mate (verb) — sense 5

“In fact, the apes live with us, and have for many ages. We call them the first men—we speak their language quite as much as we do our own; only in the rituals of the temple do we make any attempt to retain our mother tongue. In time it will be forgotten, and we will speak only the language of the apes; in time we will no longer banish those of our people who mate with apes, and so in time we shall descend to the very beasts from which ages ago our progenitors may have sprung.”