Calvin believed that all matter was created by God on the first day of creation, but that the plants and animals were not created ex nihilo. They were formed by God from a welter of preexisting materials and so were organically and materially related both to each other and to the created universe as a whole. Readers who insisted in completely distinct creations out of nothing on each of the days within the creation week were engaged in intellectual sophistry, in Calvin's view. The word created, ought not to be read in an overly literal way as far as questions of chronology are concerned; creatures were, in effect, "created" before they were formed:
Those who assert that the fishes were created from nothing because the waters were in no way sufficient or suitable for their production are nevertheless resorting to rationalization, for the fact would remain that the material of which they were made existed before which, strictly speaking the word "created" does not admit. I therefore do not restrict the creation here spoken of to the work of the fifth day but rather suppose it to refer to that shapeless and confused mass that was in effect the fountain of the whole world.
Calvin's language is difficult, but he clearly rejected the idea that the creation in Genesis only describes events and not processes. He seems to have held a kind of hyperaccelerated emergent or even evolutionary view of what happened during the creation week:
[God] created the great creatures of the sea and other fishes—not that the beginning of their creation is to be reckoned from the moment in which they received their form but because of the universal matter that was made out of nothing. So with respects to species, form only was added to them; but creation is nevertheless a term truly used respecting both the whole and the parts.
What might Calvin say, then, about highly literalistic interpretations of Genesis today in the light of scientific evidences for a very old earth and common ancestry across diverse species? The clearest evidence is given by Calvin in his interpretation of Genesis 1:16. Calvin's concern was to make clear that the language of "greater" and "lesser" lights in Genesis in no way conflicted with contemporary astronomy calculations that showed that seemingly small stars and planets such as Saturn were actually much greater in size than the "great" light of the moon ordained by God to "govern the night." While this might seem like a trivial matter to us today, it was an unsettling challenge to the biblically inspired and geocentric cosmological theories of medieval and early Reformation Europe. Calvin's approach to the problem is simple: the words of Genesis are not to be taken literally. [Ronald E. Osborn, Death Before the Fall: Biblical Literalism and the Problem of Animal Suffering (Downers Grove, IL: IVP, 2014), 100-2]
According to Osborn, while John Calvin held to a 6-day creation, he nevertheless did not believe that one should interpret Genesis literally in all aspects. However, is that really what Calvin believe? I would contend not.
For his sources, Osborn cites a republished edition of Calvin's commentary on Genesis edited by Alister McGrath and J.I. Packer. I have no idea whether that is supposed to be the best edition of Calvin's commentary on Genesis, or whether it looks nicer and have good notes by McGrath which should supposedly help the reader. Nevertheless, I will use the old commentary and there should be little difference in content.
The key indicator that something is off is when we look at the context and the verse of which Calvin was commenting. The verse was Genesis 1:21, and before that was Genesis 1:20, where God said, "Let the waters swarm with swarms of living creatures, and let birds fly above the earth across the expanse of the heavens." The form in Hebrew is the 3rd person jussive form "Let X be/do." In other words, it is not a direct command but rather a command or permission using the agency of a 3rd party. This is the context for understanding why Calvin wrote what he did. In his commentary on verse 20, Calvin wrote, "should it not be lawful for him, who created the world out of nothing, to bring forth the birds out of water?" Calvin rightly saw the jussive form of the verb and therefore interpreted the text accordingly.
In verse 21, it is stated that God created the sea and flying animals, and it is in this context that Calvin "denigrated" the idea that this was a creation apart from means. Calvin rejected those who assert without proof that the waters were insufficient to produce life, because he had already seen in verse 20 the jussive form of the verb. In other words, Calvin rejected an im-mediate (i.e. without means) creation of the sea and flying animals purely because of the text, by sticking with the literal meaning of the text. Calvin clearly believed in creation ex nihilo, not for everything but for those in the text that are explicitly said to be created ex nihilo.
The divine mode of accommodation that Calvin advocated was that God intended to show it as from our point of view. In other words, the description of events was what an observer would see if he was on planet earth from its creation. It is not scientific language, and YEC has never claimed that Genesis 1 is a scientific text. Therefore, there is nothing wrong with God's description of the moon as the lesser light and the sun as the greater light, because that is how a person standing on the surface of the earth would have perceived it. Calvin was not engaging therefore in some kind of allegorical interpretation when he claimed that the language of "greater" and "lesser" had nothing to do with astronomy. He was clearly saying Genesis was descriptive language, not scientific language. As an aside, Osborn clearly should read more history and not parrot the false statement that geocentrism is a "biblically inspired claim."
Calvin interpreted the creation account literally, even in his rejection of the immediate creation of the sea and flying creatures. Earlier in his commentary, Calvin affirmed the teaching that God created the light and darkness before the sun and moon were created, and it is clear that the "before" is one of temporal chronology. As Calvin wrote,
It [was] proper that the light, by means of which the world was to be adorned with such excellent beauty, should be first created; and this also was the commencement of the distinction, (among the creatures.) It did not, however, happen from inconsideration or by accident, that the light preceded the sun and the moon. To nothing are we more prone than to tie down the power of God to those instruments the agency of which he employs. The sun and moon supply us with light: And, according to our notions we so include this power to give light in them, that if they were taken away from the world, it would seem impossible for any light to remain. Therefore the Lord, by the very order of the creation, bears witness that he holds in his hand the light, which he is able to impart to us without the sun and moon. Further, it is certain from the context, that the light was so created as to be interchanged with darkness. But it may be asked, whether light and darkness succeeded each other in turn through the whole circuit of the world; or whether the darkness occupied one half of the circle, while light shone in the other. There is, however, no doubt that the order of their succession was alternate, but whether it was everywhere day at the same time, and everywhere night also, I would rather leave undecided; nor is it very necessary to be known. [source]
We see therefore that Calvin interpreted Genesis 1 plainly, paying attention even to the forms of the verbs. Osborn thus misrepresents Calvin's exegesis of Genesis 1, and fails in his attempt to make Calvin amenable to the compromising positions held to by modern Man.