At the time of Federation, the colonies' main source of revenue consisted of customs and excise duties (income tax being still a newer notion). Since one of the main reasons for Federation was to create a common market, inevitably authority over these taxes was vested exclusively in the Commonwealth Parliament (section 90). It was acknowledged that this would create a situation where the Commonwealth would raise much more money than it could spend, whereas the States, being still responsible for most areas of law and of social infrastructure, would need to spend much more money than they could raise (the problem now known as "vertical fiscal imbalance"). Although the framers were able to agree on a formula for distribution of the Commonwealth's surplus to the States in the first few years after Federation, they could not agree on a long-term formula. Accordingly, section 96 of the Constitution provides that the Commonwealth Parliament "may grant financial assistance to any State on such terms and conditions as it thinks fit". One result of this has been that the Commonwealth has been able to make grants to the States on terms so specific as to amount to the virtual takeover of particular fields of competence. For instance, although the Constitution gives the Commonwealth no express power over education, by means of "tied grants" it has in fact become paramount in the field of tertiary education. Although any state has the option to refuse a grant, the consequences of doing so make this unattractive. Similarly, the Commonwealth has become dominant in the field of public hospitals, and a major player in the field of roads and other major infrastructure. The Commonwealth has also come to monopolise income tax. Once the advantages of income tax were recognised, both the Commonwealth and the States levied income taxes. However, during World War II, the Commonwealth government decided to take over the collection of income taxes and return some proceeds to the States as grants. The Commonwealth passed legislation to levy income tax at a nationwide rate similar to the previous combination of Commonwealth tax and the various state taxes. Separate legislation then granted section 96 monetary grants to states if the State did not levy income taxes. In practice, it would be difficult for States to continue taxing. This arrangement was twice challenged by the States in the High Court and twice upheld. In the Second Uniform