Members of the House of Representatives were to be elected directly by the people and the members of the senates were supposed to b elected indirectly by the people. A direct election is a principle of a democratic form of representation while a indirect election is a principle of a republican form of government.
The direct election of members of the House and Senate creates a democracy where a tyranny of the majority is inevitable. We need the representatives in the senate to be chosen by and accountable to the state legislators. In this way the interests of the states and the rights of the people can be protected from the abuse of power by the central government.