Why do we find in the upper (i.e., more recent) strata of the geological column fossils of species that cannot be found in lower (i.e., earlier) strata? Why are there almost half a million different species of beetles? Why are living things so well adapted to their environment?

The generally accepted (sc., broadly Darwinian) answer to those, and related, questions—that all of today’s species originated by descent with differential modification from one or just a few initial species—is an instance of a particular kind of scientific explanation. Diachronic (or paleoetiological) theories, those which “ascend from the present state of things to a more ancient condition, from which the present is derived by intelligible causes” (Whewell, Philosophy of the Inductive Sciences, 2: 95), have been put to use in fields ranging from cosmology to linguistics, and are structurally distinguishable from such synchronic (nomothetic or structural) theories as Newtonian mechanics, chemical atomism, or Mendelian genetics.

Fr. Georges Lemaître characterized the “cosmogonic hypotheses” which underlie evolutionary histories of various aspects of the material world (e.g., his hypothesis for galaxies and stars, and, implicitly, Darwin’s for flora and fauna) as “seeking out initial conditions which are ideally simple, from which the present world, in all its complexity, might have resulted, through the natural interplay of known forces” (Lemaître, Primeval Atom, 140). He explicitly included diversity as an example of the kind of complexity that he had in mind (Lemaître, Primeval Atom, 87–89). Nineteenth-century English philosopher Herbert Spencer, who was primarily responsible for applying the word “evolution” to Darwin’s account of the origin of species, defined the “evolution” that such theories posit more abstractly, as “a change from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity” (Spencer, First Principles, 396).

The idea of a universal process of evolution, effective “from the earliest traceable cosmical changes down to the latest results of civilization” (Spencer, “Progress: Its Law and Cause,” 446–47) has found both materialist proponents (e.g., Ernst Haeckel, History of Creation, and Spencer, First Principles) and spiritualist ones (e.g., Pierre Teilhard de Chardin, Phenomenon of Man). More defensible than such theories as an evolutionary account of the history of the material world is a concatenation of distinct evolutionary processes—in cosmology, in geology, in biology, and in linguistics—unified only in the sense that the successive processes must interface at their termini.

This article will discuss only evolution as an account of the origin of biological species, offering: First, a summary of the scientific idea; second, a discussion of its consonance with Catholic theology; third, a discussion of its compatibility with Thomistic philosophy of nature and metaphysics; and fourth, a brief survey of the history of the Catholic reception of the idea, the history of the emergence of a Catholic evolutionism.

I. Evolution and Science

History

The idea that some species originated as the descendants of other species (and even that the human species has an animal ancestry) had been mooted by a number of authors in the eighteenth century (e.g., Benoit de Maillet, Telliamed, and Erasmus Darwin, Zoonomia) and by the early nineteenth had begun to receive serious (if still somewhat speculative) elaborations (e.g., Jean-Baptiste Lamarck, Zoological Philosophy, and Robert Chambers, Vestiges of the Natural History of Creation). Still, the evolutionary theory proposed, independently and in a model of amicable coöperation, by Charles Darwin and Alfred Russel Wallace in 1858–59 far exceeded in scientific quality any of its predecessors and is, in its main outlines, the foundation of the theory accepted to this day.

Central Theses

Darwin’s theory can be summarized in three central theses and a salient application.

The first thesis, perhaps definitive of biological evolution, is that species originate by descent with modification from other species. This thesis might be given the now somewhat uncommon, but still useful, name of transformism.

The second thesis, logically distinct from the first and so not a consequence of it, emphasizes the differentiating character of those transformations. A single ancestral species might produce several different descendant species as a result of different transformations of different groups within the ancestral species. A common-ancestry thesis might be limited in scope (e.g., to mammals, without claiming a common ancestry even for mammals and birds), but Darwin suggested a more comprehensive (at least nearly universal) common ancestry for all living things.

The third thesis specifies the cause of the transformations. Darwin and Wallace, recognizing beneficial and hereditable individual variation within a single species and rates of reproduction in excess of the carrying capacity of the species’ ecological niche, emphasized that the variants better adapted to (i.e., more fit for) the ecological niche in which they lived would be more likely to survive to reproductive maturity, i.e., there would be a natural “selection” of those variations. Darwin said only that “Natural Selection has been the main but not exclusive means of modification” (Darwin, Origin of Species, 6, and 6th ed., 421). Salient in this account of evolutionary change are four features.

The first and second are naturalism and actualism. Species originate as the product of a natural process currently operative in nature.

The third is gradualism. The operative process is the accumulation of changes that are in some sense gradual. How gradual? Darwin’s answer was that “specific changes may have been as abrupt and as great as any single variation which we meet with under nature, or even under domestication” (Darwin,Origin of Species, 201). This is closely related to the actualism just mentioned.

Fourth, although the accumulation will be the result of natural selection, and will therefore modify the species in the direction of greater adaptedness to its environment, the individual variations from among which nature selects do not themselves need to be (and so, at least presumptively, were not) preplanned or goal-directed in order for the process to produce the observed result: “No shadow of reason can be assigned for the belief that variations … which have been the groundwork through natural selection of the formation of the most perfectly adapted animals in the world, man included, were intentionally and specially guided” (Darwin, Variation of Animals and Plants, 1st ed., 2: 432). This ruled out not only direct divine engineering (which would also have been inconsistent with the naturalism mentioned above), but also the kind of natural, inner perfecting principle (as proposed by Lamarck and Teilhard de Chardin) or even just the kind of orthogenesis that Michael Ruse characterized as “evolution ha[ving] a kind of momentum of its own that carries organisms along certain tracks” (Ruse, Monad to Man, 261).

The salient application of the theory of evolution is to the origin of the human race, i.e., to the animal ancestry of man.

Evidence

Common Ancestry

The case for common ancestry is entirely independent of the defensibility of natural selection as the cause of transformations. The structure of the case for it is consilience of induction. Not only does common ancestry make sense of “the [taxonomic] fact that all organisms, recent and extinct, are included under a few great orders, under still fewer classes, and all in one great natural system” (Darwin, Origin of Species, 429), it is the best explanation of a wide array of observed facts from diverse aspects of the natural world.

The first of those aspects is paleontology. Not only do extinct and living species fit together into a single system, but the fauna of each period of the earth’s history is intermediate between those of the preceding and the succeeding periods. In addition, extinct species are more or less intermediate between their modified descendants (Darwin, Origin of Species, ch. 9–10).

The second is biogeography. The similarity of the fauna of a continent, even across differences in ecological niche, e.g., marsupials in Australia, can be explained by descent from a common immigrant species. The variation of fauna living in similar conditions in two different places, e.g., the faunae of the Galápagos and of the Cape Verde Islands, can be explained by descent from entirely different immigrant species into the different places (Darwin, Origin of Species, ch. 11–12).

The third is comparative morphology. Animal parts with widely different uses often show a similarity of structure, e.g., the bone patterns of the hand of a man for grasping; of the forelimb of a mole for digging; and of the leg of the horse, the paddle of the porpoise, and the wing of the bat for running, swimming, and flying (Darwin, Origin of Species, ch. 13).

Scientific research since 1859 has done much to reinforce the strength of each of those lines of argument. New fossil finds conform to the expectations of the theory. The theory of continental drift did much to resolve facts of plant and animal distribution that seemed anomalous in the nineteenth century. Biochemical and genetic similarities, e.g., a genetic code common to all living things and the occurrence of the same genes in the same order in remotely related species (synteny), complemented the similarities of gross morphology. The existence of certain kinds of non-functional DNA, e.g., birds have both the genes necessary for tooth construction and additional genes that prevent those genes from being activated, constitute an analogue to vestigial organs.

Although Darwin’s theory explains a wide array of facts from the diverse areas just mentioned, these are facts that are generally unknown to laymen. Non-biologists simply do not wonder why the structure of bat wings resembles that of mole legs and whale flippers; they do not wonder why the faunae of the Cape Verde and of the Galápagos Islands differ from each other despite the similarity of environment and why each resembles the fauna of its nearest mainland. They do wonder how a species that lacks eyes or wings could ever turn into a species that has them. This lack of wonder about the features of the world that Darwin is trying to explain, combined with the somewhat counterintuitive nature of Darwin’s solution (which, to highlight the paradox, attributes a common ancestry to beetles and whales) is responsible for much of the skepticism that Darwin’s theory continues to face.

Natural Selection and Transformism

The operation of natural selection in the modification of species is not only a deductive consequence of observed facts but is something that is easy to demonstrate, both in the laboratory and in the wild. The question is whether it can modify a species so starkly that ancestor and descendant must be recognized as distinct species, i.e., whether, to quote the title of Wallace’s seminal 1858 paper, varieties of a species really have a “Tendency… to Depart Indefinitely from the Original Type.” We can trace evidence for the affirmative answer through the evolution of the theory.

Darwin was limited by his ignorance of the cause of the variants from which selection could be made. He defended the possibility of stark interspecific transformation by addressing the challenge of what he called “organs of extreme perfection,” his example being the eye: “If numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist … then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.” He went on to identify such gradations in nature, less and more perfect eyes, each useful to its arthropod possessor (Darwin, Origin of Species, 186–89).

By the 1940’s, a cause of variation had been identified. The development of Mendelian genetics and the recognition of genetic mutation as a source of phenotypic change led to a synthesis of Darwinism and Mendelism into what is variously called “Neo-Darwinism” (though that word has also been put to other uses) or just the New or Modern Synthesis (Julian Huxley, Evolution). Experiments showed that even point mutations can produce significant changes in phenotype. Transpositions and duplications of fragments of genetic material add to the range of phenotypic changes that genetic changes can produce. These possibilities move the “numerous, successive, slight modifications” from which nature can select to the genetic level and allow the successive modifications to be phenotypically rather less slight than Darwin had imagined they would be.

If the Modern Synthesis emphasized “evolution [as] an interplay between mutation and selection, with the former providing a supply of variation and the latter acting as a fitness-based sieve,” with “mutation provid[ing] new genes [and] selection act[ing] not on genes but on phenotypes,” it left unaddressed “how we get from altered gene to new phenotype.” That process is partly genetic (i.e., controlled by genes), but partly epigenetic (i.e., altered not only by transcription factors, but even by the environment) (Arthur, “The Emerging Conceptual Framework of Evolutionary Developmental Biology”). The greater attention to that process given in evolutionary developmental biology (“evo devo”) has widely been characterized as an “Extended Evolutionary Synthesis.” Attention to the relevance of embryology to evolution is not new. Indeed, Darwin had written (to American botanist Asa Gray, 10 September 1860) that “embryology is to me by far the strongest single class of facts in favor of change of forms.” Nevertheless, evolutionary developmental biology has made real contributions to our understanding of evolution, though its revolutionary character as a new synthesis is sometimes said to be exaggerated (Laland and Wray, “Does Evolutionary Theory Need a Rethink?”). Sean Carroll summarized the situation as follows:

The architects of the modern synthesis united evolutionary disciplines by asserting that the mechanisms that operated at the level of individuals in populations and species were sufficient to account for the great differences that evolve over geological time.... The extrapolation from small-scale variation to large-scale evolution is well justified. In evolutionary parlance, Evo Devo reveals that macroevolution is the product of microevolution writ large. (Carroll, Endless Forms Most Beautiful, 291)

Application to Man

Darwin’s idea that man, no less than other biological species, owes his origin to descent (with modification) from earlier species is not so much a component of his evolutionary theory (in the strict sense of that term) as an extension of it, and one that presupposes a thesis that is as much philosophical as scientific, namely that man differs from other animals only in degree, not in kind. Success in that philosophical denial of human exceptionalism would make an evolutionary explanation of man’s origin no more problematic than was the evolutionary origin of any other species.

Darwin realized that, in order to defend this extension, what he needed to do was to attend not only to bodily structure but also to mental faculties. In the former he was successful, a success only reinforced by subsequent research. Less convincing is his claim that all of man’s mental and moral powers are the result of a gradual evolution of powers also found in animals (Darwin, Descent of Man, 105–6). He did, to be sure, offer good arguments for some general continuity in the perceptual and emotional faculties of man and animal (Darwin, Expression of the Emotions in Man and Animals), but a refutation of human exceptionalism requires attention to intellectual powers as well, a point on which he was less successful.

Darwin did seem to suspect that a purely evolutionary account of the human mind would create problems: “With me the horrid doubt always arises whether the convictions of man’s mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy. Would any one trust in the convictions of a monkey’s mind, if there are any convictions in such a mind?” (Darwin to William Graham). The comment was triggered by Darwin’s acknowledgment of his “inward conviction” that the world was not the result of chance, but it is hard to see how that “horrid doubt” would not be equally subversive of scientific reasoning itself.

Christian Anti-Evolutionism

Whether because of an excessively literal interpretation of Scripture or because evolution has often been used as a weapon in polemical attacks on religion, some Christians continue to be drawn to anti-evolutionism, of which, broadly speaking, there are two varieties currently on offer.

The first (usually called “Creation Science”) is a comprehensive anti-evolutionism, rejecting most of the paleoetiological sciences in favor of a young earth, a flood-based geology, and a direct divine role in the formation of the various kinds of living things. Supplementary to the substance of this view is its claim that scientific evidence can be adduced in favor of the idea’s central theses.

Quite distinct from that view is a second kind of anti-evolutionism, called by its proponents (e.g., Michael Behe and William Dembski) “Intelligent-Design Theory,” a theory not necessarily comprehensively anti-evolutionist, as the just-mentioned alternative is, but focused on the idea that natural selection is incapable of producing the kind of complexity found in some biological structures and processes (Behe, Darwin’s Black Box, 5, 39, 45), but with an implicit broader rejection of the “develop[ment of life] entirely by natural means” (Behe, Darwin’s Black Box, xi). The alternative offered is the design of those “irreducibly complex” features of biological organisms by an intelligent agent.

Neither view has attained any significant degree of scientific support even among Christian scientists. Either (the former explicitly, the latter tacitly) would constitute an abandonment of the most plausible theology of nature, namely the Christian naturalism defined by Belgian priest-geologist Henry de Dorlodot as “the tendency to attribute to the natural action of secondary causes all that is not excluded therefrom either by reason or the positive data of the natural sciences, and to have recourse to a special Divine intervention distinct from God’s general governing activity only if it is absolutely necessary to do so” (de Dorlodot, Darwinism and Catholic Thought, 94).

Intelligent-Design Theory is also thought by most biologists (e.g., Kenneth Miller, Finding Darwin’s God) to underestimate how much natural selection (and other natural processes) can do and to be too vague as an alternative account of the history of life.

II. Evolution and Theology

The question of whether evolutionary biology is compatible with Catholic theology can be organized around four points of putative incompatibility.

Evolution and Scripture

Do the first words of Sacred Scripture, “in the beginning God created…” contradict the evolutionary account of the origin of biological species? For a number of reasons, they do not. Although some Protestants continue to appeal to the Hexaëmeron (the six-day account of creation) in their objection to the theory of evolution, understanding the first chapter of Genesis has always been difficult. A. J. Maas wrote, in the Catholic Encyclopedia in 1910, that “all interpreters begin by feeling the need of an explanation of [the Hexaëmeron], and all end by differing from all other interpreters (Maas, “Hexaemeron,” 313).

First, textual considerations alone suggest that God gave a role to secondary causes in the origin of living things. The beginning in question in Gen 1:1 is that of the entire material world; about the origin of living things, the text later says:

Let the earth bring forth [βλαστησάτω, germinet] the green herb ... and the fruit tree. (Gen 1:11)

God created [ἐποίησαν, creavit] the great whales, and every living and moving creature, which the waters brought forth [ἐξήγαγεν, produxerant] ... and every winged fowl. (Gen 1:21)

Let the earth bring forth [ἐξαγαγέτω, producat] the living creature in its kind, cattle and creeping things, and beasts of the earth.... And God made [ἐποίησαν, fecit] the beasts of the earth ... and cattle, and every thing that creepeth on the earth. (Gen 1:24–25)

Second, the differences between Genesis 1 and Genesis 2 (e.g., in the order of the creation of living things) makes clear that the application of those texts to matters of history (or science), as opposed to the theological matters to which they are primarily addressed, must be done with caution.

How did the Fathers of the Church read the Hexaëmeron? They differed over whether its days were ordinary 24-hour days, St. Basil teaching that they were literal days (Basil, Homilies on the Hexaëmeron, hom. 2, para. 8) and St. Augustine that there was a simultaneous creation with only one day, represented in a sevenfold aspect (Augustine, Literal Meaning of Genesis, bk. 4, ch. 22, and City of God, bk. 11, ch. 9). Both sides agreed, however, on another point of great importance to the idea of evolution, namely that created things were natural (albeit secondary) causes in the formation of living things. This one finds even among the more literal (Syrian) interpreters. St. Ephrem wrote: “The earth produced everything with the aid of waters and light. Although God had no need of their help in order to produce these things, still it pleased Him to make use of them” (Ephrem, Explanatio in Genesim, 10C). St Basil, that: “He who gave the order [sc., to bring forth animals] also freely gave [to the earth] the power of bringing forth” (Basil, Homilies on the Hexaëmeron, hom. 5, para. 5, and hom. 8, para. 1). This idea was put to a use that Catholic evolutionists were later to find particularly congenial by authors more open to less literal (somewhat Alexandrian) interpretive practice, in particular by St. Gregory of Nyssa (St. Basil’s brother) and by St. Augustine.

St. Gregory’s Apologetic Explication of the Hexaëmeron, written at his other brother’s (St. Peter of Sebaste’s) request for a more scientific defense of St. Basil’s Homilies, is in fact a broader systematic inquiry into natural philosophy (i.e., science). Its goal was to effect a reconciliation of natural philosophy and Scripture. His views turned out in the end to be somewhat different from St. Basil’s. His theologically grounded cosmogony begins with an initial simultaneous creation of the material world followed by a sequential emergence (“evolution” in the etymological sense) of individual things: “By the power of the Creator everything was established simultaneously in material form; then in order to bring actual things into existence, the individual manifestation of the objects we see in the universe was completed in a certain natural order and sequence” (Gregory, Hexaëmeron, §73); “The voice of God is not a command expressed through words, but [rather] the word of God is that wise artisanal power found in everything that has come into existence, through which the wonders of existence are given effect” (Gregory, Hexaëmeron, §64).

St. Augustine suggested that God embedded in the world which He created rationes seminales(“seminal reasons”), seed-like potencies whose gradual actualization caused the adornment of the world to change over time (Augustine, Literal Meaning of Genesis, bk. 5, ch. 4), an evolution (or at least a succession) of fauna, though not an evolutionary transformation of one species into another.

Our interpretation of the Hexaëmeron must be guided by our knowledge of the nature of created things, as were those of St. Augustine and St. Gregory, and even of St. Basil. St. Augustine said that one should not imagine water acting contrary to its nature when interpreting the Biblical passage about the “firmament in the midst of the waters.” He warned against “using the testimony of Holy Scripture against people who engage in learned discussions about the weights of the elements” (Augustine, Literal Meaning of Genesis, bk. 2, ch. 1, para. 4). St. Thomas abstracted from St. Augustine’s commentary the principle that “in the initial arrangement of nature we must look not for anything miraculous, but for what the nature of things ordains” (Aquinas, Summa theologiae I, q. 67, a. 4, ad 3). Francisco Suárez said, in a discussion of the problem of evil: “One should not have recourse to the First Cause when an effect can be reduced to secondary causes” (Suárez, Summa theologiae, bk. 1, ch. 1, n. 8).

Although the Church emphasizes the importance of following the guidance of the Fathers in the interpretation of Scripture, Pope Leo XIII, in his seminal encyclical Providentissimus Deus (1893), cautioned that this must not be done uncritically:

The defense of Holy Scripture … does not require that all the opinions which individual Fathers or later interpreters have made be equally maintained, since, in passages discussing nature, some of those opinions are merely those of their own times and now seem unlikely. So, one must carefully discern what in their interpretations they pass on [tradant] to us as pertaining to faith or closely connected to it, what they pass on with unanimous consent. (Leo XIII, Providentissimus Deus, no. 19; translation mine).

The sacred writers, or to speak more accurately, the Holy Spirit who spoke through them, “did not seek to teach man the internal constitution of visible things,” but rather … sometimes described them either figuratively [translationis modo] or in terms that were in common use at the time.... [They wrote] what God Himself had said, speaking to men in a way adapted to human understanding. (Leo XIII, Providentissimus Deus, no. 18; translation mine).

Pope Pius XII also emphasized the care with which interpretive questions of this type must be approached in his 1943 encyclical Divino afflante Spiritu:

we may rightly and deservedly hope that our time also can contribute something towards the deeper and more accurate interpretation of Sacred Scripture. For not a few things, especially in matters pertaining to history, were scarcely at all or not fully explained by the commentators of past ages, since they lacked almost all the information which was needed for their clearer exposition. How difficult for the Fathers themselves, and indeed well nigh unintelligible, were certain passages is shown, among other things, by the oft-repeated efforts of many of them to explain the first chapters of Genesis. (Pius XII, Divino afflante Spiritu, no. 31)

What is the literal sense of a passage is not always as obvious in the speeches and writings of the ancient authors of the East, as it is in the works of our own time. For what they wished to express is not to be determined by the rules of grammar and philology alone, nor solely by the context; the interpreter must, as it were, go back wholly in spirit to those remote centuries of the East and with the aid of history, archaeology, ethnology, and other sciences, accurately determine what modes of writing, so to speak, the authors of that ancient period would be likely to use, and in fact did use. For the ancient peoples of the East, in order to express their ideas, did not always employ those forms or kinds of speech which we use today; but rather those used by the men of their times and countries. (Pius XII, Divino afflante Spiritu, no. 35–36)

Evolution (Transformism and Common Ancestry) and Creation

The term “creation” is, not entirely unreasonably, used in two senses, as ably pointed out by St.-George Jackson Mivart, English biologist and one of the nineteenth-century pioneers of Catholic evolutionism: “In the strictest and highest sense ‘Creation’ is the absolute origination of anything by God without pre-existing means or material, and is a supernatural act” (Mivart, Genesis of Species, 269). In that sense, God created the material world as a whole, angels, and each individual human soul, but did not create plant or animal species, or even the first human body. About the first two the Bible says that the earth brought them forth and about the third that it was “formed … of the slime of the earth” (Gen 1:11–12, 24, and 2:7). Still, they too were, in another sense, created: “In the secondary and lower sense, ‘Creation’ is the formation of anything by God derivatively; that is, […] the pre-existing matter has been created with the potentiality to evolve from it, under suitable conditions, all the various forms it subsequently assumes.... This is the natural action of God in the physical world, as distinguished from His direct, or, as it may be here called, supernatural action” (Mivart, Genesis of Species, 269). That distinction made, he went on to say:

Conflict has arisen through a misunderstanding. Some have supposed that by “creation” was necessarily meant either primary, that is, absolute creation, or, at least, some supernatural action; they have therefore opposed the dogma of “creation” in the imagined interest of physical science. Others have supposed that by “evolution” was necessarily meant a denial of Divine action, a negation of the providence of God. They have therefore combated the theory of “evolution” in the imagined interest of religion. (Mivart, Genesis of Species, 279)

The theory of evolution, being nothing more than an account of how one material thing is transformed into another, presupposes, but offers no account of, how the material world of which that thing is a part came into being. It might, as far as the theory is concerned, always have existed, or it might have been created, or it might have come into existence in some other way.

Natural Selection and Divine Providence

Another putative tension between evolution and Catholic doctrine is the question of whether the theory gives to chance a role that places it in tension with the doctrine of divine providence.

What caused the variants from among which (metaphorically) nature makes its selection? In his Origin of Species, Darwin had written: “I have hitherto sometimes spoken as if the variations … had been due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation” (Darwin, Origin of Species, 131). The question of the exact place of chance in Darwin’s own thought and work (on which, see Curtis Johnson, Darwin’s Dice) need not detain us here. What is important to note are four other points.

First, Darwin rejected one proposed cause of variation. Having written to his friend, American botanist Asa Gray, that “I am conscious that I am in an utterly hopeless muddle. I cannot think that the world, as we see it, is the result of chance; & yet I cannot look at each separate thing as the result of Design” (Darwin to Gray, 26 November 1860). Darwin later wrote that “however much we may wish it, we can hardly follow Professor Asa Gray in his belief ‘that variation has been led along certain beneficial lines’” (Darwin, Variation of Animals and Plants, 2: 432).

Second, although Darwin might have hoped to discover some other kind of biological law as the cause of variation, the idea that variation was not produced by any biological laws perhaps received support from H. J. Muller’s work in the fruit fly laboratory at Columbia University in the 1920’s, showing that irradiation produces the kind of micromutation on which natural selection could act.

Third came a transition from science to philosophy of nature. If the deliberately non-polemical Darwin, skeptical though he was about the idea of designed variation, did not push the question of whether the theory of evolution was itself anti-providentialist, others did so, claiming that Darwinism should lead us to radically anti-providentialist conclusions. Nobel Laureate Jacques Monod, for example, wrote that “chance alone is at the source of every innovation, and of all creation in the biosphere. Pure chance, absolutely free but blind, [is] at the very root of the stupendous edifice of evolution.” In the final paragraph of the book, he added : “Man knows at last that he is alone in the universe’s unfeeling immensity, out of which he emerged only by chance. His destiny is nowhere spelled out, nor is his duty” (Monod, Chance and Necessity, 112 and 180).

Fourth, Monod’s “blind chance” is not entirely consonant with the biological evidence: There is some evidence not only of mutation bias against alteration of particularly important strands of DNA and of targeted DNA repair, but of environmental stress inducing accelerated mutation rates (Monroe, “Mutation Bias,” and Belfield, “Accelerate[d] Mutation Rate”). The case for some kind of mutation bias may not yet be dispositive (Zhang, “Important Genomic Regions,” and Liu and Zhang, “Is the Mutation Rate Lower?”), but it is surely not necessary in order to reconcile the ideas of evolution and providence. Indeed, the resolution of that controversy is irrelevant to the question at hand. Even a strong version of the random-mutation thesis is entirely compatible with the doctrine of divine providence (cf. International Theological Commission, Communion and Stewardship, no. 69). St. Thomas Aquinas explicitly argued that providence not only does not exclude, but actually requires the presence of chance in the world (Aquinas, Summa contra gentiles bk. III, ch. 74). That random processes can be deliberately chosen in order to serve larger ends is clear from our ordinary experience. The Knights of Columbus use random processes at their Bingo Nights as a means of providing social enjoyment for parishioners and of raising money for charity.

Could such variations, random in the sense just specified, be of any use in the history of life on earth? Filtered and favored by natural selection, they would keep living things well-adapted to their environment when it changes (as well as giving them the features necessary to live in adjacent environments to which they were at first not well-adapted). Could God count on a fauna produced by natural selection to provide a biological foundation for the kind of rational being that it was His intention to create? Perhaps so. Cambridge evolutionary biologist Simon Conway Morris has suggested that there seems to be good reason to believe that “If ‘we’ had not emerged, then … [at least some] viviparous, warm-blooded, vocalizing and intelligent species would have done so” (Morris, Life’s Solution, 223). His claim is still scientifically controversial, but the defense of the compatibility of providentialism with the role given to chance by Darwin does not depend on Morris’ claim being true.

Given the tendency of some anti-evolutionists to use the phrase “intelligent design” as a designation precisely for their idea that direct divine design and action was necessary to the formation of various complex biological structures and processes, perhaps it is worth adding that one can hold that the universe itself was intelligently designed, a product of divine providence, without rejecting the evolutionary origin of species. Not only is divine design compatible with the concepts of derivative creation and of Christian naturalism mentioned above, but those latter concepts, when applied to the origin of species, draw particular attention to the ingenuity of God, as Catholic authors have long emphasized. In 1887, French Dominican M. D. Leroy wrote: “The genesis of the organic world through the intermediation of natural agents requires infinitely more ingenuity than does direct creation. Between a watchmaker who makes a precision watch and an inventor who creates a machine capable of itself producing the same watch, I have no hesitation; the inventor seems to me a hundred yards above the watchmaker” (Leroy, Évolution des espèces organiques, 57). In 1903, Ludwig von Hammerstein wrote: “A billiard player wishes to send a hundred balls in particular directions; which will require greater skill—to make a hundred strokes and send each ball separately to its goal, or, by hitting one ball, to send all the ninety-nine others in the directions which he has in view?” (von Hammerstein, Gottesbeweise und moderner Atheismus, 150). Finally, in 1910 German Jesuit Erich Wasmann wrote: “God’s power and wisdom are shown forth much more clearly by bringing about these extremely various morphological and biological conditions through the natural causes of an evolution of species than they would be by a direct creation of the various systematic species” (Wasmann, Modern Biology and the Theory of Evolution, 410).

Natural Selection and Catholic Moral Theology

Does Darwin’s thesis that natural selection, grounded as it is in a universal “struggle for existence,” raise moral concerns? Does it, as William Jennings Bryan once argued, make “hatred … the law of man’s development” and assert that “man has reached his present perfection by a cruel law under which the strong kill off the weak” (Bryan, Menace of Darwinism, 36)?

The idea of such a conflict between Darwinian biology and Christian ethics is based upon the idea that natural Darwinism leads to one or more of the socio-economic policies loosely associated under the vague umbrella-term “social Darwinism,” whether because of the policy’s emphasis on the value of competition in human affairs or because of its use of an idea of differential fitness. Under that umbrella fall four distinct policies in particular:

• Laissez-faire capitalism and governmental abstention from poor-relief: Spencer wrote that “Instead of diminishing suffering, [‘relief of the poor from public funds raised by rates,’ though not necessarily from ‘the spontaneous sympathy of men for each other’] eventually increases [the suffering of those whose ‘own stupidity, or vice, or idleness, entails loss of life’]. It favours the multiplication of those worst fitted for existence, and, by consequence, hinders the multiplication of those best fitted for existence” (Spencer, Social Statics, 381).

• Militarism and imperialistic war: Friedrich von Bernhardi argued that “War is a biological necessity.... Those forms survive which are able to procure themselves the most favorable conditions of life, and to assert themselves in the universal economy of Nature. The weaker succumb” (Bernhardi, Germany and the Next War, 18).

• Eugenics, “the science of improving stock”: Francis Galton said this would “give the more suitable races or strains of blood a better chance of prevailing speedily over the less suitable than they otherwise would have had” (Galton, Human Faculty and its Development, 25n). 

• Invidious distinctions between human races: Spencer wrote that “differences of complexity exist between the minds of lower and higher races” and that “some races [have] faculties that are almost or quite absent from others (Spencer, “Comparative Psychology of Man,” 258–59).

The association between these ideas and natural Darwinism was made by many of the ideas’ proponents as well as by many of their anti-evolutionist opponents. Although traces of some of the ideas can be found in Darwin’s work, the logical connections are tenuous and the ideas themselves exogenous. The exogeneity is evident from the fact that Spencer’s first presentation of his views on economics and poor relief was published in 1851, several years before the Origin of Species; the invidious racial distinctions can be found in many non- (and pre-!) Darwinian authors (e.g., in Samuel George Morton’s American-School ethnology). The tenuity is, if not proven by, at least suggested by, the contrast between Spencer’s capitalism and Wallace’s socialism or between Bernhardi’s militarism and Spencer’s pacifism.

These ideas are neither tenets of Darwin’s scientific thought nor deducible from it and many defenders of natural Darwinism were emphatic in their rejection of its extension to social policy. The alleged Darwinism of social Darwinism is grounded in two concepts (or at least phrases) that appear in his theory.

The first, “struggle for existence,” was a reality recognized, both in nature and in human society, before Darwin ever wrote. Count Buffon, Augustin Pyramus de Candolle, Charles Lyell, and many others recognized that the rate of reproduction observed in biological species made the survival of most offspring to reproductive maturity unsustainable; there must, therefore, be a competition (i.e. a struggle) for survival. Darwin’s grandfather Erasmus put the general biological situation in dramatic terms—“Air, earth, and ocean, to astonish’d day / One scene of blood, one mighty tomb display! / From Hunger’s arm the shafts of Death are hurl’d, /And one great Slaughter-house the warring world!” (Erasmus Darwin, The Temple of Nature, IV: 63–66). Charles Darwin emphasized that he used the word “struggle” in “a large and metaphorical sense”—“a plant on the edge of a desert is said to struggle for life against the drought… [and a] plant which annually produces a thousand seeds, of which on an average only one comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground” (Darwin, Origin of Species, 62–63).

The second, “survival of the fittest,” was coined by Spencer in 1864 (Spencer, Principles of Biology, §165); Darwin began using it only in 1868–69 (Darwin, Variation of Plants and Animals, 1st ed., 1:6), acknowledging Spencer’s term as a “more accurate, and … sometimes equally convenient” alternative to “natural selection” (Darwin, Origin of Species, 5th ed., 72). But as Harvard geologist, and lifelong Baptist, Kirtley Mather emphasized in the statement he prepared as testimony for the Scopes Trial in 1925, “the survival of the ‘fit; does not necessarily mean either the survival of the ‘fittest’ or of the ‘fightingest’”: “At times of crisis in the past, it was rarely selfishness or cruelty or strength of talon or of claw that determined success or failure. Survival values at different times have been measured in different terms. Ability to breathe air by means of lungs rather than to purify the blood by means of gills meant success in escaping from the water to the land” (Mather, “Statement,” 245).

However much the ideas of a struggle for existence and of the survival of the fittest might be extended by analogy to some aspects of human social life (survival of the most elegant or most explanatory theory in the history of science? survival of the catchiest neologism in the history of lexical evolution?), natural Darwinism differs from social Darwinism in being descriptive rather than prescriptive, normative and descriptive in a way that makes arguments from one to the other fallacious.

Darwin himself did not offer a prescriptive moral theory. He did accept the idea that man has a “moral sense, or conscience,” a principle of action that would “lead… him without a moment’s hesitation to risk his life for that of a fellow-creature; or after due deliberation, impelled simply by the deep feeling of right or duty, to sacrifice it in some great cause.” This, “the most noble of all the attributes of man,” is “of all the differences between man and the lower animals… by far the most important.” His interest was not a fuller elaboration of its content, but rather seeing “how far the study of the lower animals can throw light on one of the highest psychical faculties of man.” His thesis was that “any animal whatever, endowed with well-marked social instincts, would inevitably acquire a moral sense or conscience, as soon as its intellectual powers had become … well developed” (Darwin, Descent of Man, 70–72). Darwin’s use of moral sense theory may put him at odds with Catholic metaethics, according to which ethics is founded not on moral feelings, but on moral judgments about the truth of abstract propositions, but there is nothing in his theory that constitutes a challenge to Christian teaching about justice, benevolence, or self-sacrifice.

Indeed his colleague and champion Thomas Henry Huxley wrote that, although “from the point of view of the moralist, the animal world is about the same as a gladiator’s show,” nevertheless “[we must] understand, once and for all, that the ethical progress of society depends, not on imitating the cosmic process, … but in combating it” (Huxley, Evolution and Ethics, 200 and 83). Prince Pyotr Kropotkin was unwilling to accept even Huxley’s gladiatorial view of nature as particularly Darwinian, emphasizing that “mutual aid is as much a law of animal life as mutual struggle, [and], as a factor of evolution, it most probably has a far greater importance.” Even Darwin, he said, intimated that “the fittest are not the physically strongest, nor the cunningest, but those who learn to combine so as mutually to support each other, strong and weak alike, for the welfare of the community” (Kropotkin, Mutual Aid, 6 and 2).

Evolutionary Anthropogenesis and Exceptionalism, Creation, and Monogenesis

Ontological Exceptionalism

The idea that man is different in kind from animals (human exceptionalism) is not a purely religious one. Friedrich Engels wrote, in an unfinished essay that “the animal merely uses its environment, and brings about changes in it simply by its presence; man by his changes makes it serve his ends, masters it. This is the final, essential distinction between man and other animals[;] … it is labor that brings about that distinction” (Engels, “The Part Played by Labor,” 365). The Catholic characterization of the finaland essential distinction is, however, somewhat different from, and ultimately goes further than, Engels’ idea. It has two components—one behavioral, the other structural.

 The first power is conceptual (as opposed to merely perceptual) thought. Engels had recognized that “the eagle sees much farther than man, but the human eye discerns considerably more in things than does the eye of the eagle” (Engels, “The Part Played by Labor,” 361); he failed, however, to recognize the full significance of the fact that conceptual thought is not merely the ability to distinguish triangles from squares or to recognize boxes, traps, and eagles, but to distinguish chiliagons from myriagons (thousand-sided figures from ten-thousand-sided ones) or to conceptualize carnivores, logical dilemmas, and multi-dimensional vector spaces. We have concepts of things of which no image is possible. Wallace, impressed though he was by the power of natural selection to reshape plants and animals in response to environmental pressures, rightly pointed out that certain intellectual acts of human beings (e.g., the development of higher mathematics) were simply beyond the reach of that process (Wallace, “The Limits of Natural Selection,” 339–41 and 358–59).

Engels called “a contrast between mind and matter, man and nature, soul and body” a “senseless and unnatural idea” (Engels, “The Part Played by Labor,” 366). St. Thomas argued that conceptual thought was beyond the power of any purely material being (Aquinas, Summa theologiae I, q. 90, a. 2).

The second feature of human behavior is free will. The idea that some human actions are free (from determination by antecedent states of our brain and our circumstances, free even from determination by our character) and can be the result of deliberate choice is, in one sense, a more radical challenge to nineteenth-century determinism than is quantum indeterminacy. That kind of freedom is not the kind of power that a material being can possess.

Creation of the Human Soul (Phylogenetic Exceptionalism)

Since every (and a fortiori the first) human soul was created immediately by God (CCC, no. 366; Aquinas, Summa theologiae I, q. 90, a. 2), the idea of evolution by natural processes from animal ancestors cannot provide a complete account of the origin of the first human beings.

That doctrine leaves open the question of the origin of the first human body. Was it formed by direct divine action on non-living matter? Did God infuse the created human soul into an animal body fully formed by natural evolutionary processes? Or did God form the first human body not from the slime of the earth but from an evolved animal body which He had to (or at least did) modify further before infusing into it a human soul?

In 1950, Pope Pius XII, in his encyclical Humani generis, wrote that “the Teaching Authority of the Church does not forbid that, in conformity with the present state of human sciences and sacred theology, research and discussions, on the part of men experienced in both fields, take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter” (̂Pius XII, Humani generis, para. 36). Pope St. John Paul II said in 1996 that “today, more than a half-century after the appearance of that encyclical, some new findings lead us toward the recognition of evolution as more than an hypothesis” (John Paul II, Message to the Pontifical Academy of Sciences, para. 4).

Given the distinctiveness of human powers, the evolutionary origin of the human body, Catholic evolutionists emphasize, in no way constitutes a challenge to human dignity. As another of the nineteenth-century pioneers of Catholic evolutionism, Italian anatomist Filippo de Filippi, put it: “To think that the origin of man is perhaps less divine when the Biblical clod of earth turns out to be the entire organic world is a strange way of understanding human dignity” (De Filippi, Uomo e scimie, 67–68).

Monogenesis

Of greater theological concern than the possible evolutionary origin of the first human body is another question: Did the human race begin with a single first couple or with an entire initial population (monogenism vs. polygenism) and, if there was an entire first human population, was it at least a single first human group or were there many separate first human populations (monophyletic vs. polyphyletic polygenism)? The presumptive Darwinian answer was monophyletic polygenism, a view which later received additional support from the details of genetic variation (trans‐species polymorphisms) common to human beings and chimpanzees (Ayala, “The Uniqueness of Humankind”). Catholic theology, by partial contrast, has long held a monogenetic account not only in order to emphasis the specific unity of the human race, but as an implication of the doctrine of original sin as defined in the Council of Trent’s “Decree on Original Sin”: “The sin of Adam is in its origin one, is transfused into all men by propagation not by imitation, and is in each man as his own.” That doctrine seems to imply that there was a single original sinner (or couple) from whom every other human being is descended. And so, in addressing the question in his encyclical Humani generis, Pope Pius XII wrote:

The faithful cannot embrace the opinion which maintains that either after Adam there existed on this earth true men who did not take their origin through natural generation from him as from the first parent of all, or that Adam represents a certain number of first parents.... It is in no way apparent how such an opinion can be reconciled with that which the sources of revealed truth and the documents of the Teaching Authority of the Church propose with regard to original sin, which proceeds from a sin actually committed by an individual Adam and which, through generation, is passed on to all and is in everyone as his own (Pius XII, Humani generis, no. 37). 

The non-definitive character of the phrase “it is in no way apparent that” was noticed almost immediately. In the first years after the publication of the encyclical, officials at the Holy Office proposed to Pope Pius that he officially promulgate a strict interpretation of that phrase, but he declined to do so (Kemp, Origins of Catholic Evolutionism, 400–401).

 In the last half-century or so, a number of theologians have tried variations on the theme of reconciling the doctrine of original sin with polygenism, to their own satisfaction, though not to that of their critics. The variations have included the idea of a communally committed original sin or at least universal responsibility for the sin of the head of the community. Karl Rahner, for example, wrote: “The first man created in the state of original justice is nominated by God as the trustee, in respect of the justice compulsorily intended by God for all men, for all the men who follow him, whether they descend from him physically or not. This first man loses original justice for himself and all other men. Thus all are subject to original sin” (Rahner, “Theological Reflections on Monogenism,” 270).

In “Science, Theology, and Monogenesis,” Kemp has argued that the alleged conflict between biology and theology on this point is only apparent, and can be resolved by making a distinction, as this scientifically possible and theologically orthodox scenario shows: Following the evolutionary emergence of a population of primates, many of which had perceptual powers sufficiently complex to permit the abstraction of concepts if they only had a rational soul capable of doing so, God selected two of those beings and created for them (and for them alone) rational souls, with the intention of later creating rational souls for their descendants as well. The rational soul did not make the new human beings biologically incapable of interbreeding with the larger population from which they had emerged. Some of the descendants of those first human beings did in fact interbreed with the non-human members of the larger population and God infused rational souls into offspring who had even one human parent. That interbreeding introduced into the human population the range of genetic diversity cited by Ayala as evidence of polygenesis. Before long, the entire biologically human population would, even without either providential assistance or selective advantage, be descended from the first two human beings, and consequently would be fully human.

The scenario was based on the fact that a single couple could be the ancestor of all other human beings without being the sole biological ancestor of them all. It distinguished between a larger group (“merely biologically human beings”) and a smaller group (the ones with rational souls, “philosophically human beings” in virtue of their rationality and “theologically human beings” in virtue of the consequent destiny that God grants to them). Such a distinction would make the human race biologically polygenetic in the way that genetics and evolutionary biology suggest that it is while at the same time making it theologically monogenetic in the way that Catholic theology requires.

III. Evolution and Philosophy

Is the idea of an evolutionary origin of biological species compatible with the Aristotelian-Thomistic philosophy of nature (and its underlying metaphysics) which have played such an important role in the formulation of Catholic theology?

To keep that question in focus, we must distinguish it from the rather different question of whether the theory is consistent with everything that St. Thomas ever said about species and their origin. Of particular relevance are three ideas.

The first two are theological. One makes the origin of species something that occurred not over time, but only at the beginning of the world: “In its beginning, the universe was perfect with regard to its species [quantum ad species]” (Aquinas, De potentia Dei, q. 4, a. 2, ad 22); “nothing can be added to the perfection of the universe … as to the number of species” (Aquinas, Summa theologiae I, q. 118, a. 3, ad 1–2).

The second idea attributes their origin to God’s direct formation of individual species: “The first members of species were immediately created by God, such as the first man, the first lion, and so forth” (Aquinas, Super sententiam, bk. 2, dist. 1, q. 1, a. 4), and “the corporeal forms that bodies had when first produced came immediately from God” (Aquinas, Summa theologiae I, q. 65, a. 4).

A third idea, asserting the fixity of the resultant species, is a point of natural philosophy: “The thing generated is like its generator in species and form” (Aquinas, De potentia Dei, q. 3, a. 8, sc. 4).

The passages quoted above must, however, be balanced against other passages—from St. Thomas’ work and from that of St. Albert the Great—some of which are more consonant with the theory of evolution than are those just quoted. St. Thomas wrote that “when God made things out of nothing He did not at once bring them from nothingness to their ultimate natural perfection, but conferred on them at first an imperfect being, and afterwards perfected them, so that the world was brought gradually from nothingness to its ultimate perfection” (Aquinas, De potentia Dei, q. 4, a. 2) and that “the universe in its beginning was perfect … as regards nature’s causes, from which afterwards other things could be propagated” (Aquinas, De potentia Dei, q. 3, a. 10, ad 2). He also wrote:

In the first production of things matter existed under the substantial form of the elements.... In the first instituting of the world animals and plants did not exist actually.... On the day on which God created the heaven and the earth, He created also every plant of the field, not, indeed, actually, but "before it sprung up in the earth," that is, potentially [potentialiter].... God created all things together so far as regards their substance in some measure formless. But He did not create all things together, so far as regards that formation of things which lies in distinction and adornment. (Aquinas, Summa theologiae I, q. 74, a. 2 c. and ad 1–2)

St. Thomas, like Aristotle before him, also accepted the possibility of heterogenesis (including the spontaneous generation of living things from non-living matter, which will be discussed below).

St. Thomas cannot in any case be taken as the last word either on the workings of nature or on the interpretation of Scripture. On the workings of nature, St. Albert wrote that “in matters of faith and morals, Augustine is more to be believed than are the philosophers if they disagree, but if it is medicine that is under discussion, I would put more trust in Galen or Hippocrates and, if it is the natures of things, in Aristotle or in some other expert on that subject” (Albert, In II sententiarum, dist. 13, a. 2). And on the interpretation of Scripture we must bear in mind the passage from Pope Leo XIII quoted above.

Most importantly, these ideas need to be distinguished from the relevant philosophical theses at the core of Thomism—hylomorphic essentialism, proportionate causality, and teleology.

Despite the rejection of Thomism by some Catholic evolutionists and the rejection of evolution by some Thomists (both discussed below), there has, in recent years, been a strong and sustained effort to show that a Thomistic philosophy of nature can accommodate evolutionary biology (e.g., Edward Feser, Aristotle’s Revenge; Mariusz Tabaczek, Theistic Evolution; and Nicanor Austriaco, ed., Creation through Evolution). In light of the continued concern that some Catholics have expressed in this matter, a review of the points of contention is in order.

Essentialism

The term “essentialism” has been used and defined in various ways. The version central to Aristotelian philosophy is designed to underlie the recognition of the world as containing ordinary-sized objects (e.g., squirrels and oak trees, not just quarks and leptons) which are irreducible (not fully explainable by reference to the interactions of their parts), mutable (capable of persisting through some changes, though not through others); and intelligible (adapted from Stephen Boulter, “Can Evolutionary Biology do without Aristotelian Essentialism?,” 84–85). Aristotelian essentialism characterizes those objects (“substances”) as composites of matter (which individuates them) and a substantial form (which gives them their essence, making them kinds of things that they are). A substance’s essence plays both a classificatory role (comparing and contrasting it with other substances) and an explanatory one (with respect to its powers and structure). A substance can persist through changes in what are called its accidental features, but loss of its essential features would cause it to cease to exist as what it was, though its matter would persist in another substance or substances.

Over the last half-century or so, however, essentialism (in any form) has come under attack for, among other charges, being generally obstructive in biological taxonomy (David Hull, “The Effect of Essentialism on Taxonomy”). It has, however, at the same time, also had its philosophical defenders, in versions more or less Aristotelian (e.g., Edward Feser, Scholastic Metaphysics, and David Oderberg, Real Essentialism). Our particular concern here is the charge that essentialism is incompatible with the theory of evolution.

Ernest Mayr, for example, has argued that, although essentialism fits other sciences, biology is different from the inorganic sciences precisely in its focus on the uniqueness of individuals. The theory of evolution, he wrote, requires that in biology essentialism be replaced by “population thinking” (Mayr, Growth of Biological Thought, 46). This is not, however, quite correct. It is the historical, not the biological, character of the theory of evolution that requires attention to individual differences and to population averages (or to the distribution of feature variants within a population) rather than to species types. Physiology and anatomy (so, functional, as opposed to evolutionary, biology) do not require attention to those differences any more than does chemistry. In geology, while individual details are not important in crystallography, they are important elsewhere—not only in addressing questions in archeology (e.g., in recognizing the geographical source of the sarsens and bluestones used to build Stonehenge), but in more purely geological work (e.g., similarities between the Appalachian and the eastern Greenland and variations in magnetic polarity provide important evidence of continental drift). Individual differences can provide clues to history, but their importance to that task does not call into question the individual object’s possession of an essential character which explains its powers and structure.

Other alleged grounds for the claim of incompatibility are that essentialism entails fixism (i.e., progenitors’ inability to produce offspring of a species different from themelves) and clear boundaries between species, boundaries that are allegedly incompatible with Darwinian gradualism. Both implications are denied by essentialists. The former, because the idea that an individual of one species does not have the power to produce offspring of another species does not preclude its being a contributor to a confluence of causes that so form matter as to produce an individual of a different species (on which, see more below). The latter, because Darwinism requires only gradualism with respect to the genotypes of succeeding generations. Evolutionary developmental biology has shown that slightly different genotypes can undergo (or give rise to) very different developmental programs, producing very different phenotypes. Even cases of phenotypic similarity that might leave us unable to assignindividuals to their respective species (epistemic uncertainty) do not undermine the claim that each individual belongs to one species or another (metaphysical nominalism), whether we can recognize which or not.

The theory of evolution is, therefore, consistent with Aristotelian essentialism.

Proportionate Causality and Transformism

The idea of one species giving rise to another raises two additional questions that Thomistic evolutionism must answer.

Trans-specific Generation (Heterogenesis)

First, can members of species ever produce offspring of a different species, as the theory of evolution requires that they at least occasionally do? Reproductive powers, as the name itself suggests, are, in their ordinary (“for the most part”) operation, only the power to produce another individual of the reproducing species. Nevertheless, neither St. Albert nor St. Thomas ruled the production of a different species out in principle. They allowed it in several cases where the science of their day suggested that such a thing actually happened.

St. Albert identified five ways in which one plant could be transmuted into another, for example the transmutation of seeds themselves while they are in the ground. Fields sown with rye, he said, sometimes produced wheat, and vice versa (Albert, De Vegetalibus, bk. 5, tr. 1, ch. 7). In that same place, he mentioned in passing that such heterogenesis could also occur in animals, citing hybridism (“the mixing of seeds close in composition”), as in the product of mules from the interbreeding of horses and donkeys.

Aristotle thought that some lower organisms emerged spontaneously from non-living matter, e.g., testaceans from mud or insects from putrefying matter (Aristotle, History of Animals, bk. 5, ch. 15 and 1). St. Thomas attributed the emergence of lower organisms from putrefying matter in such cases to the action of “a celestial power with active and passive qualities,” a cause specifically different from, but at least in some sense (he thought) greater than, the lower organism that it produced (Aquinas, Super Sententiam, bk. 2, dist. 1, q. 1, a. 4).

The analyses of the phenomenon in most of those cases turns out to be incorrect, but the willingness to entertain them shows that they did not think that transmutation (and a fortiori, would not think the transformism at the core of the theory of evolution) was theoretically problematic.

However much the problem of the transformations included in the modern theory of evolution is somewhat mitigated by a Thomistic distinction between the “natural species” of metaphysics and the “systematic species” of contemporary biology (for a history of the concept, see James Hofmann, “Erich Wasmann [on] Natural Species”), that will not, given the relatively comprehensive version of common ancestry generally accepted in contemporary evolutionary biology, completely eliminate the question of whence the substantial forms of new species come. Insects and reptiles, for example, would not count as a natural species as most Thomists used the term. The problem is only heightened when the offspring has powers not found at all in the generator.

Progressive Heterogenesis

Can a species produce, not just another species, but a higher species? Does the idea that evolution produced such a change not violate the principle that an effect cannot be greater than its cause (Nihil dat quod non habet)?

The idea of distinguishing higher and lower forms of life was, to be sure, overextended by those who elaborated a comprehensive scala naturae in which nearly every species had a place higher or lower than every other, but the idea of progress in the history of life does not require such a fine-tuned chain of being.

The idea of progressive change is not, of course, built into the process of evolutionary change in the way that orthogenetic theories once proposed. However much evolution might preclude widespread maladaptive changes (such as beaks less capable of using the only available food source), it does not always lead to greater complexity. Nevertheless, Darwin himself did recognize the fact of a general direction from something like lower to higher in the history of life: “From the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows” and “from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved” (Darwin, Origin of Species, 490).

Aristotle laid the foundations for such a distinction in his analysis of vegetative, animal, and human life as a matter of the addition of powers. Plants have only vegetative powers; animals add the powers of sensation, appetite (or emotion), and locomotion; man adds the powers of reason and will. Whether one thinks that, say, chimpanzees are higher than corals or not, a universal version of the common-ancestry thesis leaves the problem intact.

St. Thomas did think that his philosophy of nature could accommodate a transition from a vegetative to an animal state in the course of the embryological development of animals:

In the generation of man or of animal, there are many generations and corruptions succeeding one another reciprocally, for when a more perfect form comes the less perfect form fades away. And thus, although in the embryo there is first a vegetative soul only, when it has attained a ˙greater perfection the imperfect form is taken away, and the more perfect form takes its place, i.e., a soul which is vegetative and sentient simultaneously. (Aquinas, De Spiritualibus Creaturis, a. 3, ad 13; see also Summa theologiæ I, q. 76, a. 3, ad 3, and q. 118, a. 2, ad 2)

One does not have to agree with St. Thomas’ embryology to see in this idea further evidence that he did not think that the emergence of a higher from a lower form of life was ruled out by any principle of his philosophy of nature.

Whence does the substantial form of the offspring come? How does that transformation (at least in the case of generation from a lower form) not violate the principle of proportionate causality?

A Thomist might argue that it does, i.e., that the appearance of new species requires an act of God to produce the new substantial form, but Daniel De Haan, among others, has argued that it does not, defending what he calls “Thomist naturalism”: “It is not each individual hylomorphic substance, taken on its own in isolation from all other hylomorphic substances, that has the potentiality and actuality for educing novel substantial forms. It is the cosmos’s systems of hylomorphic substances that possess both the potentiality and actuality for the eduction of the substantial forms of all hylomorphic species.” This includes “the abiogenesis and evolution of all living organisms, including the emergence of sentient animals” (De Haan, “Nihil dat quod non habet,” 90 and 67).

The Thomistic-Aristotelian philosophy of nature does, therefore, allow the transformist history of life that is the central tenet of the theory of evolution.

Teleology and Natural Selection

The third concern is whether St. Thomas’ fundamentally teleological philosophy of nature can accommodate a history of life in which a natural selection of chance variations plays such an important role. The problem disappears when one comes to see that there are two kinds of teleology.

The first is the kind of intrinsic teleology characteristic of living organisms, the idea that structures and processes are fully understood only by reference to the good of the organism (e.g., perspiration by reference to the regulation of body temperature). This is the teleology that is a central feature of Thomistic philosophy of nature, but nothing in evolutionary origins undermines this teleological aspect of living things.

The second is the kind of extrinsic teleology characteristic of artefacts. Their structure and processes do not serve anything that can really be called the good of the artefact, though they may be understood by reference to the good of the user (e.g., the structure of keys by reference to locks and those who need them). The natural world being, at root, the product of creation (and thereby being at least analogous to an artefact), a theist might reasonably expect at least some of its features to have an extrinsic teleology, to serves some purpose intended by the Creator. St. Thomas offered an example in his argument that “the multitude and distinction of things come from God”—biological diversity as comprehensible by reference to extrinsic teleology:

The distinction and multitude of things come from the intention of the first agent, who is God. For He brought things into being in order that His goodness might be communicated to creatures, and be represented by them; and because His goodness could not be adequately represented by one creature alone, He produced many and diverse creatures, that what was wanting to one in the representation of the divine goodness might be supplied by another. For goodness, which in God is simple and uniform, is manifold and divided in creatures. Hence, the whole universe together participates the divine goodness more perfectly, and represents it better than any single creature could do. (Summa theologiæ I, q. 47, a. 1)

The insight that that answer provides is both entirely consistent with and quite independent of the evolutionary-biological answer that diversity is explained by differential pressures on an animal population in different corners of its environment.

Nothing in the Thomistic philosophy of nature, however, requires that everything that happens in nature exemplifies intrinsic teleology. Natural selection itself may in fact have a purpose (relative to creation) of keeping phylogenetic lineages going, as mentioned above, but such a purpose is not required of natural selection any more than of the tides or of the phases of the moon. Some processes might be merely by-products of features that do exist for a reason (as spandrels are by-products of arches).

Alternatives to Thomistic Evolutionism

Some Catholics have rejected the attempt to synthesize Thomism and evolutionism. The alternatives they offer fall into two camps.

On one side one finds authors like Pierre Teilhard de Chardin and, more recently, John Haught, who might be characterized as placing much greater emphasis on the dynamic features of nature than Western philosophy classically has done. Though both are generally thought of as theologians rather than philosophers, their views seem best placed here.

Teilhard, in addition to his scientific work and to his less controversial early articles arguing for the compatibility of evolution and Catholic theology, gradually developed a theology of nature (e.g., in The Phenomenon of Man) that, while loosely inspired by evolutionary ideas, is in fact more eschatological than historical in its overall orientation. The Jesuit curia forbade him to publish this work, telling him to limit himself to scientific research. When his literary executors published The Phenomenon of Man after his death, the Holy Office issued a monitum warning that, “leaving aside judgments on matters pertaining to the positive sciences, … in philosophical and theological matters [certain of Teilhard’s] works are full of such ambiguities, and indeed even grave errors, as to be at variance with [offendant] Catholic doctrine.” Although Pope St. John Paul II did take the occasion of a commemoration of his life at the Institut catholique de Paris in 1981 to praise Teilhard’s “powerful poetic insight into the deep value of nature, … keen perception of the dynamic of nature, [and] wide view of the becoming of the world” (Agostino Casaroli, Letter), Casaroli also took care to acknowledge those aspects of Teilhard’s work that made it necessary to read the books with caution; shortly thereafter L’Osservatore Romano published a communiqué reminding readers that the monitum was still in effect.

However much one must acknowledge Haught’s concern to formulate an evolutionary-theistic response to the philosophical, anti-theistic naturalism of Richard Dawkins, Daniel Dennett, and others, his work also offers a critique of more traditional theologies of nature. “A Darwinian universe looks a lot different from the world-pictures in which our religious traditions were born and nurtured,” and therefore “we cannot have exactly the same thoughts about providence after Darwin as we had before” (Haught, Deeper than Darwin, 78). Thomism, he said, “cannot adequately contextualize the discoveries of evolutionary biology, cosmology, and astrophysics” (Haught, Resting on the Future, 5) and must be rejected. He looked instead for philosophical grounding to some extent to the process philosophy of Alfred North Whitehead, but more to Darwin himself, who, Haught said, “has gifted us with an account of life whose depth beauty and pathos—when seen in the context of the larger cosmic epic of evolution—expose us afresh to the raw reality of the sacred and to a resoundingly meaningful universe” (Haught, God after Darwin, 2nd ed., 2). Theology, he wrote, has long remained hostage to a Platonic “metaphysics of the past” which he said must be replaced with a Darwinian “metaphysics of the future” (Haught, God after Darwin, 2nd ed., 90–96), interpreting the world as “a still-unfinished drama” and “‘God’ [as] less concerned with imposing a plan or design than with providing a ‘vision’ for the universe, one that allows all beings, and especially human beings, to participate in the creative process” (Haught, “God After Darwin”).

His contrasts—between “a humble, promising God whose essence is self-giving love” and a “domineering force,” between divine persuasion and coercion, between “a posture of letting the world be and […] a manipulative controlling of it,” between “crudely stamping a prefabricated blueprint onto the creation” and “promoting freedom and arousing adventure in the world [rather] than […] preserving the status quo or establishing impeccable design” (Haught, Deeper than Darwin, 79–81 and 83)—will strike many more traditional theologians as a false (or at least a tendentiously formulated) dichotomy. His focus on the future leads him to ignore, if not explicitly to deny, the original sin (peccatum originale originans) in favor of an historically ungrounded universal sinfulness (peccatum originale originatum), leaving one to wonder what the point of the adjective “original” might be.

In the other camp (e.g., Michael Chaberek, Aquinas on Evolution), one finds an anti-evolutionism which places, say its critics, much less faith in natural science than did St. Thomas and which adheres too closely to ideas that are peripheral to the central tenets of St. Thomas’ philosophy of nature. Central to this alternative are the ideas of successive creation and intelligent design. Mariusz Tabaczek (“Evolution and Creation”) has offered a Thomistic critique of these ideas.

IV. The History of Catholic Evolutionism

In light of that enduring myth of contemporary secularism, the idea that the defining note of the relationship between science and religion has been conflict, if not outright warfare, it is perhaps worth reviewing the history of Catholic evolutionism—the synthesis of Christian naturalism (extended from the functional to the formational economy of the natural world, in particular to the origin of biological species), an exceptionalist anthropology, and a synthesis of evolution and creation in anthropogenesis—both by identifying its most important proponents and by reviewing magisterial reactions to and pronouncements on the new ideas as they emerged. (For a detailed history, see Kemp, Origins of Catholic Evolutionism, and Mariano Artigas, et al., Negotiating Darwin.)

Some parts of the synthesis—creationism about human souls, human exceptionalism, and arguably also monogenesis—are tenets without which the synthesis would not be Catholic evolutionism. With respect to the other parts—the salience of natural selection, the comprehensiveness of common ancestry, and some details of anthropogenesis—Catholics could, and did, choose among a range of options, based on three facts.

First, the theses are separable. Transformism does not logically imply either common ancestry or natural selection, and those two are themselves logically independent of one another. In addition, an account of the origin of plant and animal species did not require its extension to human origins.

Second, common ancestry and natural selection come in more robust and more modest versions (distinguished above).

Third, acceptance of each thesis can (and Catholic evaluations of each did) range across an epistemological spectrum: contrary to doctrine (and therefore false); presumptively contrary; compatible but false for philosophical or scientific reasons; plausible but not definitively established; definitively established (and therefore true).

Scientists and Theologians

Catholic theology of nature has long incorporated some version of Christian naturalism. The idea of laws of nature functioning as secondary causes not just in the operational, but in the formationaleconomy of the world (though not, of course, in its creation), however, was raised by René Descartes in his Discourses in 1637. It first applied in detail sufficient to recognize as a scientific hypotheses by Bl. Niels Stensen in his Forerunner of a Dissertation on a Solid Naturally Contained within a Solid in 1669. The first Catholic application of that idea to the emergent paleontological question of the origin of biological species (i.e., to fossil evidence of the appearance in more recent strata of species not found in earlier strata, “faunal succession”) was made by Jean-Baptiste d’Omalius d’Halloy in 1831.

Catholic evolutionism found both defenders and critics in the pages of the Catholic press, in its encyclopedias, and in its textbooks. The synthesis advanced along two fronts.

First, Catholics made contributions to the new scientific theory. Particular mention should be made of Albert Gaudry (1827–1908), generally recognized as the father of evolutionary paleontology, and Erich Wasmann, SJ, (1859–1931), who made significant contributions to our understanding of the evolution of ants. Those contributions were themselves in no way distinctively Catholic, but both authors, in addition to providing a kind of living witness to, also offered arguments for, the general compatibility of evolutionary biology with Catholic doctrine.

More important to this encyclopedia than the scientific contributions made by Catholics are those explicit defenses of compatibility, both in general and with specific regard to anthropogenesis. They constitute a second front.

Catholics were not deeply concerned about the theological acceptability of the idea of an evolutionary origin for plants and animals. Earliest Catholic reaction even to the publication of Darwin’sOrigin of Species focused on possible application of the theory to man, commenting with amusement on the fact that “science,” which had a few years before had had difficulty with the idea of common ancestry for the Chippewa, Hottentot, Chinese, and English now proposed that those same Chippewa and Englishmen shared a common ancestry with plants and animals.

The earliest attempts to extend the theory of evolution to human origins came with de Filippi’s 1864 lecture “On Man and Apes,” in which he argued that the evolutionary origin of the human body is compatible with human exceptionalism. This was followed by Mivart’s Genesis of Species in 1871, the first explicit articulation of the idea of the infusion of a created soul into a human body.

Twenty years later, Zeferino Cardinal González, by then retired primate of Spain, pointed out that all the evidence citable in favor of Mivart’s view could be accounted for by a more modest alternative according to which God formed the first human body not by directly shaping non-living matter (“the slime of the earth”) but by modifying an evolved animal body. That alternative, he thought, better respected Biblical concerns about divine involvement in anthropogenesis (González, Biblia y ciencia, 1: 514–15).

Although some Catholics (e.g., John Zahm, Evolution and Dogma, 369–70) thought that no divine supplement to the work of evolutionary processes was necessary, others (e.g., Robert de Sinéty, “Transformisme.” 1837–47) were content with the mixed account. Science, after all, could hardly provethat God had not put any finishing touches on the animal-human body before infusing into it a rational soul.

The Official Church

For many years the offices of the universal Church chose not to issue any doctrinal commentary on the question of evolution.

A provincial council held at Cologne in 1860 had declared “clearly opposed to Sacred Scripture and to faith” the idea that “man, even considering only his body, was brought forth by the spontaneous change of a less perfect nature into a more perfect one in a way that is continuous and culminates in a human nature.” Although Catholic anti-evolutionists sometimes cite that statement as an authoritative endorsement of their view, it is not clear that it condemns the Catholic evolutionism that emerged a few years after the council met (as opposed to the more problematic pre-Darwinian views at which it was aimed). Its authority is in any case limited. The issue received little notice at the Vatican Council held ten years later.

The Holy Office declined to address the question of evolution on several occasions when it could have done so.

The Pontifical Biblical Commission, in its statement on the historical character of the first three chapters of Genesis, included among statements with respect to which “the literal historical sense was not to be called into doubt” both “the distinctive [peculiaris] creation of man and the formation of the first woman from the first man.” Catholic evolutionists argued that creation of the soul would make the creation of man distinctive even if the body which it informed was the product of evolutionary processes; at least some Catholic evolutionists who defended the evolutionary origin of Adam’s body were explicitly willing to accept direct divine action in the formation of Eve, an issue not raised in later magisterial statements on evolution. (On theological views of the origin of Eve, see Thomas Motherway,“The Creation of Eve in Catholic Tradition.”)

In 1948, in response to a question about the 1909 decree that Emmanuel Cardinal Suhard, then archbishop of Paris, had sent to Pope Pius XII, the Commission replied that, its earlier responsa not having precluded a later, more scientific treatment of the problem of the literary character of the first chapters of Genesis on the basis of recent research, there was no need for them to issue a new statement at that time. They added that the problem was “obscure and complex”: “These literary forms do not fit into any of our classical categories and cannot be judged by reference to Greco-Latin or modern literary genres. One can neither deny nor affirm their historicity en bloc without wrongly applying the standards of a literary genre under which they cannot be classified” (Pontifical Biblical Commission, Letter, 46–47).

Two particularly materialist (and highly speculative) accounts of biological evolution had been placed on the Church’s Index of Prohibited Books in the century before the publication of Darwin’s Origin of Species. Explicit official condemnation of such books continued into the early twentieth century when they were drawn to the attention of the Index or of the Holy Office. In addition, the Index prohibited books that addressed the evolution question by drawing what it thought was too sharp a line between various parts of the Bible (as, in their judgment, Rafael Caverni had done in his Nuovi studi della filosofia).

Neither Mivart’s book nor González’ was ever delated to, and so was never reviewed by, the Congregation of the Index. Under Pope Leo XIII, the Index instructed two authors (Leroy and Zahm) to withdraw from circulation books in which they had defended something like Mivart’s view, but the books were never listed on the Index itself. When the issue came back for review (by the Censorship Section of the Holy Office, which had by then replaced the Congregation of the Index) in the 1930s, the book in question being Messenger’s Evolution and Theology, Pope Pius XI requested a review of the scientific state of the question; in the end no action was taken against the book.

The first magisterial statement on the question was in Pope Pius XII’s encyclical Humani generis in 1950—a provisional acceptance of the orthodoxy of the evolutionary origin of Adam’s body and a firm (if not unqualifiedly definitive) rejection of polygenism (as detailed above).

Pope St. John Paul II also addressed the question, though in statements less authoritative than is an encyclical, on two occasions in particular. The first was in his General Audience of April 16, 1986, where he said:

from the point of view of doctrine, there is no apparent difficulty in explaining the origin of man, as far as concerns the body, by the hypothesis of evolutionism. However, it must be added that that hypothesis puts forward only a probability, not a scientific certainty. Doctrine, on the other hand, invariably affirms that man’s spiritual soul is directly created by God. That is, it is possible, according to the hypothesis just mentioned, that the human body, as a result of the order impressed by the Creator in the powers of life, was gradually prepared in the forms of antecedent living beings. The human soul, however, on which man’s humanity ultimately depends, being spiritual, cannot have emerged from matter. (John Paul II, “Created Things Have a Legitimate Autonomy,” 216–20)

The second was in an address to the Pontifical Academy of Sciences in 1996, in which two points of comparison with Humani generis are worth noting. First, his comment that “new knowledge has led to the recognition of the theory of evolution as more than a hypothesis” (John Paul II, Message, no. 4), was less provisional about acceptance than Pope Pius XII’s had been. Second, he chose not to mention the issue of monogenesis.

The Catechism of the Catholic Church , while it does not address the question of evolution directly, suggests in two ways that the theory is not per se problematic. First, by what it says: “The question about the origins of the world and of man has been the object of many scientific studies which have splendidly enriched our knowledge of the age and dimensions of the cosmos, the development of life-forms and the appearance of man” (CCC, no. 283). Second, by what it does not say: Two paragraphs later it notes that “since the beginning the Christian faith has been challenged by responses to the question of origins that differ from its own” and it goes on to list some, ranging from pantheism to emanationism to materialism (CCC, no. 285). The theory of evolution is not on that list.

The Catechism does not address the question of polygenism directly but its presumption of monogenism and of the historicity of Adam can be found in two passages: “Because of its common origin the human race forms a unity, for ‘from one ancestor [God] made all nations to inhabit the whole earth’” (CCC, no. 360, quoting Acts 17:26) and “By his sin Adam, as the first man, lost the original holiness and justice he had received from God, not only for himself but for all human beings (CCC, no. 416).