Notwithstanding anything to the contrary contained in the provisions of this Act relating to the computation of income chargeable under the head "Interest on securities", "Income from house property", "Capital gains" or "Income from other sources", or in section 199 or in sections 28 to 43B, the profits and gains of any business of insurance, including any such business carried on by a mutual insurance company or by a co-operative society, shall be computed in accordance with the rules contained in the First Schedule.