Mbh 3.148.1c

And men practise truth, and devote themselves to religion and religious rites. And thence sacrifices and various religious observances come into existence. And in the Treta Yuga people begin to devise means for the attainment of an object; and they attain it through acts and gifts. And they never deviate from virtue. And they are devoted to asceticism and to the bestowal of gifts. And the four orders adhere to their respective duties; and perform rites. Such are the men of the Treta Yuga. In the Dwapara Yuga, religion decreaseth by one half. And Narayana weareth a yellow hue. And the Veda becometh divided into four parts. And then some men retain (the knowledge of) the four Vedas, and some of three Vedas, and some of one Veda, while others do not know even the Richs. And on the Shastras becoming thus divided, acts become multiplied. And largely influenced by passion, people engage in asceticism and gifts. And from their incapacity to study the entire Veda, it becomes divided into several parts. And in consequence of intellect having decreased, few are established in truth. And when people fall off from truth, they become subject to various diseases; and then lust, and natural calamities ensue. And afflicted with these, people betake themselves to penances. And some celebrate sacrifices, desiring to enjoy the good things of life, or attain heaven. On the coming of the Dwapara Yuga, men become degenerate, in consequence of impiety. O son of Kunti, in the Kali Yuga a quarter only of virtue abideth. And in the beginning of this iron age, Narayana weareth a black hue. And the Vedas and the institutes, and virtue, and sacrifices, and religious observances, fall into disuse. And (then) reign iti[41], and disease, and lassitude, and anger and other deformities, and natural calamities, and anguish, and fear of scarcity. And as the yugas wane, virtue dwindles. And as virtue dwindles away, creatures degenerate. And as creatures degenerate, their natures undergo deterioration.