Matsyapurana 39.19

39.19

puṇyāṃ yoniṃ puṇyakṛto viśanti pāpāṃ yoniṃ pāpakṛto vrajanti
kīṭāḥ pataṃgāśca bhavanti pāpān na me vivakṣāsti mahānubhāva //

Yayâti.—“Just as a sleeping animal maintains life, similarly the soul after vacating the corpse, enters another body according to its virtue or sin. A virtuous soul is reborn (as a sequence of its good actions), in the form of a pious being, and similarly a vicious soul takes its birth as a reptile or insect. It is not, for me, to enumerate the sins, the commitment of which leads the soul to be born as a reptile or an insect. Aṣṭaka! I have explained to you, briefly, the reasons of the soul becoming a quadruped, lower animal and man. Now tell me what more do you wish to ask me?"—18-20.