Science in Christian Perspective
and the Theory of Evolution Part II
T. H. LEITH
Department of Natural Science
Toronto, Ontario, Canada
From: JASA 32 (September 1980): 156-163.
[Part 1 of this two-part paper was published is the March 1980 issue,
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,
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
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
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
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
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
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.
-"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.
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,