It is often a very difficult matter to determine when a foreign corporation may be said to be "doing business " within the State, as a corporation, or simply engaged in individual interstate commercial transactions in the State.

The courts have not been able to lay down any general rule for determining this question, but have been compelled to decide each case upon its own merits or facts. The more important of these specific adjudications will be discussed in the next sections, in which will be considered the taxing powers of the States with reference to interstate commerce.