I. Problem and Thesis: Is the evolutionary origin of new species possible on Aristotelian principles of nature? My thesis is that it is possible and reasonable, given the Aristotelian notions of chance (including substantial generation by chance) and instrumental causality in animal reproduction.

II. First Objection to Thesis: For Aristotelian natural philosophy nature is prior to chance but evolution requires that chance be prior to nature.

III. Second Objection to Thesis: On Aristotelian grounds new species would be without an adequate efficient and formal cause, the identical-in-kind substantial form of the male, and so would in fact come into being ex nihilo, which is impossible.

IV. Explanation and Defense of the Thesis, Step 1--Chance and substantial generation by chance:

A.] Chance events are not uncaused but are events in which two natures not of themselves specifically ordered to interact do so in this or that case. Nature is therefore is ontologically prior to chance--there can be no chance events where there are no natures.

B.] So that not just anything comes to be by chance from just anything, there must be a proportionality of active and receptive powers in the natures interacting in the chance event, which proportionality of natures arises not from the specific formalities of the natures but from more general and generic characteristics and formalities they have. [IV.A&B answer first objection]

C.] Thus there can be substantial generation by chance, in which the off-spring is different in kind from the parent, without violation of the requirement of sufficient formal causality in the agent.

V. Explanation and Defense of the Thesis, Step 2--Instrumental Causality, and Sperm and Ovum as that by which a form different in kind from the parents is generated:

A.] An instrument contributes in the order of efficient and formal causality to the production of the effect, and it does so according to its own formalities and powers, but the formalities and powers of the instrument are themselves inadequate to produce the effect.

B.] Although Aristotle had no scientific basis for asserting that semen could have formal powers and potencies not directly derived from the male, we now know that via chance mutations there can exist significant formal differences between the form of the parent and the form potential in the semen or ovum as instruments of reproduction. The formality of sperm and egg as instruments are sufficient to produce by substantial generation by chance an off-spring different in kind from the parents. [V.A&B answer second objection]

VI. Considering evolving generations by analogies from motion and substantial change, since there is not a change in biological species in one generation:

A.] Intermediate generations during evolutionary processes can be thought of as imperfect participations in the posterior and subsequent species.

B.] The evolutionary course from subsequent to new species is analogous to the alterations that an individual material thing undergoes prior to substantial change.

VII. The Aristotelians' failure to consider that animal species do evolve arose from at least three factual lacunae, which have only been filled in the last half-century:

A.] The complex bio-chemical properties of the sperm and ovum

B.] The convergent evolution making biological evolution a practical certainty

C.] The explanation of plate tectonics which accounts for a vastly changing terrestrial environment.

VIII. Conclusion: Aristotelian natural philosophy can accommodate the facts of a stable ecosystem in which no new species arise or one in which new species arise by evolution.

One of the great contributions of Aristotelian thought over that of others, according to leading Aristotelians like Albert the Great and Thomas Aquinas, was that it could account for the regularly ordered substantial changes of the world with natural principles. But if we accept as true the evolutionary origin of new species, then we must ask whether Aristotelian natural philosophy can account for such substantial change as the origins of new species, or whether evolutionary fact has proven Aristotelian theory wrong. Is the evolutionary origin of new species possible on Aristotelian principles of nature? My thesis is that it is possible and reasonable. I shall build my argument on the notions of instrumental causality and chance events, especially what Thomas Aquinas calls substantial generation by chance. These two notions allow, on Aristotelian grounds, the kind of variation between parent and off-spring required for evolutionary change. I shall then suggest analogies drawn from motion and substantial change to help our understanding of the progression of species through intermediate forms to a new species. Throughout, for ease of presentation, I will refer to the origin of new animal species. My conclusion is that Aristotelian natural philosophy is open to a world of evolving biological species or to a stable world in which species have not evolved, although it seems to me that a world of evolving species is more what one would expect on the principles of an Aristotelian natural philosophy.

Let me propose two initial objections, that is, two arguments that an Aristotelian in natural philosophy cannot accept the natural, evolutionary origin of new species. The first argument is based on Physics II.8, in which Aristotle rejects non-teleological, mechanical accounts of substantial change. Organic development is either for the sake of something or by chance; it is not by chance, since chance outcomes are irregular and organic outcomes are regular; therefore, organic development is for the sake of something and not by chance. But it is a principle of the contemporary evolutionary explanation that new species arise by chance and not by nature: they are not aimed at by the natures of previous species. Although Aristotle obviously allows for chance events, the point of this first argument is that for Aristotelian natural philosophy nature is prior to chance but evolution requires that chance be prior to nature.

The second objection or argument is this. Aristotelian natural philosophy maintains that in animal reproduction and substantial change the offspring's new substantial form neither comes to be ex nihilo nor is induced ab extra by an immaterial cause; rather, new substantial form is educed from the potencies of matter by efficient causes, prior in being to the effect, which have adequate formal reality to produce that effect. In works of human art, the adequate formal reality pre-exists in the efficient cause as the skill of the artist, who first conceived the end product in his mind according to his art. In natural biological reproduction, the adequate formal reality pre-exists as the substantial form of the parents. Specifically--and ancient and medieval Aristotelians said this on account of their imperfect chemical and biological knowledge--the substantial form of the male is the efficient cause whose formal reality is adequate to produce an effect of the same kind. The male's substantial form accounts for the substantial form of the offspring. Therefore, it would seem impossible for the substantial forms of subsequent generations to differ from the substantial forms of previous generations, because the principle of sufficient causality requires that the substantial forms of each generation be the same. New species are impossible on Aristotelian grounds because new species would be without an adequate efficient and formal cause, the identical-in-kind substantial form of the male, and so would in fact come into being ex nihilo, which is impossible. Thus runs the second argument.

Let us turn to an Aristotelian answer to the first objection by examining in Aristotelian natural philosophy the notion of chance interactions of natural bodies. The physical world is not an abstract, mathematical world of a priori necessary connections. It is a dynamic world of natures interacting in an environment, so that chance is a necessary corollary of nature. Chance is not a denial of nature and causality but arises immediately from them. The active powers and receptivities of natures are ordered to one another in an environment. Chance events occur when two substances not ordered by their natures to interact do so under the particular circumstances of the prevailing environment. Because individual chance events are singular events not part of the proper ordering of nature, they are generally unpredictable, but they are often causally knowable in retrospect, by reasoning that Aristotelians call "suppositional." It is important to emphasize that by "chance" the Aristotelian does not mean "uncaused," as much current use of the word "chance" would have it. Chance events are ones in which two natures not of themselves specifically ordered to interact happen to do so in this or that case. The cause of the outcome which has arisen by chance is the joint operation of the two natures which happened to interact.

Chance events, then, depend on natures. Nature is ontologically prior to chance, and this conclusion is as true for an evolutionary world as a world of stable species. For, chance events are not so completely random that any two natures can interact in any chance way to produce any outcome whatsoever. There must be some correlation and proportionality of active and receptive powers of the two natural things which interact. If chance is the interaction of two natures not specifically ordered to one another, then there must be natures in order for there to be chance.

The correlation and proportionality of natures interacting by chance cannot lie at the level of the specific character of the natures. The requisite proportionality of active and receptive powers in the interacting natures arises not from the specific formalities of the natures but from more general and generic characteristics and formalities they have. Thus cosmic rays mutate a gene not qua gene but qua the gene's more generic molecular properties. It is not the teleology of a gene to be mutated but to be stable. But because of the kind of chemical bonds, , etc., that a gene has, it can be mutated when acted on by ultraviolet radiation, for example. On account of the specific formalities and character of natures, they are stable and tend to reproduce an offspring of the same kind. But because of the generic formalities, characteristics, and potencies things have, they are open to chance, and to a substantial difference-in-kind between parent and offspring.

To explain this last point, Thomas Aquinas distinguishes substantial changes which happen by nature from those which happen by chance. [Thomas, In 7 Metaph L6, C:1382] Substantial changes by nature are the regular, ordered changes of "like producing like." Substantial changes by chance fall outside the normal, regular orderings of natures, but still they are not completely random. They are caused by the active and passive potencies of the agent and patient, and as we shall see, their instruments. Thomas uses the case of the horse and ass producing the mule as an example of one kind of substantial change by chance. He says,

... man begets man, and similarly a horse begets a horse, and each natural thing produces something similar to itself in species, unless something beyond nature <i.e. by chance> happens to result, as when a horse begets a mule. And this generation is beyond nature, because it is outside of the aim of a particular nature ... since in the generation of a mule the sperm of a horse cannot induce the form of a horse in the matter, because it is not adapted to receive the form of a horse, it therefore induces a related form. Hence in the generation of a mule the generator is similar in a way to the thing generated; for there is a proximate genus, which lacks a name, common to horse and to ass; and mule is also contained under that genus. Hence in reference to that genus it can be said that like generates like; for example, if we might say that that proximate genus is "beast of burden," we could say that, even though a horse does not generate a horse but a mule, still a beast of burden generates a beast of burden. [In 7 Metaph L7, CC 1432-1433]

Agents sometimes interact in an environment with other natures with which they share no ordered teleology. From those chance interactions an important difference in substantial form can arise between parent and offspring.

Someone might acknowledge that Aristotelianism can account for monsters without thereby acknowledging that it can account for the evolution of new species. Let us turn, therefore, to a consideration of the instrumental causality of the semen and menstrual material in reproduction.

The male as efficient cause does not act immediately on the female's menstrual matter but acts through an instrument, the semen. The semen, by virtue of its own chemical and elemental properties and powers, which it has from the male, educes the new form of the offspring from the potencies of the female's menstrual material. To avoid any preformationism, which would hold that the offspring's substantial form exists somehow in re in the semen itself and is induced into the maternal matter, Aristotelians emphasize the semen's nature as the male's instrument in reproduction.

An instrument has its own powers and nature different from that of the primary efficient cause which employs it, and it operates according to its own form in producing the effect. Thomas Aquinas emphasizes that what makes an instrument an instrument is, on the one hand, that it has a nature of its own by virtue of which it acts in production, but, on the other hand, that the effect produced has a formal reality beyond the limits of the nature of the instrument itself. The instrument contributes in the order of efficient and formal causality to the production of the effect, and it does so according to its own formalities and powers, but the formalities and powers of the instrument are themselves inadequate to produce the effect. Semen, Aristotle speculates, has a chemical constitution of living heat and pneuma, formalities by virtue of which it acts in the order of efficient and formal causality in animal reproduction. But what semen does as an instrument acting on menstrual matter is not animal reproduction itself; it is only a part of the process of animal reproduction. The chisel's striking the marble is effective efficiently and formally in causing the statue to come to be, but chisel striking marble is only part of the entire artistic productive process. Similarly, semen's educing form from menstrual matter is only part of the process of animal reproduction resulting in healthy offspring. Semen acts instrumentally in reproduction, that is, it acts through its own proper powers, formalities, and nature to achieve an effect beyond the limits and powers of its own nature, viz. the reproduction of healthy offspring.

We have said that an instrument, to be an instrument, must have its own formality through which effects are brought about. The formality of the instrument itself makes a great difference to the outcome of efficient causality. Aristotle and Thomas understood little of the true complex formal structure of semen and ova, speaking only of menstrual materials suited or not to receive the form of the male parent. Only in the last half-century, through the study of DNA, have we begun to understand the biochemical formalities of the instruments of animal reproduction. Although Aristotle had no scientific basis for asserting that semen could have formal powers and potencies not directly derived from the male, we now know that via chance mutations there can exist significant formal differences between the form of the parent and the form potential in the semen or ovum as instruments of reproduction. Mutations can arise from chance encounters with x-rays, for example, or in ways we do not yet fully understand in the process of bio-chemical union of sperm and egg. The point to emphasize is that these chance mutations in the genetic packages of the instruments of reproduction, and so differences in kind between the parents and offspring, are not alien to an Aristotelian natural philosophy. One reason Aristotelians of early centuries did not recognize the dynamism of biological change was that they did not understand the complex formality of the instruments of reproduction, sperm and egg. But this error was only factual; the Aristotelian notion of nature, chance, and efficient and instrumental causality can accommodate the new facts without falsification of the underlying principles of Aristotelian natural philosophy.

Thus, it seems to me, the second objection of a lack of sufficient formal causality is answered. Let us now try to draw some comparisons between motion and evolving that will make the latter set of processes clearer.

The difference in kind between parent and offspring, which arises by substantial generation by chance, does distinguish the substantial form of the parent and offspring, but it does not constitute a new species in the biological meaning of the term "species." Biology asks no natural philosophy to account for so radical a change in one generation. The origin of a new biological species from a progenitor species is more gradual, over many generations. Some have asserted that we should envision such a process mechanistically, as the accumulation of many new attributes which in the end add up to a new species. I think an Aristotelian natural philosophy can accommodate the facts more reasonably than that, however.

I suggest that an aspect of an Aristotelian understanding of substantial change can be applied analogously to the problem of the origin of species through evolution. The mutations essential to evolutionary change are the result of chance interactions of parent and instrument--the genetic package of sperm and egg--with bodies in the environment. When the environment changes over time, some of the changes arising from mutations can have adaptive value. These mutations, being changes of attributes, can be expressions of a substantial form different from that of the parent generation, since attributes depend for their existence on substantial form. Differences in significant properties are caused by substantial differences. However, these different substances are not at first a new biological species; they are intermediate and imperfect participations in the posterior and subsequent species. These intermediate forms are analogous to--not the same as but analogous to--the alterations that an individual material thing undergoes preparatory to substantial change. In the end there may arise offspring substantially different in kind from the original species, a generation which is the first generation of a new species. This first generation of a truly new species is related to the immediately preceding generation as perfect to imperfect participation in the new substantial form of the subsequent species. Such a new species has come about not by infusion of form from without, but by numerous substantial changes by nature and substantial changes by chance taking place over time in a complicated and dynamic environment.

My position is, then, that Aristotelian natural philosophy, especially with its understanding of chance and of instrumental causality, allows for the random mutational variations contemporary biology has discovered. The understanding of substantial generation by chance described by Thomas Aquinas maintains the Aristotelian priority of nature in events of the world and corrects misunderstandings of the notions of "random" and "chance" that crop up at times among contemporary philosophers and biologists. The variations are random with respect to the specific teleologies of the beings which interact, but the variations which do occur still depend on the generic natures of those beings. The analogical application of Aristotelian notions of the imperfect participation in a form by intermediate states and of alterations as preparatory to substantial changes can help us to conceive better the course of evolutionary change from one species through intermediate generations to a new species. It seems to me there is no reason in principle that an Aristotelian natural philosopher could not hold for the evolutionary origin of new species, except of course for the special case of the origin of the rational human species. Furthermore, my account should not be taken as abrogating Physics VIII and the need for immaterial intelligences at work in the evolving world.

If there are no theoretical impediments in Aristotelian natural philosophy to an evolutionary world view, then the reasons for past Aristotelians' failure to see the biological world's evolutionary character must have arisen from their errors of scientific fact. I see at least three. As I argued above, Aristotle lacked the scientific knowledge that has emerged, through the study of DNA, of the complex formality of the instruments of reproduction, sperm and egg, which would allow him to recognize the variety of sources of chance variation in animal reproduction. Second, Aristotle lacked the convergent evidence from comparative anatomy, embryology, biochemical analysis of enzymes and blood, taxonomy, fossil records, and the like, that has become available in the twentieth century and which makes it unreasonable to hold any but an evolutionary view in the science of biology. Third, as Benedict Ashley has pointed out, for many, many centuries Aristotelians falsely held that the terrestrial environment is unchanging, and an unchanging environment means no general evolutionary change. I think that it was only in the 1960's, with the explanation of plate tectonics, that a compelling causal understanding of our changing environment has been available.

Aristotelian natural philosophy can accommodate the facts of a world in balance in which no new species arise or of an evolving biosphere in which new species arise; it was more reasonable for Aristotelian philosophers like Aristotle and Thomas Aquinas, given the scientific information available, to conclude that the natural world does not evolve. It is more reasonable for natural philosophers today to hold and understand the evolutionary origins of new species with Aristotelian natural principles.