Karmaṇā daiva-netreṇa jantur deha upapatti (SB 3.31.1). By karma everyone is working, but even (everyone) is working in three modes of nature. This material world, there are three modes of nature: sattva-guṇa, rajo-guṇa, tamo-guṇa. So if you are working in tamo-guṇa, then you are preparing a body of tamo-guṇa. If you are working rajo-guṇa, then you are preparing a body for rajo-guṇa. Similarly, sattva-guṇa. Everything is described in the Bhagavad-gītā. What is the different result? Ūrdhvaṁ gacchanti sattva-sthāḥ. Those who are working in sattva-guṇa, they will go to the higher planetary systems. There are Janaloka, Maharloka, Tapoloka, Brahmaloka, Satyaloka, Siddhaloka. You'll go there. Ūrdhvaṁ gacchanti sattva-sthā madhye tiṣṭhanti rājasāḥ (BG 14.18).
And if you want to remain in the middle planetary system, then that is rajo-guṇa. And jaghanya-guṇa-vṛtti-sthā adho gacchanti tāmasāḥ. And those who are working in tamo-guṇa, jaghanya, most abominable behavior, they will go to the animal kingdom or the lower planetary system. This is the process. As you are contaminating, kāraṇaṁ guṇa-saṅgo 'sya sad-asad-janma-yoniṣu (BG 13.22). Sat and asat-janma. Why there are so many varieties of life? Cats, dogs, trees, aquatics, birds, beasts, human beings, demigods, civilized, uncivilized, so many. Why there are so many varieties? Kāraṇaṁ guṇa-saṅgo 'sya. As he is associating with different types of qualities, he is getting a different types of body. This is going on.