Charles Ellicott Commentary Genesis 1:23

"And there was evening and there was morning, a fifth day." — Genesis 1:23 (ASV)

The fifth day. —Upon the work of the first four days, geology is virtually silent, and the theories regarding the physical formation of the world belong to other sciences. But regarding the fifth day, its testimony is ample. In the lowest strata of rocks, such as the Cambrian and Silurian, we find marine animals, mollusca, and trilobites; higher up in the Devonian rocks, we find fish; in the Carbonaceous period, we find reptiles; and above these, in the Permian, those mighty saurians, described in our version as great whales.

Traces of birds, even in these higher strata, if existent at all, are rare, but indubitably occur in the Triassic series. We thus learn that this fifth day covers a vast space of time, and, in accordance with what has been urged before regarding vegetation, it is probable that the introduction of the various genera and species was gradual. God does nothing in haste, and our conceptions of His marvellous working are made more clear and worthy of His greatness by the evidence which geology affords.