One might also prove that none of the things constituted by nature comes to be or has come to be by reference to a model by the following argument. If the world is eternal, and if none of the things that now come to be in accordance with nature comes to be by reference to an Idea as model, then neither would anything have come to be in this way in the past. But the world is indeed eternal as they say; and none of the things that now come to be in accordance with nature comes to be by reference to an Idea as  model, as I shall prove. Therefore, neither did any of the things that came to be naturally in the past come to be as [if] by reference to a model; for all things always come to be in the same way in the world, since it is eternal. But if this is so, the Ideas would not be models.
[103,12] But that none of the things that now come to be naturally comes to be by reference to an Idea [as] model is clear from the following. All the things that come to be are particular instances of particular things that exist, and are generated by a particular instance of particular things, so that things [generated] naturally also [come to be in this way]: this man, for instance, is naturally generated by this man, and this horse by this horse, and this vine by this vine. But surely each of the things too that produce something naturally does so in virtue of the nature in it, i.e. in virtue of the cause and principle and power in it to move in some way. For those things both exist naturally and have a nature that have a principle of movement within themselves inasmuch as they are also these particular things, but not per accidens; and the horse in fact produces the horse in virtue of this sort of principle in it. But certainly neither a man nor a horse nor any other of the things that generate and produce naturally produces the thing it generates while looking to a model; for it is not by giving any thought whatever to how the thing that is to be should come to be that they set about producing what they produce, as we see in the case of the arts. For it is for this reason that the arts are only in those capable of exercising this kind of foresight over the things that will be as a result of art, for they are in men; but many things are generated in accordance with nature both by irrational and even by inanimate things.
Therefore, none of the things that now produce something in accordance with nature does so by reference to a model; but if not now, not in the past either, for the generation of things that come into being naturally is always similar.
[103,31] There is both truth and falsity in saying that the things that come to be naturally do so according to certain numbers, determinate and arranged in a fixed order (tetagmenos) (for they do not come to be as a result of chance or spontaneity, for it is for this reason that a man generates a man), and in thinking on this account that nature produces its effects by reference to a kind of model or plan (logos) while looking to this latter. For all natural things come to be according to a certain order and certain determinate numbers, and not by chance or spontaneity, but surely this does not mean that they also come to be by reference to a model. For it is not by reflecting (ennoein) that nature produces what it does (for it is an irrational power), but it is responsible for the fact that [generation] takes place in an orderly progression of movements, so that a first movement is followed in orderly sequence by a second, although not as the result of any reasoning process, and this second movement is followed in turn by a third, until the movements have progressed to the end for the sake of which they occurred. It is this order that art imitates, for it puts things together in a rational way and [thus] produces its object. Hence art is a rational power,  but nature is irrational. But it is not correct to say that nature, being a kind of divine art, produces nothing irrationally, nor to think that, because it is divine, it possesses from the gods this gift of producing what it does [by looking] to some fixed and determinate model. For it is not in this way that nature is called a divine art, as if the gods were employing this art, but because, being a power from the gods, it is capable of preserving the right order of movement according to a certain harmonious sequence, not in virtue of any reasoning process of thought (noesis), but because it is from the gods. But the gift it possesses from the gods could not be that of producing by reference to a model, for how could it produce anything by reference to a model of which it has no knowledge whatever?
[104,12] One might, however, more reasonably say that nature has from the gods this similarity to the [rational] agent, in whom the thing coming to be exists and from whom it takes its origin, that nature produces according to the regular order (eutaxia) of movements that is without reason. Again, it is possible to discover [this] regular order even in evil things and in those that come to be in a way contrary to nature, such as abscesses, wounds, boils, and periodic illnesses. But the generations of certain living things too are in fact orderly, but not by reference to an Idea, those e.g. of worms, gnats, and grubs.