On the nature of things (books 1 - 3)   

Mother of Gods, if only he will spare
To taint his soul with foul religion.

So, too, the wooly flocks, and horned kine,
And brood of battle-eager horses, grazing
Often together along one grassy plain,
Under the cope of one blue sky, and slaking
From out one stream of water each its thirst,
All live their lives with face and form unlike,
Keeping the parents' nature, parents' habits,
Which, kind by kind, through ages they repeat.
So great in any sort of herb thou wilt,
So great again in any river of earth
Are the distinct diversities of matter.
Hence, further, every creature- any one
From out them all- compounded is the same
Of bones, blood, veins, heat, moisture, flesh, and thews-
All differing vastly in their forms, and built
Of elements dissimilar in shape.
Again, all things by fire consumed ablaze,
Within their frame lay up, if naught besides,
At least those atoms whence derives their power
To throw forth fire and send out light from under,
To shoot the sparks and scatter embers wide.
If, with like reasoning of mind, all else
Thou traverse through, thou wilt discover thus
That in their frame the seeds of many things
They hide, and divers shapes of seeds contain.
Further, thou markest much, to which are given
Along together colour and flavour and smell,
Among which, chief, are most burnt offerings.

Thus must they be of divers shapes composed.
A smell of scorching enters in our frame
Where the bright colour from the dye goes not;
And colour in one way, flavour in quite another
Works inward to our senses- so mayst see
They differ too in elemental shapes.
Thus unlike forms into one mass combine,
And things exist by intermixed seed.

But still 'tmust not be thought that in all ways
All things can be conjoined; for then wouldest view
Portents begot about thee every side:
Hulks of mankind half brute astarting up,
At times big branches sprouting from man's trunk,
Limbs of a sea-beast to a land-beast knit,
And Nature along the all-producing earth
Feeding those dire Chimaeras breathing flame
From hideous jaws- Of which 'tis simple fact
That none have been begot; because we see
All are from fixed seed and fixed dam
Engendered and so function as to keep
Throughout their growth their own ancestral type.
This happens surely by a fixed law:
For from all food-stuff, when once eaten down,
Go sundered atoms, suited to each creature,
Throughout their bodies, and, conjoining there,
Produce the proper motions; but we see
How, contrariwise, Nature upon the ground
Throws off those foreign to their frame; and many
With viewless bodies from their bodies fly,
By blows impelled- those impotent to join
To any part, or, when inside, to accord