SB 12.2.39

kṛtaṁ tretā dvāparaṁ ca
 kaliś ceti catur-yugam
anena krama-yogena
 bhuvi prāṇiṣu vartate
kṛtam — Satya-yuga; tretā — Tretā-yuga; dvāparam — Dvāpara-yuga; ca — and; kaliḥ — Kali-yuga; ca — and; iti — thus; catuḥ-yugam — the cycle of four ages; anena — by this; krama — sequential; yogena — pattern; bhuvi — in this world; prāṇiṣu — among living beings; vartate — goes on continuously.
The cycle of four ages — Satya, Tretā, Dvāpara and Kali — continues perpetually among living beings on this earth, repeating the same general sequence of events.