2? re Roman senate and plebeian power

Couple of things I have not understood from anything I've read. Anybody care to enlighten me?

By 715 BC, according to Livy (1.17), the Senate existed already existed during the time of the Kings; and, in fact, they gained the authority to ratify choice of King.

How did the Senate actually come into being? It seems a rather odd departure from other ancient political entities (other than maybe Greece), and yet the first time we hear of it, it is already full-formed.

The Plebeian Assembly gained ground, perhaps not steadily, but in a general trend, during the early and middle Republican days. It is not clear to me, really, how they accomplished this. There were two recorded secessions, but I have not read so far why these were effective at bending the patricians to their will. In various places I have read things implying that the plebeians "forced" the patricians to concede rights to them, or they waited out the patricians, but I have not read the actual mechanisms by which the plebeians were able to win these contests. I just don't get it - seems like the patricians could have put down the plebeians forcefully whenever they seemed to be getting too big for their britches.

Thanks.

Read Engels: "Family, Private Property and the State". He deals with the political structures of tribes and early city-states.

The Senate, as it's name implies means council of elders and is in origin a tribal element. The Senate is very influential, though the decissory power is in the comitiae: the assembly of the people/warriors.