Page "Genesis Rabbah" Paragraph 11

But with the notoriously loose construction of the haggadic exegesis it became easy to string together, on every verse or part of a verse, a number of rambling comments ; or to add longer or shorter haggadic passages, stories, etc., connected in some way with the exposition of the text.

This process of accretion took place quite spontaneously in Genesis Rabba, as in the other works of the Talmudic and midrashic literature.

Between the beginning and the completion of these works — if ever they were completed — a long period elapsed during which there was much addition and collection.