Srimad Bhagavatam Canto 12, Chapter 02, Text 39

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SB 12.2.39

krtam treta dvaparam ca
 kalis ceti catur-yugam
anena krama-yogena
 bhuvi pranisu vartate
 
Translation: 
 
The cycle of four ages — Satya, Treta, Dvapara and Kali — continues perpetually among living beings on this earth, repeating the same general sequence of events.
Srimad Bhagavatam Canto 12, Chapter 02, Text 38
Srimad Bhagavatam Canto 12, Chapter 02, Text 40