Like the Swiss tribes, the early inhabitants of Mexico depended much upon their fishing, for which their position gave them great facilities.
If you look at the arms of the Mexican Republic, on a passport or a silver dollar, you will see a representation of a rock surrounded by water.

On the rock grows a cactus, and on the cactus sits an eagle with a serpent in his beak.

The story is that the wandering tribe preserved a tradition of an oracle which said that when they should find an eagle, holding a serpent, and perched on a cactus growing out of a rock, then they should cease their wanderings.

On an island in the lake of Tezcuco, they found eagle, serpent, cactus, and rock, as described, and they settled there in due course.

What fragment of truth is hidden in this myth it is hard to say.