Through Hera's agency, the goddess Ilithyia retarded Alcmena's delivery, and Eurystheus, who also was a Perseid, was born a seven-month child before Heracles 1.
Now, the words of gods differ from those of mortals in that neither intention nor deed are divorced from them, a circumstance or quality that some call integrity: thought, word and deed constituting what is integrated in harmonious oneness. That is why Zeus did not go against his own word, although he did seize Ate by her hair, and having whirled her round his head, cast her out from Heaven and down to earth, where she may still be found among men. Instead Zeus, wishing to take care of both word and son, persuaded Hera to agree that while Eurystheus should be king (for being the first born Perseid, as he had proclaimed), Heracles 1 would be allowed to serve him and perform twelve LABOURS, to be prescribed by Eurystheus himself. But that after he had performed them, Heracles 1 should be given immortality.
Previous differences on earth
This was the nature of the relationship that Heaven established between Eurystheus and Heracles 1. Before them, however, differences had aroused between Heracles 1's stepfather Amphitryon, and Eurystheus' father Sthenelus 3. The background of it all may be said to be the infiltration of the Pelopides, who succeeded, through Sthenelus 3 and Eurystheus, in replacing the dynasty of the Perseids on the throne of Mycenae. For although Eurystheus was a Perseid on his father's side, he opened the way for the dominance of the Pelopides, his mother being daughter of Pelops 1. The conflict expressed by Eurystheus and Heracles 1 continued after their departure from this world, and only ended when the Perseids, renamed HERACLIDES, returned to the Peloponnesus, and took possession of what they regarded as their legitimate inheritance.
Parada's scheme of the three key ancestors -- Deucalion, Atlas, and Io -- is summarized here. Euripides made the Heraclides the basis of his play about the children of Heracles.