Carry forward of speculative loss: permitted only against speculative business income and requires timely return filing. Losses from speculative business can be carried forward only if the return for the year in which the loss arose is filed on or before the due date under section 139(1); carried-forward speculative losses may be set off only against income from speculative business in subsequent years. Unabsorbed depreciation is governed by separate provisions and is not subject to the speculative loss set-off and carry-forward rules.