Is Seniority decided as per Merit or Date of Joining?

In a catena of decisions, the Hon'ble Supreme Court has made it clear that in case more than one candidate is selected, the seniority is as per ranking of the direct recruits in the merit list (of course, subject to the adjustment of the candidates selected on applying the rule of reservation).

However, simply reporting to duty early would not alter the ranking given in the merit list. 

Citation: 

1994 SCC (6) 301, 1994 SCALE (4)307