Corporations for the construction and operation, or the operation alone, of steam railways, interurban railways, and street railways, or for the transaction of the business of life insurance, may be formed to endure fifty years; those for other purposes, not to exceed twenty years; provided, however, that in addition to the power herein granted to incorporate for a period of years, corporations hereafter organized or now existing may have perpetual existence by so providing in the articles of incorporation or by amendment thereto pursuant to section 491.20.