These neat symmetries, you begin to realize, turn up everywhere, as a well-known passage from Book 3 makes clear:

Divine providence in its wisdom created all creatures that are cowardly and that serve as food for others to reproduce in great numbers so as to assure that some would be left despite the constant consumption of them, while it has made sure that those animals which are brutal and aggressive predators reproduce very few offspring. The hare, for example, is hunted by every kind of beast, bird, and man, and so reproduces prolifically. Of all animals, she is the only one that conceives while she is already pregnant. . . . But the lioness, since she is the strongest and boldest of animals, gives birth to only one offspring in her entire life, for when she gives birth she expels her womb along with her young. . . . Likewise, if vipers and the Arabian winged serpents were to live out their natural life spans, humans could not survive at all.

For Herodotus, virtually everything can be assimilated into a kind of natural cycle of checks and balances. (In the case of the vipers and snakes he refers to, the male is killed by the female during copulation, but the male is “avenged” by the fact that the female is killed by her young.) Because his moral theme is universal, and because his historical “plot” involves a world war, Herodotus is trying to give you a picture of the world entire, of how everything in it is, essentially, linked.