You must have heard that Dharm stands with his four legs in Sat Yug, with his three legs in Tretaa Yug, with his two legs in Dwaapar Yug and with his one leg only in Kali Yug. Have you ever thoght why does it happen so? This is because Dharm is cursed to be like this. Whose curse it was? Let us read this story.
There was a Muni Pippalaad. He was the son of Rishi Dadheechi. You must have heard the name of Dadheechi also. He is the same Rishi who gave his bones to Indra to make Vajra (thunderbolt) to kill Vritraasur. Pippalaad Muni was the incarnation of Rudra. Dadheechi performed a severe penance and asked Shiv to be born as his son.
When Dadheechi gave his bones to Devtaa, his wife was not there. When she came back to the Aashram and found her husband dead, she got very angry. She cursed Indra and other Devtaa; got pyre ready to become Satee. The then a Divine voice said - "You are carrying Rishi's child so do not think of being Satee." But she wanted to go to her husband, so she tore her stomach and the embryo came out. As it came out it was shining like Sun in Tri-Lok. When she came to know that he was Rudra's incarnation, she offered her prayers to him, left him near a fig tree and went to her husband. Brahmaa Jee named him Pippalaad because he was born under a Peepal tree.
Once Pippalaad was taking bath in Pushpbhadraa River, that a he saw a beautiful princess Padmaa, incarnation of Paarvatee. He got attracted to her and asked her hand from her father. First the king did not want to marry his daughter to that old Rishi, but seeing his Tej he married his daughter to him. Padmaa was very beautiful and a very Pativrataa woman.
One day Dharm Dev wanted to test the Paativrat Dharm of Padmaa. So he came to her near the river and tried to distract her pointing out again and again the old age of Rishi. At this Padmaa got annoyed and cursed Dharm to diminish soon. Hearing this curse Dharm Dev appeared in his real form. Seeing Dharm Dev, Padmaa asked for his apology and told him that she cursed him unknowingly, but she could not take her curse back. She then modified it - "In every Yug one part of Dharm will be destroyed, thus in Sat Yug Dharm will be present in all the four forms, in Tretaa Yug Dharm will be present in three forms, in Dwaaapar Hug Dharm will be present with two forms and in Kali Yug it will be present only in one form."
That is why Kali Yug is so short of Dharm. Dharm was satisfied with Padmaa's Paativrat Dharm. he blessed her with 10 sons and made her husband a handsome youth. Both .lived for long time enjoying their life. They had 10 sons and all of them were great sages like their father.