Dr. John Nieto: “On the Definitions of Evolution”

A Tutor Talk by Dr. John Nieto

Note: Periodically members of the Thomas Aquinas College teaching faculty or chaplaincy present informal lectures, followed by question-and-answer sessions, on campus. These late-afternoon gatherings afford an opportunity for speakers to discuss topics of great interest to them and to share their thoughts with other members of the community. Dr. John Nieto delivered the following talk on September 30, 2015:

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Ave Sedes Sapientiae

1. What we commonly call evolution has been the object of tremendous investigation and debate on the part of scientists, philosophers, and others, learned and unlearned alike, for well over a century. Yet, to my knowledge, these thinkers have spent little of this effort upon definitions of evolution. Such definitions are clearly at work, since many propose to prove the fact of evolution, its likelihood or improbability or its necessity or its impossibility. Clearly all name by evolution at least a definite kind of generation or coming to be, especially in living substances. Still, these thinkers leave unclear both the character of this generation and its real principles, according to their kind.

2. I propose in these remarks to offer several definitions of evolution and various elements involved in evolution. I will do so while making distinctions in the kinds of definition. I will not offer every kind of definition I think possible or necessary. Some belong to other disciplines that I am not sufficiently master of, though I may even here refer to a definition or quasi-definition as an example. None of these remarks propose to establish whether evolution has or has not occurred, much less the certainty possible in such questions. Rather, as Socrates proposed to establish what virtue is before determining whether someone can teach virtue, so I propose to state what sort of generation we name by the word ‘evolution’ and the nature necessary to the principles that would bring about such a generation.

3. Furthermore, the philosophical definition I offer assumes various natural principles implied by Aristotle’s understanding of nature and worked out by him and Saint Thomas Aquinas. The ability or inability of other philosophical schools to propose a definition of evolution is not my present concern. Yet the present remarks suggest to me that Aristotle’s ability to grasp and define movement in a manner superior to other philosophical efforts make his thought uniquely able to consider the notion as well as the real possibility of the sort of generation we now call evolution.

4. Let me begin by distinguishing a ‘real’ from a ‘nominal’ definition. First, this is not a distinction between the definition of a word or name and the definition of the thing named. We do call the nominal definition the significatio nominis or ‘meaning of the name’. But this does not imply that the name’s signification or meaning does not define the thing. It merely determines the thing sufficiently to account for having assigned a distinct word to name the thing, perhaps merely a chimera. So the ‘phoenix’ is a ‘bird that dies every five hundred years and rises from its ashes’.

5. In the strictest sense, the real definition proposes an account of something from the principles that give rise to its existence in reality. A nominal definition does not go so far. Yet a nominal definition need not belong to something imaginary or doubtful. Nor do they fail to touch upon the real principles of the things defined. The definitions by which we use names correctly without a scientific or philosophical resolution to its principles can be called a nominal definition.

6. So the understanding of the soul as ‘that by which the living live’ is a nominal definition. Aristotle uses this definition to draw a real definition of soul, not through its principles but as a certain kind of principle to living substances. Similarly, Euclid’s definition of equilateral triangle, on this understanding, is a nominal definition. He then demonstrates the dependence of the equilateral triangle upon two equal circles sharing a common radius. This allows us to gather a more perfect, real definition that accounts for the equilateral triangle through its real, quantitative principles: a rectilinear figure whose base is the common radius of equal circles and whose sides are radii drawn to their common point of intersection.

7. Further, one must recognize that sometimes things have a common nominal definition that cannot come under a common real definition. I will give several examples that I believe will illustrate this principle, though I do not insist upon each of them. First, logic clearly proposes that number and the continuum fall univocally under the genus of quantity or ‘so much’. But, if there is no universal science of mathematics, but the science immediately divides into arithmetic and geometry, this results from the inability to bring these two ‘species’ of quantity under one kind of ‘real’ definition. We cannot define them by principles of the same kind. Number resolves to the indivisible; the continuum to the ever-divisible.

8. Second, the word ‘energy’ in modern physics brings many things under one name sufficient for certain mathematical equations: heat, electricity, magnetism, local motion, and so on. But, even should we suppose all these forms of energy involve one kind of locomotion or another, we cannot give these definitions that resolve to immediate experience through principles of the same order. Heat arises through local motion of a body’s molecules. The local motion of a whole body arises in relation to the body’s weight or mass. Again, the name energy includes potential as well as kinetic energy. But, though a quantitative equality arises from the proportion of one to the other, these cannot be resolved to the same real principles, should one attempt to define them by real intrinsic principles, such as movement and the disposition to movement.

9. Here I propose to give a nominal definition of evolution as well as several ‘real’ definitions. I do not, however, understand the real definitions to determine the reality of evolution, but merely the kind of reality implied in our understanding of evolution. This will, perhaps, exclude some understandings of evolution and it may imply the ‘possibility’ of evolution, though, again, this is not my immediate purpose. All I propose is that the ‘real’ definitions resolve such a coming to be to principle known to natural philosophy or the empirical science of biology. Further, I would insist that evolution might admit of one nominal definition, though it cannot come under one real definition.

10. Note also here that I am speaking principally of the evolution of living things. What I say will bear, both implicitly and explicitly, upon the ‘evolution’ of inanimate substances, especially the elements. But my principal concern here is with our usual use of the word ‘evolution’, to distinguish a certain kind of generation or coming to be in living substances.

11. Let me begin by distinguishing what I understand to be the first meaning of the word from many other, closely-related meanings. I will do this by pointing out that these other meanings imply the meaning I speak of as ‘first’. In fact, I do not think this order in the meanings of the word to be controversial. But there would certainly be significant disagreement were one to ask which of the various things we call evolution is first in reality. Though I do not intend to address this second question directly, I do hope my distinction of several meanings of the work will illustrate the distinction I am making, that between the order in the meanings of the word and the order between the realities named by the word.

12. I have already proposed that evolution first names some kind of coming to be or generation. This use of ‘evolution’ names the coming to be of the member or members of a species that has not yet existed from something other in species. Note also that such a generation belongs, according to this meaning of the word, not only to the individual generated but also to the species that has not previously existed.

13. One might wish to clarify this definition by stating that evolution only occurs when the species that comes to be is capable of continued existence. Such a continuation might depend upon the stability of something’s intrinsic principles or its conformity to the environment in which it comes to be. The presence of this aspect in the nominal definition becomes clear, especially in living substances, insofar as people readily distinguish mere mutations or monsters, or what was once called a ‘sport’, from an evolution. The one cannot establish a new species for one reason or another, while evolution occurs when individuals have come to be sufficient to establish offspring for so many generations—the number is ultimately relative to the perspective—and in circumstances, actual or possible, favorable to such offspring.

14. I would also add that the ordinary use of evolution limits these generations, at least implicitly, to those arising from natural causes as such. On this account, evolution is something done by nature. Some even hold that natural substances necessarily ‘evolve’. This notion does not necessarily exclude some corresponding human or even divine action. It certainly does not exclude chance interactions among merely natural substances.

15. I believe this understanding of evolution does exclude the use of mind to exploit powers not immediately available to the natural beings that generate some member of another species. One example would be the combination of genetic material from two plants or animals unable to unite such material through the reproductive organs they actually possess. I realize that mind can intervene in natural operations in many different ways and I do not intend to attempt to distinguish all of these or to put them in order. I will state later the manner in which I believe mind, at least the divine mind, does not ‘interfere’ with natural operations but is in fact necessary for natural operations. Such an involvement of mind, if necessary in the manner I will suggest, obviously does not detract from the ‘naturalness’ of the generation of natural substances.

16. I wish briefly to point out other meanings of the word ‘evolution’ that I consider ‘posterior’ to the one I have proposed. Most obvious is the use of the word in two or three ways to name the series of species that have come to be by evolution or the principles and causes that bring these species into being. We speak, for example, of evolution as the ordered array of species from some simple original life form to man or even from hydrogen to man. The order may be merely historical and happenstance or it may imply that the higher things are somehow implicit in the lower.

17. Again, the word sometimes signifies the powers in natural substances that bring about new species. We say that evolution has introduced certain organs, that it has changed this animal or has adapted it to a certain environment. Sometimes the word signifies this power to ‘evolve’ natural substances in a manner abstracted from present limitations. According to this use, ‘evolution’ almost has the character of a god, something more purposeful and determined than ‘Mother Nature’ but of the same order.

18. One more relevant use of the word evolution depends upon two already mentioned, namely, the particular generation of a new species or a series of species that have come about in this way. In this use we speak of the evolution of a particular organ or behavior, especially as this might distinguish one species from another. So we speak of the evolution of an organ sensitive to light or how such a cell might become what is properly called an eye. Again, one might name a whole series of conditions that an organ might have taken on as various species have evolved: the evolution of the eye or the brain, the evolution of DNA; the evolution of the genome.

19. I consider all these uses of the word to be posterior to its use in naming a particular generation in which the member a species comes to be from a being of another species, in such a manner as to constitute some principle by which the species might multiply and thrive. The other uses either name things that have come about by such generations, the powers that effect such generations, or the form or expression of that generation in some part of the animal or its actions. These other uses may well name a reality that is prior to evolution in the sense in which I intend to define it, but this use of the name allows it to name those realities.

20. Now this definition implies several things often ignored by those most interested in furthering the ‘theory’ of evolution. The most important of these is the real distinction of species. Many, among those who support evolution and those who oppose it, suppose that evolution demands that there is no real distinction of one species from another. The distinctions we make in species, on this position, correspond only to some distinction in our language or thought. Even Darwin himself seems to encourage this philosophical tenet.

21. Here I will defend the real distinction of species only as it is necessary to the notion of evolution. I leave aside at present the evidence for such distinction found in philosophical considerations as well as the manner in which particular conceptions of species used by some evolutionists imply a real distinction of species. I propose that precisely what some see as marvelous in the notion of evolution and what others think to be beyond the power of nature is precisely that a being of one species should be the real origin to a being of another species.

22. There are certainly those who propose that immediate offspring are always so similar to their parents that they cannot constitute another species. Darwin himself seems to have held this. But such a generation is not sufficient in their understanding to constitute an ‘evolution’. Rather, they admit, evolution appears only to those who look at many generations, when the differences are so great that the individuals cannot constitute a single species. Note here, that the position admits that each generation agrees in species with the previous, though the first and the last we are concerned with do not agree in species. I am not arguing merely with someone who states that we cannot tell in which generation the difference in species arose.

23. Many things might be said against the position stated, namely that each generation agrees in species with the previous, though the first and the last we are concerned with do not agree in species. I will point out only one. For those who stand by the principle of contradiction, the agreement in species and the lack of such agreement cannot be taken in the same respect. Most often, in my experience, those arguing in this way state that species is an imposition of our mind. They usually understand intellect to demand some likeness for its own purposes of sorting things into sets. In fact, this position holds, the differences e use to distinguish species are no ‘deeper’ than the differences we use to distinguish varieties of a species, though there may be more of them or they may be greater in degree.

24. I will challenge only one aspect of this position. I hold, against this position, that if evolution exists, it is more wonderful than they would propose. They hold that what evolution does in bringing about what we call new species only differs from what happens in the daily generation of offspring in degree, in the mere accumulation of differences. If this is so, the evolution of a species is no more real than the rising of the sun. It is a manner of speaking that we must understand to be reduced to some other reality, which may or may not be worthy of wonder. On this view, the fact that monkeys bear monkeys may be more wonderful than the fact that a ‘monkey’ might beget an offspring that begets a man. Again, what might be more wonderful on this position is that material things, especially the living, interact with one another ‘according to kind’. Or perhaps most wonderful is that men experience the world as if it contains kinds. I leave these questions behind.

25. To sum up, I am proposing that evolution names an act of generation by which the member of one species brings into being one or more members of a species that has not previously existed. I insist that the distinction in species that defines evolution cannot be merely apparent. If the name ‘evolution’ distinguishes one kind of generation from other, the distinction of species must be real.

26. Again, on this view, evolution is a phenomenon over and above the mere variation that occurs in a population over time. The rise and fall of the races of men, the ethnic variations that change through innumerable causes, certainly deserve the name ‘evolution’ in a weaker sense of the word than the one I presently attempt to define. I do not deny that these racial differences are real. I do not deny that they come to be and pass away over time through various acts of reproduction. What I deny is that they constitute something one through itself—a per se unum—in the category of substance or even in any category of accident.

27. The history of generations of this order, the history of generations that constitute populations differing only according to some identifiable collection of accidents, cannot be of immediate interest to natural philosophy as it constitutes a demonstrative science or even the parts of empirical science that immediately serve this part of natural philosophy, however useful such history may otherwise be. As such, this history is in each generation completed in a becoming that one might understand as an evolution compared to another, if only it is far enough removed in time. Again, such a history may include certain remarkable generations, such as the first mixture of Spanish and American Indian ‘blood’. But these are finally remarkably different from other generations in this line only in degree not in kind.

28. To clarify this question further, I wish to give some definitions of species that will be useful in what follows. The first of these is a logical definition, the second biological. These differ as nominal and real definitions for reasons that I will mention without significant defense. Recall here, however, that a nominal definition does not imply that it does not touch upon real principles. Nor does it imply that it cannot become ‘real’ when founded in scientific inquiry to more complete definition.

29. Logically species is one of the answers to the question ‘what is it?’ in distinction from the question ‘how is it?’ ‘Property’, ‘accident’, and ‘difference’ answer the question ‘how is it?’; genus and species answer the question, ‘what is it?’ Species answers this question such that it can be said of many distinct in number but not in kind; it is thereby the most ‘proper’ answer to such a question. Note, however, that species is not merely a name, the reality named bears the notion of species properly speaking. The reality bears this notion insofar as the thing signified is the foundation in reality to that name and has some existence in that name as the signified is in the sign.

30. A ‘real’ definition of species proposes to resolve species to its ‘principles’ as these exist in reality. The ‘real’ definitions offered here concern only those principles involved in becoming, as these will influence the evolution of species. Further, for reasons I will not presently defend, I cannot provide one ‘real’ definition of species. I will propose a definition of species proper to the living and another proper to the inanimate.

31. A species of living substance is here understood ‘biologically’, through the reproductive power most proper to the living, so that each member of the species—in the strictest sense—has the power, by itself or with another of the appropriate sex, to bring forth another of the same species. Note that those living things, such as the workers among ants and bees, as well as the immature of any species, do not possess their species completely—they are not complete members of that species. This fact underscores the difference between the logical and real consideration of species. A boy or a worker bee belongs logically to its species no less than a man or the queen bee. Real or biologically, however, the former are immature or incomplete participants in the species taken as a class, because they do not possess in a complete manner the intrinsic character that constitutes ‘species’ in each member of that class.

32. So taken, species is the intrinsic character by which one shares completely—or as completely as possible—in the nature or essence proposed in answering the question ‘what is it?’ The species of a living being is the substance as constituted by its body and soul. Since the proper operation of any species of being that shares in vegetative, bodily life is to produce another of the same kind, only those possess such a species completely who have reproductive power. This reproductive power is principle or part of the principle—male or female—through which the species and nature maintains its existence through time beyond the present existence in this singular.

33. Similarly, for the present purposes, the species of non-living substance, simple or compound, is the compound of matter and form determined by the principles through which it comes to be and remains in existence. These same principles accidentally cause the corruption of the substance insofar as they are principles in the becoming of other substances, usually other in kind. So hydrogen is understood to be the single proton, for example, insofar as it arises, as supposed here, and remains in existence from the powers instantiated in quarks. Such a proton exists, as supposed here, in various relations to an electron. These various ‘states’ of such substances—namely the way they arise from quarks and their relations to electrons—seem to establish the substance as distinct principles in the generation and movement of other substances. In asserting this, I assume something I do not here defend, that quarks and electrons are not themselves substances in their own right, though they may well, each in its own way, have some kind of existence in separation from hydrogen and its various compounds.

34. Now this distinction between species taken as logical and as real implies another distinction pertinent to this question, the distinction between the subject genus and the predicable genus. For this reason, the distinction of these genera also pertains to that between a nominal definition and a real one. I will speak briefly to this distinction.

35. Predicable genus is genus as discovered in Aristotle’s Categories. This is the ‘highest’ answer to the question ‘what is it?’ It is the ‘last’ insofar as it also answers the same question, when other, prior answers are taken as subject. ‘What is Socrates?’ ‘A man.’ ‘What is a man?’ ‘An animal.’ Species taken logically is placed in this genus. As such, species is tied to a definition that gives the significatio nominis, the meaning of the name. The genus is the principal part of this definition. At the same time, insofar as the species, as well as the individuals, about which we predicate the genus are united and one in that genus, the genus is a kind of whole containing these species.

36. Subject genus appears in one way in Aristotle’s Posterior Analytics. In another way, subject genus appears only in the particular sciences. In these sciences some determinate subject constitutes the ultimate subject of demonstration—in itself or in its various parts. Plane geometry is about plane surface and principally the circle and then the triangle. Physics or natural science is about mobile being and principally about being as its changes place or perhaps as the living being moves itself—perhaps each in different sense of principal. Metaphysics or first philosophy is about being as such, being as being. But principally it is about substance, since substance exists first and foremost. So considered, subject genus appears in each science as the determinate nature or reality which the science will investigate and about which it will offer demonstrative knowledge.

37. As the subject genus or its principal part constitutes the ultimate subject of demonstration, it brings together into one science the various attributes or predicates that each science proves to belong to that subject. Arithmetic considers the odd and the even, the prime and composite, because these belong to its subject, number. Geometry considers the various relations of sides and angles that triangles admit and the various ways that straight lines might touch or cut the circle as so many attributes that belong to these parts of its subject.

38. In each case, the conclusion predicates the attribute about this subject through some middle term or cause. This middle term—as logic shows—is a real definition of the subject that manifests its principles to be the cause of the attribute in question. Only by offering a real definition of the subject in the middle term as the cause of the reality signified by the attribute do we see that attribute as belonging per se, through itself, to the subject. Thus logic reveals the subject genus as a whole that unites the attributes of a science through common resolution to one or more real definitions of that subject.

39. Species taken as defined by a real definition belongs to subject genus rather than to the predicable genus. In my understanding, it may belong to that genus in one of two ways. Above all, a species belongs to the subject genus as the principal part of that genus, or even as a member of that principal part. So the circle is the principal figure which plane surface admits. Again, number is the subject of arithmetic, but no species of number belongs to it more than another, unless one considers particular attributes of number. But a species may also belong to the subject genus as a secondary part of that genus. So the equilateral triangle constitutes part of the subject genus of geometry insofar as the geometer discovers, uncovers, attributes that define that triangle, its boundary by three equal straight lines, in the circle—more exactly in two equal circles sharing a common radius.

40. I have dwelt upon these distinctions because they concern the real definition of evolution. Most controversy about evolution, in my understanding, arises from one of two causes. The first and most obvious is the assumption that a generation deserving the name evolution brings about new species without divine causality. Though I think this cause of controversy is related to the other, I will address this cause later.

41. The other cause is the tendency on both sides of controversy to resolve this question to species as logically defined and thus as resolved to a predicable genus. On either side, for evolution or against, the consideration involves only realities insofar as they are predicated of or attributed to a subject. So considered, these realities are abstracted from the subject’s matter. Controversy naturally resolves to the mode of predication or attribution: is this predicate substantial and essential or accidental. This ‘devolves’ into philosophical disagreement about the possibility that so many accidental differences can constitute a substantial or essential difference. Obviously, these disagreements offer dialectical profit of many different kinds, even for the scientific consideration of evolution. But dialectical or logical argument cannot establish scientific truth.

42. I trust that I have established myself on one side of such a philosophical or dialectical disagreement. I think it impossible that accidental differences considered as such, considered precisely as accidental, can constitute an essential or substantial difference. No difference merely accidental to an individual can establish it in another species and no difference merely accidental to a species can establish it in another genus. If one conceives evolution only through species considered logically, one cannot form a definition of it appropriate to natural science. This would be like attempting to define the void in natural science through quantity’s logical character as an attribute or accident. Instead, Aristotle defines it in relation to movement, though his intention is to prove no such thing can exist.

43. Similarly, a natural definition of evolution must conceive such a generation as an act of the composite of form and matter. As with any other generation, evolution cannot be considered without attention to the role these principles play in generation. We do not even see the possibility of generation without considering these principles, as Aristotle makes clear in Physics 1.7. Along these lines, consideration of any real distinctions in generation demands attention to some determination of form and matter appropriate to whatever characterizes that generation. In other words, only an attention to the composite of form and matter as these have some proportion to the act of generation in question and to each other will allow one to conceive of and consider scientifically the kind of generation we call evolution.

44. This is to consider the thing generated at least as belonging to a subject genus of generable substances. In the case of living substances, one must consider the one generated and the one generating as composed of body and soul. If one were to define the evolution of the living from the non-living, one would have to consider the proportion of these composites to one another in various ways. It would be similar if one were to define the evolution of a species of living substance from another that bears the name ‘living’ only analogously and not univocally, say the evolution of something sensitive from something vegetative. One would have to consider the proportion of reproductive powers in the one to the disposition in another of its organized matter to the soul bearing another species.

45. Now I will proceed to discuss generable substances in a manner that will form the immediate foundation—with one or two distinctions—for my definitions of evolution. I will not propose definitions of evolution of the living from the non-living or, in any precise way, of living things from other that bear the name life only analogously, as man from something merely animal or the animal from something merely vegetative. Rather, I will propose three definitions: one for the evolution of one inanimate species from another and two for the evolution of living things from other living things.

46. To determine the common subject genus, generable substance, I note that even common experience makes clear that one substance does not come to be just any other substance. Even among the ancients, Aristotle established the science of chemistry in his On Generation and Corruption, though attention to the fact that the generation of one substance depends upon the corruption of another. This corruption implies the dependence of the thing coming to be upon the matter of the thing undergoing corruption. The substance that corrupts does not become just anything because its matter has some disposition and proportion to one form and nature rather than another.

47. Chemistry studies the order of substances insofar as the matter of one, in suitable conditions, bears such a disposition and proportion to another. As this science makes clear in many ways, among the various generable substances some stand out as the ‘first’ generable substances through which and from which all other generable substances come into being. We call these substances, whatever they are, elements. This is to say that the matter of the elements has certain properties through which matter has some disposition to the forms and natures of generable substances that are not elements. Further, the matter in these elements under such properties rightly proportioned is some part, that is the matter, of all composite substances.

48. The ancients considered the qualities hot and cold, wet and dry, to be these most fundamental properties. We now propose, with a probability of an order altogether different, that the ‘valence’ of one element, its attraction or inclination to enter into composition with other elements, arises from its constitution from certain parts utterly unknown to the ancients and even from those first investigating these elements. The valence of an element depends upon the character of these parts—which may be integral parts or only potential parts—though we have almost no independent experience or knowledge of what these parts are or what they are like. I abstract here from considering whether these integral parts, namely electrons and protons or quarks, and perhaps neutrinos and so on, are themselves the more fundamental substances and therefore elements. For the sake of philosophical investigation of the subject genus, I assume as at least reasonable the position that what we still call elements are the most fundamental of generable substances. In this way they are first in the subject genus of generable substance.

49. Again, among these substances, one, hydrogen, seems to be first in another way. With a probability that few would gainsay, the parts just mentioned, as the proper matter of hydrogen, seem to constitute, under certain conditions, the proper matter of the other elements. So hydrogen becomes helium. But a difference arises. When hydrogen and oxygen become water, certain aspects of the matter’s proper disposition to hydrogen and oxygen remain. The matter of water retains a distinction of some sort into nuclei having protons disposed to become hydrogen or oxygen. When hydrogen becomes helium, its matter does not retain this disposition to become hydrogen. Its disposition is much more stable because the protons and neutrons of helium constitute one nucleus, unlike those in water.

50. I have entered into these details for one purpose, to suggest the sort of determination proper to the philosopher and scientist who considers things according to their proper subject genus and not merely through their predicable genus, as does the dialectician for purposes not immediately scientific. I will add only one comment regarding the elements. In my understanding, empirical research has resolved the generation of composites from the elements to the electron as a proper principle, while it has resolved the generation of the other elements from hydrogen to what we call quarks.

51. I have no wish in saying this to suggest that I know things I do not know. I wish only to illustrate the claim that scientific consideration must resolve the attributes proper to its subject genus to the definition of that subject genus. Here the generable character of all substances composed from many substances differing in species seems to arise from the electron as an integral part or perhaps as a ‘force’ present in the matter of those substances that do not come to be from many substances differing in species. The generable character of the elements themselves seems to arise from another integral part or ‘force’, the quark.

52. Now, while Aristotle established the science of chemistry to the extent that he identified the aspect of generation used to define its subject, modern investigation seems clearly to have gone much farther and with much more certitude in determining the material disposition of generable substances. This includes an empirical distinction that corresponds to one proper to philosophy that is also more fundamental and better known. Chemistry has two major parts corresponding to the distinction of living and non-living known immediately from common experience. Its first and principal part, inorganic chemistry, considers the material disposition by which composite substances come to be from the elements. Another and secondary part, organic chemistry, studies the material dispositions by which living substances subsist in and come to be from their proper matter.

53. To my mind, three aspects of organic chemistry deserve notice in defining evolution. The order in which I offer them is not the order proper to them. First, organic chemistry considers its subject as being a cell or being from some number of cells. The cell arises from the matter known to inorganic chemistry insofar as that matter has dispositions that give rise to various integral parts, such as a nucleus and membranes, in an order proper to the cell. The being that arises from a congregation of cells does so in dependence upon the order among these cells, which in turn depend upon the disposition and order in their various integral parts.

54. Second, organic chemistry has identified one integral part, D.N.A., either alone or in some relation to R.N.A., to be, among the constituent parts of a cell, the part most formal with respect to the various species of substance. To say it is the most formal part is to say that, among the various parts entering into the same level of composition, other parts stand to it as some kind of matter, awaiting further determination. This formal part, however, completes the whole so that the entire matter, of which the formal part, here D.N.A, is only part and not the whole. So in the art of music, the third step in the scale, if it makes one ratio with the first, ‘makes’ the whole scale as major in mode. If it make another, lesser ratio with the first, it ‘makes’ the whole scale minor. Thus, organic chemistry proposes as its subject genus beings whose matter is organized into one cell or into a compound of cells. Further, it understands the species and nature of these cells to depend, at least materially in the first formation of such substances, upon the organization of those parts of the cell described as D.N.A.

55. I have made reference to the substance’s first formation because I have been given reason to think that the D.N.A in living things may change in various ways, of which I am mostly quite ignorant. I think that one of these ways will enter into an empirical account that I will use to support my principal definition of evolution. But others may occur in a manner not dissimilar to the way in which a man’s general temperament, physical and psychological—or even an animal’s—depends upon the overall constitution of the matter from which and in which he comes to be. Yet a serious and prolonged sickness may alter his constitution so that his temperament changes in a significant and even fundamental manner. Similarly, the elements, from hydrogen on, seem to be defined in one way by the number of protons and electrons from which they come to be, at least in the most stable way, yet there are various isotopes of these elements apparently having the same nature but with electrons differing in number or character.

56. The third aspect of organic chemistry I point out is that its very distinction from inorganic chemistry implies the philosophical distinction between animate and inanimate beings. This is most clear from the very evidence used in this science. Any judgment made in organic chemistry must resolve explicitly or implicitly to a direct or indirect sensible experience of something the scientist considers alive according to a definition of life brought immediately or through other definitions back to the nominal definition of the living, something that moves itself.

57. The same fact, namely the dependence of the empirical distinction of organic chemistry from the general study of chemistry, is also clear in another way. Those who identified the cell as in some way the first member of its subject genus insisted upon that the cell, and therefore the science, could not be reduced simply speaking to the principles of inorganic matter. Both Schwann and Virchow—however much each would reject final causality or a non-material principle of life—recognize that the living cell constitutes a distinct order of substance and being. This implies some awareness, however vague or even contradictory, that the living substance involves some principle distinct in order from those that do not live.

58. In keeping with its empirical character, organic chemistry itself identifies this principle as the constitution or order among the cell’s integral parts. For this reason, some thinkers, such as Harold, insist upon the primacy of the whole cell and its constitution. Philosophy, however, identifies this principle not merely in the material organization of the cell but in the unique character of one of the parts that constitute its substance. The word coined for this principle is ‘soul’, though I use it here in utter indifference to any question of its ability to exist in separation from its matter, that is, its body. If soul has such an ability, it is no way reflected in its power of integrating the parts of the cell and even cells themselves in vast numbers into a body whose operations belong properly to the whole and cannot be reduced to merely to its various parts.

59. I would dare say that thinkers such as Harold admit implicitly that there must be such a principle. He says that the cell is a whole ‘greater than the sum of its parts’. In considering his words, I abhor this statement insofar as it abuses the very notion of whole and part. In considering his meaning, I admire the force of truth. Though he pays distinct attention only to the composition of the whole substance from the integral parts of its matter, he must recognize there a whole proper to another order composition, the composition of the matter he studies with its form.

60. Here in living substances the natural power of the intellect goes farther than Harold allows himself to go. By nature men distinguish body and soul long before they can distinguish matter and form as these exist precisely in the category of substance. Now history abounds with the errors, deceptions, and follies that arise when those who distinguish soul from the body fail to grasp further and more deeply in substances the distinction of form from matter.¹ Still, the fact that men naturally make this distinction reflects a truth about living substances grasped scientifically, in my understanding, only by first philosophy or metaphysics.

61. This science of metaphysics shows, according to its own demonstrative power, that the soul is a substance, that is, a principle of substances in the first meaning of that word, according to more complete and more strict sense of the word than other forms or natures. Both form and matter, both body and soul, are properly called substances in the sense that they are principles of substance. Yet forms or natures other than soul never distinguish themselves from their matter so distinctly that they have both an active power over their matter, as the soul moves the body. Soul of every kind is said to live in as many ways as it can ‘move’ its body: by feeding and growing it, by changing its place, and further insofar as it does these things by sensing or thinking or desiring. This arises from the significatio or meaning of soul: that by which the living live.

62. In all these ways, soul stands out among the principles of substance as capable of using matter for its own purposes, which alone are the purposes of the whole substance. In the present study this touches immediately the very distinction of the subject genus I am principally concerned with. Living substances as endowed with reproductive powers from their souls constitute a subject genus distinct from that generable substances simply speaking. All generable substances whatsoever must arise in some way and in some order from the elements. The fact that organic chemistry is subordinate to inorganic chemistry implies this fact. But living substances, precisely because they possess more noble forms than others, forms deserving the name soul, come to be in such a manner that some individuals are distinct agent causes to other of their coming to be.

63. Further, this subject genus is not merely part of the subject genus of generable substance. In some way, the generation of living substances may be merely a species of generation taken as a predicable genus. The generation of something may arise through another substance as the active principle of the same nature or it may not. Each bear the name generation and I will not insist that this name is equivocal. At present, I insist only upon the fact that scientifically one cannot resolve these two kinds of generation to principle of one kind. Arithmetic and geometry have principles irreducible to one another and cannot constitute one science. So, the scientific consideration of the generation of living substances must explain such generation through principles not intelligible through the principles involved in the scientific consideration of generation merely insofar as it comes to be from elements or even the parts of elements.

64. The principles proper to the scientific consideration of the generation of living substances are the reproductive powers of a substance that arise immediately from soul insofar as it possesses aspects of substance not found in the elements. These aspects are higher than those found in the elements and their immediate compounds. We see them most distinctly as they manifest themselves through the difference of sex, male and female, in the animals immediately known to us. These differences exist less distinctly in the plants we immediately experience. There the distinction of male and female organs usually exist as parts of the same individual substance. Apparently, the vast majority of living substances possess their reproductive powers in a manner even less distinct. In such substances the reproductive powers allow the substances to act upon their own bodies so that these bodies become two cells, each a substance distinct in number. We therefore distinguish cellular reproduction from sexual reproduction.

65. Further, this power that expresses itself most distinctly in cellular reproduction exists in substances that reproduce sexually not immediately as a means of reproduction but as the means of growing and perhaps repairing the body. In cancer, we see this power exhibiting harmful and even destructive effects. Again, the very division by which the embryo develops is a dramatic exhibition of this power.

66. So, in defining the evolution of both inanimate and animate species, one must in some way reduce this coming to be or generation, on the side of the one generated, to the intrinsic principles by which matter has its disposition to the form that determines the composite to its species. The ultimate principle by which the matter of inanimate substances has such a disposition to the various forms of the elements seems to be the quark. The ultimate principle by which these elements are disposed to the forms of composite substances seems to be the electron. The ultimate principle by which the matter of living substances has such a disposition seems to be D.N.A. The differences proper to each of these principles must on this account be general conditions for the possibility of evolution and for its reality, if it does exist.

67. All real definitions of evolution, on this understanding, must in some way identify those material principles in the nature of the substance undergoing generation by which it bears some disposition to the form of some substance of another nature and species. This is merely to say that a particular generation will not bear the notion of evolution unless the matter of the one generated can from the powers and inclinations natural to its original species arrive at the form of the new species. Whether it concerns the living or the inanimate, any definition of evolution must identify these principles and, as such definitions may be philosophical or empirical, and the latter may be more or less immediately adapted to the sensible evidence, each definition must identify these material principles in a manner appropriate to its kind.

68. In the evolution of living species, one must reduce this generation in some way to the reproductive powers of some member of the prior species. Not only must the matter be capable of taking on the disposition to the form of another species. The reproductive power of one living substance, or the male and female powers in two living substances, must be able to bring about the matter disposed in this way to the form of another species. The generation of one living substance by another living substance or substances will not bear the notion of evolution unless the reproductive power by which the member of the prior species properly and per se produces another member of its own species can in some way bring into being matter disposed to the posterior form and species.

69. The following distinctions will each enter immediately into the definition of evolution I will offer. First, two comments Aristotle makes about species are necessary to the very notion of evolution. He says in one place that substances or essences are somehow as numbers and in another that the various species of material substances differ among one another little by little, ascending continuously, as it were, from the lower to the higher.

70. Now, in Metaphysics 8, Aristotle discusses the composite of matter and form. In the first chapter he shows first how it depends upon matter. He then shows how it is determined by its form. Here he first shows, in chapter two, that form is in itself proportioned to the specific difference. Then, in chapter three he considers form through its relation to names and then through its likeness to number. This likeness, I would suggest, he takes as evident from the fact that certain of the wise, most notably Pythagoras and Plato, have go so far as to say that number is the substance of things.

71. Here Aristotle says: ‘It is also clear that, if substances are somehow numbers, they are so thus and not as some say [numbers] of units.’ (1043b32-4) The adverbial pronoun ‘thus’ refers to the previous conclusion, that ‘part [of the definition] must be as matter, part as form’. (1043b31-2) He goes on to give four reasons that substance and essence having some part of its definition as form demands that it be like number. First, he notes that both a number and a definition have parts and that these parts are not divisible ad infinitum.

For the definition too is a certain number—for it is divisible and into indivisibles, for the formulas are not infinite and a number is such. (1043b34-37)

72. Next, Aristotle notes that mere addition or subtraction of any of these parts destroys the number or the definition.

And just as some of the units of a number taken away from or added to that number, it is no longer the same number but another, should even the least be subtracted or added, so neither the definition nor what was the thing’s being be any longer, with something taken away or added. (1043b37-1044a2)

73. Then, he draws the third likeness, that the difference which completes the number, the definition, and the substance, must make it something one.

And there must be something by which the number is one…For either it is not one but a kind of heap or if it is [one] one should say what sort of one from many. And the definition is one…and the substance is one thus and not as some say as being a unit or a point but each is completeness and some nature.

Later he will make clear that this unity is founded upon the proportion form has to matter, that of actuality to potency.

74. Finally, Aristotle concludes this passage with a fourth likeness:

And just as number does not have more and less, neither the substance according to its species, but if so, the substance with its matter does. (1044a2-11)

I would quickly draw your attention to the distinction Aristotle makes here: on the side of form the substance does not have more and less, though it may when taken together with its matter. This may itself imply the fact that material substances in some way stand to one another as numbers and in another way as parts of a continuum.

75. I cannot overstate the importance of this paradox for the notion of evolution. There will not be one species and another unless these species differ as numbers, and we conceive of evolution as a generation ‘between’ such species. Yet again, the first species will not have anything of the second within its reproductive powers unless the one species differs from another in a manner almost continuous.

76. This is the point of the next passage I quote from Aristotle’s History of Animals. Aristotle introduces this passage by noticing that some things in men and animals are simply the same, some are nearly so, others only analogous. He then defends this with this weighty comment:

Thus nature goes little by little from the lifeless to animals so that in its continuity their boundary hides as does where the mean lies. Thus, next after lifeless things comes the plant, and of plants one will differ from another as to its amount of apparent vitality; and in a word, the whole genus of plants, whilst it is devoid of life as compared with an animal, is endowed with life as compared with other corporeal entities. Indeed, as we just remarked, there is observed in plants a continuous scale of ascent towards the animal. (588b1-12)

I would make clear that I do not understand Aristotle to refer here to a perfect or a single continuity. Cats as a kind may differ almost continuously from other things along one ‘line’, while they differ almost continuously among themselves along another ‘line’.

77. My principal definition also distinguishes the active and passive powers of natural substances from these powers taken with respect to the divine ideas. Saint Thomas makes this distinction in explaining that ‘Saint Augustine fittingly calls the active and passive powers [virtutes] which are principles of generation and of natural movements seminales rationes.’ I would translate this more or less by the phrase ‘seminal notions’ or even ‘seed-plans’, but the term ‘seminal reason’ already has some currency. He begins by noting that ‘such active and passive powers can be considered in a multiple order.’ I call attention to the fact that he does not merely say they can be considered in many ways. He emphasizes that the distinction he will make involves several orders.

78. In fact, he mentions four orders. Each order that follows depends upon the prior. First, these powers can be considered insofar as ‘they are principally and originally in the Word of God, according rationes ideales’, that is, ideal notions. Secondly these powers ‘are in the elements of the worlds, where they were produced together from the beginning, as in universal causes.’ Third, these powers ‘are in the things that have been produced from the universal causes according to the succession of times, as in this plant and this animal, as if in particular causes.’ Fourth, these powers ‘are in the seeds are produced from animals and plants. And these again are compared to other particular effects as the primordial universal causes to the first effects produced.’ (1 Q.115 a.2 c.)

79. First, I wish to point out Saint Thomas’ principal claim: the powers of the elements, both in themselves and according to the form they take in the substances produced from them, have a universal power to bring forth all the things made from them. Many agree with this to the extent of thinking that everything in a composite substance should arise merely from its elements. Even if this opinion exaggerates the truth, these powers still have the character of ‘seeds’ from which all other things are ‘born’.

80. Second, Saint Thomas does not say here that these powers have the character of seminal rationes unless one considers them in the order mentioned, namely, as depending upon the ‘ideal notions’ that exist in God. In the elements and in the compounds made from them taken in themselves, these only have the character of certain powers with determinate effects proper to their nature and no farther. Hydrogen, for example, has some power to enter into this composite or that according to its ‘atomic structure’. Considered according to the nature of the element in question, the powers of the elements do not have the ‘plan’ or ‘notion’ of plants and animals in them, not even ‘plans’ for single-celled life forms. So Saint Thomas says later that ‘even if [these powers] cannot be called rationes insofar as they are in corporeal matter, they can be called rationes by comparison with their origin, insofar as they are drawn from the rationes ideales.’ (1 Q.115 a.2 ad 1) I would emphasize here that the dependence of these powers upon the divine ideas to which Saint Thomas refers is a present dependence, a dependence in every moment that these powers exist and exert their causality.

81. This dependence demands attention to certain distinctions in divine causality. I would distinguish these aspects of God’s causality: as creator, as first mover, as an exemplar cause, and as providence. Insofar as God creates, stated simply, his action has no mediation on the part of creatures. Insofar as God is the first mover, the nature of each substance mediates his action. The first inclinations and movements of each creature—the inclination of the eye to see, the cell’s appetite for nourishment, the falling of the rock—come forth from each mobile insofar as God moves it toward its ultimate end. Taken as they are proportioned to the mobile, these movements arise from its powers precisely as these powers belong to the mobile’s nature. These same movements, however, have a real origin in the divine mind, in its understanding and plan not for merely for the particular but for the whole universe. So taken, in relation to the divine ideas as exemplars or paradigms of creatures, these powers are so many ‘seed-plans’, rationes seminales, of all the things that can arise from them. I would only note here that God’s action in creatures also has the notion of providence insofar as we refer that action to the determinate plan that God has for the history of this world.

82. In passing, let me say that this should suffice to make clear that I think it philosophically inconceivable that evolution should occur in such a way to eliminate or obviate the need of divine causality. I think it clear enough that movement as a whole must be reduced to God as first mover. As will be clear in the principal definition offered below, I think each instance of generation depends upon God in certain ways that become only more clear if that generation should constitute the ‘evolution’ of a species.

83. Now, I distinguished the active and passive powers in created substances as they are in themselves and as compared to divine ideas and for this reason I distinguished several ways of considering God’s action. But these natural powers in the elements and their compounds involve another distinction. The active and passive powers of the elements or their parts must be considered as they act through the proper form of a substance and as they act through a ‘virtual’ form rather than one that actually informs that substance.

84. The powers belonging to any substance from its proper form is clear enough in many ways: dogs bark, cows moo, and so on. But, as Saint Thomas points out in the ninth of his Disputed Questions on the Soul,

Since the forms of things are as numbers—in which there is a diversity of species, unity being added or subtracted, as is said in Metaphysics 8—one must understand the diversity of natural forms, according to which matter is constituted in diverse species, from the fact that one [form] adds a perfection over the other. As, say, one form constitutes [the matter] in bodily existence only…but another more perfect form constitutes the matter in bodily existence and gives it further vital existence, and further another form gives to it bodily existence, vital existence, and sensitive existence, and so with others. So one must understand that the more perfect form, insofar as it constitutes matter in the perfection of a lower grade, at once composed with its matter, [this form] is understood as material with respect to a further perfection…as first matter, insofar as it is already constituted in bodily being is matter with respect to the further perfection which is life.

The point Saint Thomas makes immediately is that, when it is composed with matter, the higher form or nature brings forth in that matter the perfections appropriate to lower forms. Our intellectual soul not only makes each of us rational. This soul also makes each of us sensitive and vegetative. The same soul also makes each of us to be a body. For this reason, we are heavy bodies. The human soul and species includes these lower perfections within it as a number includes all lower numbers.

85. I want here to consider this same truth in another way. The forms of lower substances exist virtually within us. We see this generically to the extent that we have sensitive and vegetative powers and from the fact that, all thing being equal, our bodies fall and otherwise move as other inanimate mobiles. We also see in many ways that the elements, various compound molecules, cells of many kinds, and various organs exist within us. While the elements exist in us in a particular way, as the ultimate principles by which we hold together as a compound body and by which we will corrupt into other substances, many of these others, even living integral parts of us—I refrain here from comment upon their proper natures—can exist at least for some small space of time in separation from the whole.

86. My attention to this truth differs from the attention given it by Saint Thomas to the extent that I am not merely saying that our proper form causes lower perfections but that insofar as those lower perfections exist in us certain lower forms, proper in some way to less perfect substances, exist virtually, albeit not actually within us. While the separation of certain parts of us may exhibit how close these forms are to actual existence, the power they exercise as integral parts of us flows from their virtual presence.

87. What we rarely notice about a form existing virtually within a substance is that its power is not wholly dominated by that substance’s proper form. Matter perfected to this lower degree, perhaps precisely as it informs some integral part of the composite, is ‘ready’ to act according to the lower nature. This is clear with the elements to the extent that we notice that the powers of attraction by which they cling to one another and thereby keep composite substances in existence are the same powers by which composite substances literally ‘fall’ apart. In the moral order, man experiences this to the extent that his sensitive appetites come forth according to their own ‘law’; they follow their own nature.

88. What matters for the purposes of defining evolution is that the parts of a material substance—its elements, the inanimate compounds within it, and its integral parts—insofar as these have a nature less complete than the whole exhibit active and passive powers proper to that lower order. And to some extent they exhibit these powers with an indifference to the order of these powers to the higher nature. These same active and passive powers referred to the divine ideas, as certain rationes ideales or ‘ideal plans’, have an order not merely to their proper effects but to all effects to which those ‘ideal plans’ understand them to have an order. As ‘seed-plans’ or rationes seminales, these natural powers have an order to whatever effects the divine power might bring forth from them immediately or in concert with other created natures.

89. Now I will present two definitions of evolution. The first defines only a secondary kind of evolution. The second defines what I consider evolution ‘as such’. These are philosophical definitions, though I will mention, at least with the second, various matters of concern to an empirical definition.

90. What I am calling evolution in a secondary sense depends upon the elimination of intermediate varieties. Anyone who has seen the diagrams in Darwin’s Origin of Species should readily comprehend this conception. By whatever means, the descendants of an interbreeding population of living substances have become several well-defined varieties, as various determinate varieties among dogs in our day. Should intermediate varieties be eliminated in such a way that the remaining varieties no longer have the power to reproduce with one another, as, for example, a Great Dane cannot, to my knowledge, reproduce with a Chihuahua, these will constitute two species brought about by some first.

91. Before restating the definition in as formal a manner as I can, I will make three comments about the matter defined. First, one might think the members of these two populations I have described do not in fact have distinct species but have one species in a varied form from the fact that their gametes, if united by artifice, such as artificial insemination or even some genetic modification, might well produce viable offspring. I do not deny this possibility and I suppose that in the example mentioned that this is so. But the power by which these gametes come together in the objection does not belong to the members of the species. The gametes even have this common offspring within their power when compared with human reason or insofar as they are rationes seminales. But, considered precisely according to the power present in them, the gametes are the proper instruments of the reproductive power possessed by the plants or animals which form them. The species of these animals—defined by reproduction—follows this proximate reproductive power. In the strictest sense only such proximate power deserves the name ‘power’, as Aristotle shows in Metaphysics 9.

92. Second, the objection arises that the members of each of these species seem to be the same in species with their distant ancestors but not with each other. I consider this evolution in a second and weaker sense precisely because the remote parent shares in some way in the species of its descendant. The objection rests upon the fact that the remote parent should be the same in species with both species of descendent.

93. Now I agree with this claim. The remote parent here is the same in species with each species of descendent but not according to the full power of the parent species. I use the term ‘full power’ here only with respect to the breadth of attributes that power can bring forth. Here we should keep in mind that an individual member of a species rarely, if ever, possess all the powers proper to that species. The complete distinction of male and female in higher animals makes clear enough that each has only a complementary part of the species’ reproductive power, as Aristophanes points out so vividly in Plato’s Symposium. A Christian audience should appreciate the suggestion that the reproductive power of the human species existed in its most complete form only in our first parents, Adam and Eve. The communication of this power to their children must be limited in their various children. The various races seem to be distinct insofar as they develop certain aspects of that power. The past destruction of certain races in our past may well have reduced the power now present in the human race.

94. Such limitation and even division of a parent species’ reproductive power in its descendent species might be one of the ways in which species and essence, when taken together with its matter, seems less like number and more like the continuum.² Aristotle noted the possibility that so taken substance might fall away from its fourth similarity to number: ‘And just as number does not have more and less, neither the substance according to its species, but if so, the substance with its matter does.’ (1044a2-11)

95. Third, this case seems to pertain only to the living and perhaps there only with those in which there is distinction of sex. I do not see a similar manner in which some ‘evolution’ could arise among inanimate species, but I am very hesitant to suggest this.³ Similarly, I hesitate to suggest that species using cellular reproduction cannot encounter a situation of this sort.

96. What follows is my attempt at a formal definition of evolution through the elimination of intermediate varieties. Two varieties (taken here to name the character in each member by which the variety receives or deserves its distinct name) once being one in species insofar as they were capable of common generation only through members of other varieties, having lost this power by elimination of the intermediate varieties, each constitutes a species. I should find it diffcult to believe that the various species of ‘cat’ did not come into existence in this way. In fact this suspicion has led me to doubt for many year that the lion, tiger, jaguar, leopard, and the mere cat are real species. Perhaps this understanding of evolution allows me to stand by my ‘gut’ reaction, that they really are distinct species, though they may be so only by sharing in the power of a species once existing and in some way more complete.

97. I turn now to the second of the real definitions of evolution. I have said this definition considers a generation deserving the name ‘evolution’ in a manner more strict than the previous. There two species might evolve from one species by the limitation and division of its power, though each has whatever powers it has through some agreement of species with its remote parent. A remote parent of this kind has the individuals of these species in its proper power, if it does not have the species as such distinctly in its power. Here, in what I call evolution as such, a particular generation brings about some individual substance of a new species from some substance or substance of another species that lacks the power to produce a substance of that species.

98. At some point I will distinctly apply this definition to the evolution of inanimate species before applying it to the living. I do not think such generations can be called ‘evolution’ univocally. But then I will also state other reasons for doing so. Before this, however, I wish to place both kinds of evolution in one genus. Having done so, I must make two distinctions.

99. Now the genus in which I place evolution is one mentioned by Aristotle in his most general division of coming to be in Metaphysics 7. There he says, Now, of things coming to be some come to be by nature, others by art, yet others by chance.’ (1032a12-13) The third of these things coming to be by chance are those I understand to be the subjects of ‘evolution’ in this sense. I should point out that he later seems to make clear that he does not intend as what comes to be by chance something that comes to be accidentally, such as a ‘tall grammarian’. (1034a25) Later I will say how I think this differs from coming to be by chance.

100. In dividing things coming to be by chance from those coming to be by nature and by art, Aristotle implies differences he does not explicitly state. Nature and art both bring something into being by a determinate intention. Such intention implies something common to nature and art as agent causes in becoming. Each has some likeness to what comes to be. The natural agent has the same form, nature, or species with what comes to be. The artist has some ‘species’ in his soul by which he makes what comes to be, and this species is the substance and being of what comes to be. But the artistic species is substance without matter, while the species of the natural agent is substance with matter.

101. The matter upon which the natural agent acts must have according to its own nature some determinate proportion and order to the species that begins the movement. The proportion and order in the matter upon which the artist works depends upon human intellect and power, something beyond the order of matter and form. Insofar as the efficient principle in natural becoming is intrinsic to the order of patient to agent, while that in artistic becoming is extrinsic to this order, these differences exhaust the possibility for generation according to a determinate intention. What remains is something that comes to be without such intention. What comes to be must have some indeterminate order to what it will become. To this extent, whatever brings it into being does not, as such, actualize a determinate potency. I will say more of this in a moment.

102. Now, Aristotle makes several comments about this manner of becoming that I consider the foundation of the definition. His first and ‘definitive’ comment about it compares what comes to be by chance to something coming to be by art. He seems to have in mind as an example the fact that someone might get a massage, for relaxation or after exercise, and find that he has been at the same time healed of some infirmity. He says,

But if [the making is] by chance [it is] by that which sometimes begins the making for the one making by art, just as in the healing too perhaps the principle is from heating, but he makes this by rubbing. The heat then, that in the body, is either part of the health or something which is part of health follows it itself or through many things. But this last is what makes and what exists in this way is part of the health and the house, like the stones, even with other things. (1032b23-30)

Of significance here for the definition that follows is the claim that chance becomings begin from some part of the making that would constitute art, were it done by intention. This agrees with his claim in the Physics that things occurring by chance and luck fall in the genus of things happening for some end. One aspect of the significance of this claim will be immediately clear; another I will point out later.

103. Two chapters later, as part of the same discussion, Aristotle proposes an aporia, an ‘impasse’: ‘But someone might be at an impasse, why some things come to be both by art and by chance, like health, but some do not, like a house.’ (1034a9-10) He makes several distinctions to find his ‘way’ through this impasse. First (1034a10-14), the matter upon which the art works is sometimes capable of moving by itself and sometimes is not. So the animate body upon which the doctor works can move of itself, while canvas and paint of the painter cannot move one to the other. Second (1034a14-21, 25-30), the matter able to move of itself may be able to move in the manner necessary for the end intended by art or it may not. Those that cannot move in the manner necessary—as well as those incapable of moving at all—can only come to the end proposed if they are moved by one having the art. Those that can move in the manner necessary, as the animate body can sometimes, by chance, begin the movements that will arrive at the same end proposed by art, such as health.

104. I would emphasize this aspect of things coming to be by chance. The question at hand is why things such as health come to be both by art and by chance, while others do not. Aristotle resolves the question by reference to and development of his initial consideration of things that come to be by chance. There he said that these ‘makings’ begin with some part of the movement that the artist initiate. A doctor might warm the body through rubbing some part. Should the masseur do so for another purpose, healing might follow from the movement he has begun. Here Aristotle points out that the matter upon which an art might work can sometimes of itself begin such a movement. Such a movement must not of course arise within this ‘matter’ through a natural power ordered to the end intended. If one part of the body heats another as part of the body’s natural inclination to heal itself, this does not come about by chance. Should, however, an animal in pursuit of relief from pain position itself so that its broken bone has the opportunity to heal, it has moved itself in such a way by chance, yet as a veterinarian might.

105. Now, before stating the difference proper to a generation that is properly an evolution, I will make some comments about first matter and its appetite for form. For obvious reasons I will say nothing about first matter’s appetite for forms that can belong to it only by art or by God’s supernatural power. Here I speak only about its appetite for forms that first matter can also arrive at by nature.

106. In his commentary on Boethius’ De trinitate, Saint Thomas gives a very lofty conception of first matter most appropriate to the present consideration. There he says,

Since first matter is pure potency, while God is pure act, matter is perfected by the act which is its form in no other way than insofar as it shares [participat] some likeness [similitudinem] of the first act, albeit imperfectly, so that thus that which is already composed from form and matter is a mean [medium] between pure potency and pure act.

This implies that the composite of form and matter remains in potency to all other likeness of the first act that can be possessed by matter. In each animal and plant there remains some potency to the form every other material substance. This is why we can eat them and why they can corrupt into their elements. In each atom of hydrogen or carbon there in some way remains some potency to become not only all the things that can be composed from them, but even to the forms of other elements, as well as the substance composed of them. This is why, in the contemporary understanding, all elements other than hydrogen have ‘evolved’ from hydrogen.

107. But the potency in a particular composite to the forms other than its own exists in that composite in a certain order. Two aspects of that order are of present concern. First, the potency exists under a determinate form. This determination by form involves some order in its proper matter: the various organs and cells in living things, the order among the elements in all things composed of elements, and even in the elements themselves the order that we grasp under the ‘form’ of electrons, protons, and neutrons. Because the form determines the matter, the matter has an immediate or proximate potency to one or more forms. Hydrogen has a proximate potency to become water, and the gametes of various species of plant or animal have an proximate potency to a form of that species. Its potency to other forms is only remote. The matter must undergo some kind of alteration. So the lizard’s digestive system alters the eggs laid by the praying mantis. Carbon must in some way enter into some compound before it can be a molecule that plays a role in some bacterial cell or in the cell of a living substance more highly organized. To this extent, the matter in a composite substance does not have a proximate potency to just any form. And I would add that Aristotle shows in Metaphysics 9 that only such a proximate potency deserves the name potency in the strictest sense of that word. The composite is simply speaking only in potency to the forms to which its matter has some immediate order.

108. The second order pertinent to this question arises from a truth mentioned earlier. The species of things considered in themselves differ as do numbers by the addition or subtraction of some ‘indivisible’. For this reason, one species stands to another in the same order as less actual or more actual.

109. I will make only two comments in this regard. First, since man has a form or soul with an immaterial intellective power, this form is more actual than any other form able to exist in matter. In this way, matter’s appetite for form has an order above all to man’s soul. Only this form can actualize matter as completely as I can be actualized. I mention this principally in relation to other meanings of evolution. If evolution, taken as an historical series or as some general inclination in material substances, has an ascending order, this follows from the excellence of man’s form.

110. Second, the remote potency of any material substance to a higher form depends by nature upon another substance of that form. The matter of such a substance can only attain a higher form through an actual substance possessing that form, if it will take that form through a natural generation. There remains the possibility that it might take that form through human art or through chance. In saying this I mention art for the sake of an illustration I will use in defining evolution. I refer only to the higher form here, not because I think that it can attain a lower form through a becoming altogether natural, but because I have in mind here the principal difficulty facing the concept of evolution: how the higher and more actual can come from the lower and less actual.

111. With these things in mind, I come to the definition of evolution ‘simply speaking’. Here I will first describe the ‘mechanism’ currently proposed for evolution, genetic mutation. Then I will state the philosophical understanding of the principles involved in such a mechanism. In this way I conceive this definition as taking rise from genetic mutation but not tied immediately to it. Neither ‘genetic mutation’ nor anything that corresponds to it in the elements will appear in the definition that follows.

112. As the name suggests, genetic mutation involves some mutation of a gene or genes, understood as the immediate instruments of D.N.A. Such mutation can be brought about by X-rays in the laboratory as well as by chance. This or something similar is the basis, in my understanding, for what we call genetic modification. Significant evidence suggests, again in my understanding, that such mutation occurs even without such intervention from outside a substance. Cancer cells in animals may be the most prominent example. Such mutation depends, in current thought, upon the powers proper to the ‘elements’ of these genes, atoms themselves or perhaps subatomic parts. The details here are irrelevant to what follows.

113. Most instances of such mutation—by far the most—have no noticeable effect in the composite substance. Some result in what we call monsters. Evolutionary theory understands very tiny number of genetic mutations to be responsible for evolution in the sense I am attempting to define. On this understanding, some genetic mutation in the gametes of certain individual parents or in some single-celled animal, through multiplication by which cells divide, become the proximate principle by which these gametes or the single cell constitute the matter disposed to another, perhaps a higher species. In discussing the definition I will distinguish the ‘causality’ involved in monsters from the that involved in evolution.

114. Now I will form the definition. I will state the definition. Then, I will distinguish the monster from an ‘evolving’ species. Finally, I will state some principles implicit in the definition. The definition is as follows: a second species ‘evolves’ from a first insofar as some member [or members] of the first species—through some alteration of the whole or some part of this substance or substances—brings forth by chance the matter proportioned to the form of a second species to which it has no proximate potency, if the production of that matter arises through some action proper to some form existing virtually in the first species, insofar as the powers of that virtual form stand to second species as ‘seminal reasons’ or ‘seed-plans’.

115. Such an offspring differs from a monster because the monster belongs, simply speaking, to the first species. The monster may well be ‘occasioned’ by genetic mutation. But the occasion causes only in an imperfect way. Some other natural power must complete the effect which the occasion ‘offers’. Whatever brings about a ‘sixth’ digit in the human hand gives an occasion for the generative powers in the human soul to produce the monstrous hand. In chance becoming the result is beyond both the intention and the power of the nature and species that cause it accidentally. For this reason Aristotle distinguishes occasions from chance events in the Physics, as Saint Albert and I have pointed out, by stating: ‘but this too is other. For of the other [chance and luck] the cause is outside, but of this [the occasion] it is within.’(197b35-37)

116. The following comments will make explicit aspects of the definition. First, Matter is capable of this only insofar as species differ from one another in one way as a continuum (History of Animals 588b5) and in another as numbers (Metaphysics 8.3, 1043b33). I stated earlier that the matter of any composite is determined by its form. Here the matter suffers an alteration that introduces a proximate potency to a form not within the power of that composite. This cannot occur except insofar as the order or organization of the matter of the first species is ‘near’ or ‘very like’ the matter of the second. This can happen among many species only if these differences are sufficiently continuous. Further, this cannot happen unless the particular alteration, such as that of some particular gene, stands to what remains of the matter of the first species as the addition or subtraction of an indivisible. The differences, for example in protons or in the genetic structure of D.N.A. must stand as units added or subtracted.

117. Second, the definition refers to a ‘virtual form’. This belongs to the composite through its proper and actual form. This form is ‘proper’ to some lower body existing potentially in the composite, perhaps the atoms in the gene or, when an element ‘evoles’ the quarks in an atom. The power proper to the virtual form acts in such a generation in a manner proper to the lower body or part. But this action is indifferent to the nature and intention belonging to the composite through its proper and actual form.

118. Third, such a generation results in a matter lacking the proper proportion to the first species and beyond its intention, as happens in situations that result in monsters.. Neither the nature and species of the composite nor that of the less perfect body having the virtual form in question is sufficient either to explain or to complete the potency in the resulting matter to a form of the second species. Precisely for this reason, the action of the virtual form cannot result in a new species except insofar as the powers of that virtual form have the character named ‘seminal reasons’ or ‘seed-plans’ by Saint Augustine. In this way, the proper cause of the new substance must be God, even though this generation has been understood to arise by chance through the instrumentality of natural causes, even should some angelic causality be at work.

119. I will not discuss this causality in detail at present. Rather, I will close by commenting on something Aristotle says with reference to monsters that I believe to have some place in this discussion:

…the monster is something against nature, against not every nature but that which is for the most part. For regarding the eternal nature and the nature that is of necessity nothing happens against nature, but [only] in things coming to be thus for the most part but able also [to come to be] otherwise. Since even of these [monsters], in whichever [ones] it turns out against such an order yet [it turns out] always not just as it chances, it seems less to be a monster through the fact that even what is againt nature is in a certain way according to nature, when the nature with respect to form does not rule the nature with respect to matter. (The Generation of Animals 770b9-19)

I make only one comment here. The evolution of a species, if it occurs, happens ‘as it chances’ yet not ‘just as it chances’. It accords with nature because ‘the nature with respect to from does not rule the nature with respect to matter.’ And further it happens in accord with ‘the eternal nature’ regarding which ‘nothing happens against nature’.

NOTES

1. Just such a failure seems to have kept thinkers before Aristotle from grasping the proper unity to the study of the soul.
2. Perhaps the development and exploitation of just such limitations explains the sort of continuity we find among material substances. A kind of continuum among the cats, for example, and an-other higher continuum in which the cat is a certain mean between other animals, taken more or less generically.
3. Perhaps the limitation and division of the original active and passive powers such that they exist in a manner less universal could distinguish species in this way.