e, 501,
:13, 1966, pp. 147- 153; J. Stern, "The Meaning of 'Adaptation' and its H
elation the Phenomenon ,,f N atnral Selection," in 1'. Dobzhansk ~et 'al
(eds.), Lcolutionary Biology, Vol. 4, Meredith. Ness' York, 1970, ph.
39-66; I".
Ayala, ''Biological Evolution: Natural Selection or Ran (loin \\'ilkE', Amer.
Scientist, Sol, 62. 11-12/74, pp. 692-701; and C. K, laIn,
"Population Genetics:
B i'i'saltoitnni if Cenetir Variation," Scirn,'c, Vol. 184, 26/4/74. 'p.
452-454.
"'Compare SI. Smith, "The Stains of Nc'o.l)arsrinisin."
"See B. hn'nsn'h, "The Eases of Evolution," in S. 'lay
led.), Evolution
After
Dneiemn, Vol. 1, i'nns'. of Chicago Press, Chicago. 1959, qpp. 95-116;
Vi'oorlgi'r, The As-b,n,atin' Method in Biology, Cambridge Unix.
Press, Cambridge,
1937; L. Loetgren, "On the Fornmalizahilitsi of Learning and
Evolution"
in P. Soppes et al (ads,), f.ogie, Methodology and Philosophy of Science, Vol.
4, North-I lollannl Pill). Co., Anisierclam, 1973. pp. 647-658 in addition to
Reference 4~
`See for esanmple C. Si iti1islui, Si' nipi' & Slot/n' in £
total itoy
Columbia tOy Pi i'ss, New York. 1944 and The Major Features of Euolntioll, Columbia
P nis - Press, New York, 1953. ()it rates of n's',,lntn,nari change
see also 'I'.
Sr'lsopt et al ''f.ln'n two,' yersns Morphological 11-air's of Es'olnti,,n: Influence
of Morphological (1,,n,pln'sihi ,'' Pab'ol,in,logy, Vol. 1, 1975, pp. 6:5-71)
and the folios,ing nhsc'os.si,,n in \',d. 2, 1976, lilt. 174-179.
"The Major Peat art's of Evolution, 1). :376.
''B. Coldsn-lnniilt, The Material Basis of Et elation, Yale 1' ms.
Press, Ness
Haven, 1940; 0. Scfnndess'oIf, Grit ntifragerm nlrr Pndiiontologze,
E. Srhwrmz.rrharl,
Stuttgart, 1950; his Paleonmo/ogir, Orbrnder Burntrsi'gi'r, Berlin, 1936: his ''Nt'ocatastrophisni?'' reprinted in (alas
irolitifit Geology. Vol. 3, N11. 1, 1975, pp. 9-21; and, though not lil-tended
for this purpose, tin' discussion on sali;ili,nis in XI. NN hife's
Sloth's of Spm'n'iation. Compare "The 11i'ti,rn of the
IIo1iel,il Monster"
fix, J. Could, Natural ilist., \',,l. Sti. 1(177, p. 22; J f1it,,lcl & N.
Elnlrr'ge,
''Piinc'tn:iin'd Epulihria; 'I'll' Tempo & Mode of I'.vihlani Recon-sidered",
Pnli'olnology, S_li1. 3, 977, pi. 1l5-151;S. Stanles , ''ClironosIecit'' t,ongeyihs
. the 0, igol of Genera, & the P,o,c'tnali,nial Model of
Evolution/' Pali',tlsitili'gy,
Vol. -4, 1978. p,. 26-411, and I". B,oikstc'in ii a).,
"lIii'rarc-lsictd
t.inn'ar Modeling of the 't'enmiiit and XI,,(](' of Es ol,,iioti,''
in the sante s,Il,ina' pp. 1211-134.
5f Ilnnlss'ic'k, '"the lrifr'ri'tsci' of I":nmc'ti,us Iron, Structure
in Fossils,'' Orb, Jour. Phil. Sri_ Viii, 15, 1964, pp. 27-41); I). Kitts, The
Sl,an'lart' of Cn't,/t'gy, qpp. 162- 166; Xl. Grenn', '''I'sc,, Es ,il,itaai,irc
Theories,--t!i-it, Jour. P/nh. Sil , S il. 9, 1959-61), pp. 110127 and 185-193;
her "The Is gin' of Bi,ili igc" in The Logic of lit.rsotil
En,, iclt'ilgi',
Bootledge n& Ki'gsti God, London, 1961, pp. ltll-2115; her
''Statistic's & Si'li'ctii,n.'' Bill. J,'ar. Plo!. St'i., Vol. 12, 1961, ,. 25-12; 55. Book, C.
X'ois St ,mlilert, I,. San N ,ilen, in,(] NI. f1-iii', '''I'ss'i, Es
ol,itittu,mri
'I
nii's: A Oise,,ssion,'' Brit, J'oo. Phil. 5,-i., VI, 14. 11)63-4,
lip. 14/1-153;
and C. Carter and 'sI. Cretin'. -------si i, Es ,tlnti,,usrs' Theories. hi XI
(1ri'nm'; .5 Further Discnssioti,'' ibid., q,. ;145-;),5 1
'•Xl Crn'ne, '''I'sci, Es'olnti,,nary 'I'Ia'ories" and I).
Kills, The Stractact' of (ii'o/Igy, p. 16:3, Compare I. Kriihn. "Hole ,,f
Ideas in Advancing l'ale,,nittl,igy," l'ale,ibiology. N il. 5. 19711. pp, 67-Tti,
Is on, case sly I. 'l',,fla'rs;mll told N Elnlrc'clgi', ''Fact,'!
li,'i,ns . minI
l"antis\ in I li:nm,ni l'ale,irmt,l,gi .'' ,Aoit'i'it',oi S,'i,'nlist, \ oi.
(iS, 1977. pt'. 21)4211. See also J, Ditrli,iti,, '''1' It(, luc,,n:1iIi'tn'nn'ss
I,) Our K,,,,\, ledge ,i the Fossil I1,'n'orni,'' Jour. Patittt'/,:gy, So!. 11,
No 3, 1967, pp. /511.5fi5 and Xl. I hi-hI it 0 1i'nls , Major
P"at",,is
in \'cocl,rali' Li',,l,,o,,n, Helium, N.')., 1977,
163