The specific importance of California to the North and South.
Explanation:
In any well-written text, there is a natural progression between one topic and the next. This ensures that a text is understandable and easy to follow. In this case, the author ends his paragraph by stating that California had many qualities which made it desirable for both North and South. Therefore, it is likely that he will go on to elaborate on this, and describe the specific importance of California to the North and South.
