Where a bequest, devise, conveyance, transfer or gift is made to a class of persons subject to fluctuation by increase or diminution of its number in consequence of future births or deaths, and the time of payment, distribution, vestiture or enjoyment is fixed at a subsequent period or on the happening of a future event, and any member of the class dies before the arrival of that period or the happening of that event, and has issue surviving when the period arrives or the event happens, that issue shall take the share of the property that the member so dying would take if living, unless a clear intention to the contrary is manifested by the will, deed or other instrument.