In the genus

that combines all viviparous quadrupeds are many
species, but under no common appellation. They are only named as it
were one by one, as we say man, lion, stag, horse, dog, and so on;
though, by the way, there is a sort of genus that embraces all
creatures that have bushy manes and bushy tails, such as the horse,
the ass, the mule, the jennet, and the animals that are called Hemioni
in Syria,-from their externally resembling mules, though they are
not strictly of the same species. And that they are not so is proved
by the fact that they mate with and breed from one another. For all
these reasons, we must take animals species by species, and discuss
their peculiarities severally'