Said R' Yehoshua, “I have heard that the substitute of a pesach is offered, and that the substitute of a pesach is not offered, but I cannot explain [it].”  Said R' Akiva, “I will explain”: [If] the [original] pesach-offering was found before the slaughtering of the [substitute] pesach, it must be left to pasture until it develops a blemish, is [then] sold, and a peace-offering is bought with its proceeds, and so is [the rule regarding] its substitute. [But if it was found] after the slaughtering of the [substitute] pesach, it is offered as a peace-offering, and so is [the rule regarding] its substitute.