SECTION 7 REGISTRATION. The legislature shall enact a registration law, and shall require a compliance with such law before any elector shall be allowed to vote; Provided, that this provision is not compulsory upon the legislature except as to cities and towns having a population of over five hundred inhabitants. In all other cases the legislature may or may not require registration as a pre-requisite to the right to vote, and the same system of registration need not be adopted for both classes.