The various classes into which the gods are divided, are definitely fixed by the schools of theology that, as we shall see, take their rise in the Euphrates Valley.
The rivalry, on the one hand, between the Babylonian empire united under one head, and the Assyrian empire on the other, alone remains to bring about an occasional exchange of places between the two gods who stand at the head of the great gods of the Babylonian and Assyrian pantheon respectively.

The attempt has been made by Amiaud[113] to arrange the pantheon of this oldest period in a genealogical order.

In Gudea's long list of deities, he detects three generations,--the three chief gods and one goddess, as the progenitors of Sin, Shamash, Nin-girsu, Bau, and others.

The gods of this second division give rise to a third class, viewed again as the offspring of the second.

Professor Davis, taking up this idea of Amiaud, has quite recently maintained[114] that the family idea must form our starting-point for an understanding of the pantheon of Lagash.