Charles Ellicott Commentary Genesis 1:21

"And God created the great sea-monsters, and every living creature that moveth, wherewith the waters swarmed, after their kind, and every winged bird after its kind: and God saw that it was good." — Genesis 1:21 (ASV)

God created great whales. —Whales, strictly speaking, are mammals and belong to the creation of the sixth day.

But tannin, the word used here, means any long creature, and is used of serpents in Exodus 7:9-10 (where, however, it may mean a crocodile), and in Deuteronomy 32:33; of the crocodile in Psalms 74:13, Isaiah 51:9, Ezekiel 29:3; and of sea monsters generally in Job 7:12. It thus appropriately marks the great Saurian age.

The use, too, of the verb bârâ, “he created,” is no argument against its meaning to produce out of nothing, because it belongs not to these monsters, which may have been “evolved,” but to the whole verse, which describes the introduction of animal life; and this introduction of animal life is one of the special creative acts that physical science acknowledges to be outside its domain.

After their kind. —This suggests the belief that the various genera and species of birds, fishes, and insects were from the beginning distinct, and will continue so, even if there is some amount of free play in the improvement and development of existing species.