On the nature of things (books 4 - 6)   

Wherefore the more are they at pains to hide
All the-behind-the-scenes of life from those
Whom they desire to keep in bonds of love-
In vain, since ne'ertheless thou canst by thought
Drag all the matter forth into the light
And well search out the cause of all these smiles;
And if of graceful mind she be and kind,
Do thou, in thy turn, overlook the same,
And thus allow for poor mortality.

Nor sighs the woman always with feigned love,
Who links her body round man's body locked
And holds him fast, making his kisses wet
With lips sucked into lips; for oft she acts
Even from desire, and, seeking mutual joys,
Incites him there to run love's race-course through.
Nor otherwise can cattle, birds, wild beasts,
And sheep and mares submit unto the males,
Except that their own nature is in heat,
And burns abounding and with gladness takes
Once more the Venus of the mounting males.
And seest thou not how those whom mutual pleasure
Hath bound are tortured in their common bonds?
How often in the cross-roads dogs that pant
To get apart strain eagerly asunder
With utmost might?- When all the while they're fast
In the stout links of Venus. But they'd ne'er
So pull, except they knew those mutual joys-
So powerful to cast them unto snares
And hold them bound. Wherefore again, again,
Even as I say, there is a joint delight.

And when perchance, in mingling seed with his,
The female hath o'erpowered the force of male
And by a sudden fling hath seized it fast,
Then are the offspring, more from mothers' seed,
More like their mothers; as, from fathers' seed,
They're like to fathers. But whom seest to be
Partakers of each shape, one equal blend
Of parents' features, these are generate
From fathers' body and from mothers' blood,
When mutual and harmonious heat hath dashed
Together seeds, aroused along their frames
By Venus' goads, and neither of the twain
Mastereth or is mastered. Happens too
That sometimes offspring can to being come
In likeness of their grandsires, and bring back
Often the shapes of grandsires' sires, because
Their parents in their bodies oft retain
Concealed many primal germs, commixed
In many modes, which, starting with the stock,
Sire handeth down to son, himself a sire;
Whence Venus by a variable chance
Engenders shapes, and diversely brings back
Ancestral features, voices too, and hair.
A female generation rises forth
From seed paternal, and from mother's body
Exist created males: since sex proceeds
No more from singleness of seed than faces
Or bodies or limbs of ours: for every birth
Is from a twofold seed; and what's created
Hath, of that parent which it is more like,
More than its equal share; as thou canst mark,-
Whether the breed be male or female stock.