On the nature of things (books 4 - 6)   

Perforce there perished many a stock, unable
By propagation to forge a progeny.
For whatsoever creatures thou beholdest
Breathing the breath of life, the same have been
Even from their earliest age preserved alive
By cunning, or by valour, or at least
By speed of foot or wing. And many a stock
Remaineth yet, because of use to man,
And so committed to man's guardianship.
Valour hath saved alive fierce lion-breeds
And many another terrorizing race,
Cunning the foxes, flight the antlered stags.
Light-sleeping dogs with faithful heart in breast,
However, and every kind begot from seed
Of beasts of draft, as, too, the woolly flocks
And horned cattle, all, my Memmius,
Have been committed to guardianship of men.
For anxiously they fled the savage beasts,
And peace they sought and their abundant foods,
Obtained with never labours of their own,
Which we secure to them as fit rewards
For their good service. But those beasts to whom
Nature has granted naught of these same things-
Beasts quite unfit by own free will to thrive
And vain for any service unto us
In thanks for which we should permit their kind
To feed and be in our protection safe-
Those, of a truth, were wont to be exposed,
Enshackled in the gruesome bonds of doom,
As prey and booty for the rest, until
Nature reduced that stock to utter death.

But Centaurs ne'er have been, nor can there be
Creatures of twofold stock and double frame,
Compact of members alien in kind,
Yet formed with equal function, equal force
In every bodily part- a fact thou mayst,
However dull thy wits, well learn from this:
The horse, when his three years have rolled away,
Flowers in his prime of vigour; but the boy
Not so, for oft even then he gropes in sleep
After the milky nipples of the breasts,
An infant still. And later, when at last
The lusty powers of horses and stout limbs,
Now weak through lapsing life, do fail with age,
Lo, only then doth youth with flowering years
Begin for boys, and clothe their ruddy cheeks
With the soft down. So never deem, percase,
That from a man and from the seed of horse,
The beast of draft, can Centaurs be composed
Or e'er exist alive, nor Scyllas be-
The half-fish bodies girdled with mad dogs-
Nor others of this sort, in whom we mark
Members discordant each with each; for ne'er
At one same time they reach their flower of age
Or gain and lose full vigour of their frame,
And never burn with one same lust of love,
And never in their habits they agree,
Nor find the same foods equally delightsome-
Sooth, as one oft may see the bearded goats
Batten upon the hemlock which to man
Is violent poison. Once again, since flame
Is wont to scorch and burn the tawny bulks
Of the great lions as much as other kinds