Thus, in an isolated system, conservation of total energy and conservation of total mass each still holds in modern physics, as the same (apparently) separate laws that these appeared to be, in the ninteenth century.
Thus, in an isolated system, conservation of total energy and conservation of total mass over time, each still holds in modern physics. When stated in terms of mass and energy, they remain in modern physics as the same (apparently) separate laws that they appeared to be in the ninteenth century.