TYRANNOSAURUS: See, my fellow fossils: the humans have closed up the museum, and have left us to our own devices. For a few hours, at least, we may shed our roles as silent witnesses to the mysteries of Earth's past, and may speak among ourselves of such matters as we deem significant. For my own part, I should like to use this time to hear what Cephalaspis has to say, for he, as we know, is the wisest of us all. What say my brother archons?

DINOCERAS: I agree. Cephalaspis's thoughts are always worth hearing.

INOSTRANCEVIA: That is certainly so.

TYRANNOSAURUS: Cephalaspis, it seems we are all of one mind. Tell us, if you will, what thoughts are uppermost in your mind this evening.

CEPHALASPIS: It is curious that you should ask, my lord. In truth, my mind is often such a whirl of thoughts that it is difficult to identify the uppermost, but, in the case of tonight, there is one question that I find myself pondering with particular interest.

TYRANNOSAURUS: What is that?

CEPHALASPIS: Did you notice, early this afternoon, a group of young humans examining the diagram on the back of my case – the one that discusses the evolution of fishes?

TYRANNOSAURUS: Yes. As I recall, they found it laughable that anyone should believe such a preposterous thing.

CEPHALASPIS: Exactly. Now, we all know that the theory of evolution is accepted by nearly all the learned humans in this region, and is taught in all their centers of learning. It would seem, then, that to doubt it must be the act of a fool or a crackpot – and yet the humans examining my case did not seem to me to be fools or crackpots. Indeed, they seemed, for the most part, to be very sensible creatures for their age. When they expressed their doubts of evolution, therefore, it inspired me to wonder whether there was any reason for doubting it that a rational being might accept.

TYRANNOSAURUS: And is there, Cephalaspis?

CEPHALASPIS: I am not sure. But I think I would like to discuss the matter with some creature who knows a great deal about it, and might answer some questions that have arisen in my mind.

TYRANNOSAURUS: That can easily be arranged. Rensselaerina has just returned to us after a six-month sojourn in the office of Professor Alderdice, who holds the chair of evolutionary biology at the university near here. During that time, she must have overheard many learned discussions on the subject of evolution, and learned a great deal about the matter thereby.

RENSSELAERINA: That is certainly true. In Professor Alderdice's office, an hour rarely goes by without evolution being mentioned half a dozen times. If mere data are what you desire, Cephalaspis, I can certainly provide them.

CEPHALASPIS: You relieve my mind, Rensselaerina. Let me begin, then, by asking what we mean when we speak of "evolution".

RENSSELAERINA: We mean the theory that the vast diversity of forms that life takes on Earth (a fact of which we ourselves are evidence) is due to the accumulation of the changes introduced into species by mutation. Every so often, new attributes appear within the population of a species; most vanish after a few generations, or else recur only spasmodically, but some persist, and may even flourish. Over time, individuals are born with so many of these divergent attributes that they can no longer be said to be members of the original species. It is this that we mean when we speak of the evolution of a new species from an old.

CEPHALASPIS: Fine! Your answer is quite clear and lucid, Rensselaerina. There is only one aspect of it that troubles me.

RENSSELAERINA: And what is that, Cephalaspis?

CEPHALASPIS: You have spoken several times of evolution as being defined in terms of species. As you say, a new species evolves from an old. And this, I believe, is in accord with the traditional thinking on the matter, for I seem to remember that the text in which the human Darwin first proposed the theory was entitled, On the Origin of Species.

RENSSELAERINA: Just so.

CEPHALASPIS: It seems, then, that the existence of different species is the fact that the theory of evolution endeavors to explain.

RENSSELAERINA: It certainly does seem so.

CEPHALASPIS: And that, if the term "species" cannot clearly be defined, the theory of evolution ceases to be useful – for one can scarcely discuss different forms of a thing when one does not know what the thing itself is.

RENSSELAERINA: That is quite true, Cephalaspis.

CEPHALASPIS: Well, then, how do those who discuss evolution define the term "species"?

RENSSELAERINA: To be truthful, Cephalaspis, I am not quite sure. I know that they speak of something called "taxonomy", which is a system of classifying living creatures according to their various attributes. Thus, for instance, you and I both belong to the kingdom of animals, but within that taxon you belong to the phylum of chordates, while I belong to the phylum of brachiopods. In this system, a species is the smallest significant taxon, and so I conclude that the taxonomists must have some understanding of what a species is – but just what that understanding is, I am unable to say.

CEPHALASPIS: Well, then, let us leave their understanding aside for the moment, and see how far our own will take us. You say that there is a system by which creatures are grouped into species?

RENSSELAERINA: Yes, and many larger groups, each of which contains all the smaller ones. For instance, a genus may contain several species, and a family may contain several genera.

CEPHALASPIS: And this system, you say, is based on the attributes of each living thing. The species of a creature, then, is in some way related to its attributes.

RENSSELAERINA: Yes, that is so. Indeed, I should almost have said that a species is simply the group of all living creatures that possess every attribute in a given list, but I fear that that will not answer.

CEPHALASPIS: Certainly, such an answer would be quite unworthy of you, Rensselaerina. For it is evident that many of the attributes that would define a species are attributes that can be lost over the course of life. For instance, it is an attribute of most spiders to have eight legs, but it is quite possible for a given spider to be injured and lose one. But you would not then say that that unfortunate creature's species had been changed.

RENSSELAERINA: No, Cephalaspis, you are quite right. I should have to say that a species is the group of all creatures that begin life with certain attributes, rather than only those that possess them at any given moment.

CEPHALASPIS: Indeed, that answer does seem to solve the problem of injury. But does it not create problems of its own? Consider that, when you defined the theory of evolution, you spoke of it as being due to the accumulation of mutations.

RENSSELAERINA: So I did. I am quite sure that that is how the humans believe evolution to occur.

CEPHALASPIS: I should not dream of doubting you, Rensselaerina, for you doubtless know more about the matter than I. But is it not true that a single mutation can change an essential attribute of a species?

RENSSELAERINA: Not only can it, but very frequently it does.

CEPHALASPIS: And mutation occurs at conception, so that the mutated creature begins life with the changed attribute.

RENSSELAERINA: Of course.

CEPHALASPIS: Then, according to the definition you give of the species, a single mutation can suffice to place a creature in a different species from its parent. But you have specifically said that there must be an accumulation of mutations before a new species arises. It seems that you have contradicted yourself, and that therefore either your definition of species or your understanding of the theory of evolution must be faulty.

RENSSELAERINA: I cannot dispute what you say, Cephalaspis. And since I am certain that my understanding of the theory of evolution is correct in this matter, I must allow that there is a flaw in my definition of the species.

CEPHALASPIS: If that is your belief, Rensselaerina, then it seems that we must continue our search for a definition if we are to understand evolution. Let us consider, then: why should a mutant not be considered a member of a new species, supposing its mutation to be sufficiently great?

RENSSELAERINA: I suppose because such mutations are not necessarily propagated to the offspring. As I have said, many mutations recur only spasmodically, if at all, among the mutant's descendants. Albinism, for instance, is a mutation consisting in the lack of all skin pigmentation, but the descendants of an albino may well have pigmented skin. Such descendants would then possess all the attributes of the albino's ancestral species, and would therefore be members of that species – and, if the albino is descended from members of the species, and can in turn propagate members of the species, it seems that it must itself be a member of the species.

CEPHALASPIS: Well said, Rensselaerina! It seems that we must modify our previous statement. We said that the species of a creature is related to its attributes – but now it seems that it is rather related to its ability to propagate those attributes to its offspring.

RENSSELAERINA: That seems reasonable, Cephalaspis.

CEPHALASPIS: But let us pause a moment. What of creatures that are naturally infertile? They cannot propagate any attributes to their offspring, for the simple reason that they cannot have any offspring. Must we say, then, that they belong to no species?

RENSSELAERINA: I do not think there is much difficulty there, Cephalaspis. After all, we are discussing evolution, and evolution is concerned chiefly with living creatures propagating themselves. A creature that cannot reproduce is of little moment in such a connection.

CEPHALASPIS: I fear, Rensselaerina, that you have misunderstood the point at issue. The theory of evolution, to be in any way useful, depends on the definition of the species. It is that definition that we are now discussing. Evolution cannot be called upon to assist us, for, if we define evolution in terms of the species and the species in terms of evolution, then we can understand nothing of either.

RENSSELAERINA: You are quite right, Cephalaspis. I confess that I was in error. We are to consider, then, merely the question of how an infertile creature can be considered part of a species, if we define a creature's species in terms of its possible offspring?

CEPHALASPIS: That is the issue, as I see it.

RENSSELAERINA: Then I should say that, to answer that question properly, we must introduce the idea of heredity. You are doubtless familiar with that idea, Cephalaspis?

CEPHALASPIS: I believe that I have some understanding of it, but I should be grateful if you would explain its precise nature.

RENSSELAERINA: Well, as I understand it, to speak of heredity is to say that every living thing possesses certain innate qualities, known as genes, that dictate roughly what attributes it shall possess (though these may sometimes be obscured by the accidents of the creature's environment), and also what attributes it may transmit to its offspring. Even if the creature should prove, through an accident of birth or a later injury, to be incapable of producing young, its genes remain its own.

CEPHALASPIS: I see. You would say, then, that the essence of a creature's species is to be found not in its external attributes, but in its genes?

RENSSELAERINA: I believe that is so, Cephalaspis. It is a gene, after all, that enables the albino to transmit the attribute of pigmentation to its offspring, though it does not itself possess that attribute.

CEPHALASPIS: You seem, then, to be proposing to define the species as the group of all living creatures that possess the genes that could enable them to transmit certain attributes to their offspring.

RENSSELAERINA: That is correct.

CEPHALASPIS: In that case, I see only one possible difficulty. You would assert, I think, that a living creature can only belong to one species?

RENSSELAERINA: Most assuredly I would, Cephalaspis.

CEPHALASPIS: Then let us consider the albino again. It differs from its parent in possessing a new gene, not possessed by that parent, which renders it void of pigmentation. Is that so?

RENSSELAERINA: Quite so, Cephalaspis.

CEPHALASPIS: And, because it possesses that gene, it can transmit the attribute of albinism to its own offspring.

RENSSELAERINA: Indeed it can.

CEPHALASPIS: But it also possesses, in some form, the gene of pigmentation, for we have already stated that it can also transmit that attribute to its offspring.

RENSSELAERINA: That is so.

CEPHALASPIS: Then let us suppose that a learned human were to take the list of the defined attributes that pertain to the albino's parent's species; strike the attribute of pigmentation therefrom; introduce, in its place, the attribute of albinism; and declare the resulting list to constitute the attributes of a new species, closely related to the original, but nonetheless distinct.

RENSSELAERINA: Why should a learned human do that, Cephalaspis? Albinism is quite a common mutation, and never has it been argued that it constitutes a difference in species.

CEPHALASPIS: It is not with the motives of the action that I am concerned, but simply with its possibility. Under our most recent definition of the species, is such an action possible or impossible?

RENSSELAERINA: I suppose that it is possible.

CEPHALASPIS: Were such a species to be identified, then, it seems that the albino that we have been considering would belong to two different species: that of its parent, and also the new species. Both species would be defined by lists of significant attributes, all of which the albino would possess the necessary genes to transmit to at least one of its offspring. Which seems to contradict the principle that each living creature must belong to no more than one species.

RENSSELAERINA: Certainly, Cephalaspis, such a state of affairs would be quite intolerable.

CEPHALASPIS: It seems, then, that we must establish some sort of rule to regulate the construction of a species' list of attributes, so as to remove the possibility of such an occurrence.

RENSSELAERINA: We must, indeed. Moreover, I believe I see how this may be done.

CEPHALASPIS: Excellent! Tell us, Rensselaerina.

RENSSELAERINA: It is quite simple. If we desire to avoid having the defining genes of two different species exist in the same living creature, we must require that every pair of species must be defined, at least in part, in terms of mutually incompatible genes – genes that cannot coexist in the same creature.

CEPHALASPIS: A most interesting notion, Rensselaerina. You would desire, then, that, before a learned human could announce the discovery of a new species, he should first be obliged to construct a list of all the defining attributes of this species, and to verify that at least one of these attributes was genetically incompatible with an attribute of every other species known?

RENSSELAERINA: That is essentially my idea, Cephalaspis. Nor do I believe that this would be unduly burdensome to the learned humans, for I would also desire that such lists exist for all the taxonomic ranks above the species: for all kingdoms, all phyla, and so on. The various kingdoms would be defined in terms of the most fundamental incompatibilities; each phylum of a given kingdom would contain the kingdom's attributes in its definition, together with other attributes based on less fundamental incompatibilities; and so on. Thus, when a new species were discovered in a genus already known, the list that existed of the genus's defining attributes would suffice to distinguish the species from all species in other genera, and the only attributes that would need to be specifically defined would be those that distinguished the species from others in its own genus.

CEPHALASPIS: Well, then, it seems to me, Rensselaerina, that, if what you have described is, indeed, possible, you have answered the question of what a species is, and we may begin to contemplate the subject of evolution.

RENSSELAERINA: Alas, Cephalaspis, I fear that it will be difficult to contemplate evolution based solely on what I have said. I do not in the least understand the nature of genes; I do not know whether their construction is such as to make what I have described possible, nor, if it is possible, what sort of attributes would result from mutually exclusive genes. I feel certain that what I have said is true, but I do not know what it means.

CEPHALASPIS: Indeed? That is most unfortunate. I suppose, then, that we must call upon some fossil who has had occasion to learn the nature of genes, and who may be able to enlighten us as to the meaning of your words.

TYRANNOSAURUS: So we must, Cephalaspis, and we shall. But this will have to wait for another evening – for time passes quickly for us who are preserved in stone, and the morning sun is already beginning to rise. Let us therefore fall silent once again, as befits fossils when living minds are present.

ALL: Indeed, let it be so.