"SECTION 1. Any person who within a period of ten years from the date of his release or of his last conviction by the courts of this country of the crimes of robo, hurto, estafa, embezzlement, or forgery, is found guilty of any of said crimes a third time, or oftener, shall be deemed a habitual criminal and shall be sentenced as follows:
"(a) Upon a third conviction of any of said crimes he shall be sentenced to the penalty provided by law for the last crime committed and, in the discretion of the court, to an additional penalty of not less than three nor more than nine years of imprisonment;
"(b) Upon a fourth conviction of any of said crimes he shall be sentenced to the penalty provided for the last crime committed and, in the discretion of the court, to an additional penalty of not less than ten nor more than fifteen years of imprisonment;
"(c) Upon a fifth conviction of any of said crimes he shall be sentenced to the penalty provided for the last crime committed and, in the discretion of the court, to an additional penalty of not less than sixteen nor more than twenty years of imprisonment; and
"(d) Upon a sixth or additional conviction of any of said crimes, he shall be sentenced to the penalty provided for the last crime committed and, in the discretion of the court, to an additional penalty of not less than twenty-one nor more than thirty years of imprisonment: Provided, however, That in none of the cases provided for by this Act shall the total imprisonment imposed upon a habitual criminal exceed thirty years."