And God blessed them, saying,
Be fruitful, and multiply, and fill the waters in the seas, and let fowl multiply in the earth.
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he blessed them: The blessing of God gives power to multiply by generation, and, could be understood of the beasts of the earth, without requiring to be repeated. The blessing, however, is repeated in the case of man, since in him generation of children has a special relation to the number of the elect [Cf. Augustine, Gen. ad lit. iii, 12], and to prevent anyone from saying that there was any sin whatever in the act of begetting children. As to plants, since they experience neither desire of propagation, nor sensation in generating, they are deemed unworthy of a formal blessing.
Before sin matrimony was instituted by God, when He fashioned a helpmate for man out of his rib, and said to them: "Increase and multiply." And although this was said also to the other animals, it was not to be fulfilled by them in the same way as by men. As to Adam's words, he uttered them inspired by God to understand that the institution of marriage was from God.