Charles Ellicott Commentary Genesis 1:22

"And God blessed them, saying, Be fruitful, and multiply, and fill the waters in the seas, and let birds multiply on the earth." — Genesis 1:22 (ASV)

Be fruitful, and multiply. —This blessing shows that the earth was replenished with animal life from a limited number of progenitors, and probably from a small number of centres, both for the flora and for the fauna.