Complex sentence: Up to and including the 1880 census, the country had a frontier of settlement, but, by 1890, the unsettled area had been broken into by isolated bodies of settlement to the extent that there was hardly a frontier line.
Split into simpler sentences: Up to and including the 1880 census, the country had a frontier of settlement. By 1890, isolated bodies of settlement had broken into the unsettled area to the extent that there was hardly a frontier line.