Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 5
रुचिर्यो भगवान् ब्रह्मन्दक्षो वा ब्रह्मण: सुत: ।यथा ससर्ज भूतानि लब्ध्वा भार्यां च मानवीम् ॥ ५ ॥
rucir yo bhagavān brahmandakṣo vā brahmaṇaḥ sutaḥyathā sasarja bhūtānilabdhvā bhāryāṁ ca mānavīm
O holy sage, tell me how the worshipful Ruci and Dakṣa, the son of Brahmā, generated children after securing as their wives the other two daughters of Svāyambhuva Manu.
All the great personalities who increased the population in the beginning of the creation are called Prajāpatis. Brahmā is also known as Prajāpati, as were some of his later sons. Svāyambhuva Manu is also known as Prajāpati, as is Dakṣa, another son of Brahmā. Svāyambhuva had two daughters, Ākūti and Prasūti. The Prajāpati Ruci married Ākūti, and Dakṣa married Prasūti. These couples and their children produced immense numbers of children to populate the entire universe. Vidura’s inquiry was, “How did they beget the population in the beginning?”