It shall be the duty of the county highway commission and said commission shall have the power to locate, lay out, designate, construct and maintain, subject to approval of the state highways and transportation commission, a system of county highways not exceeding in the aggregate at any given time one hundred miles in any county, by connecting by the most practical route the several centers of population in the county, in such manner as to afford a connection with such of said centers of population as are not now located on any state highway with such state highway, and so as to afford, as nearly as may be done, a connection with county highways connecting the centers of population of adjoining counties, to the end that all parts of the county shall be connected with the state highway system as now laid out and designated, and that the inhabitants of the county generally shall have and enjoy a system of highly improved farm-to-market roads. If any part of this county one hundred mile highway system has been, or shall hereafter be taken over by the state highways and transportation commission and become a state highway, then an equal amount of new mileage, to take the place thereof, may be placed in the county one hundred mile system.