On the nature of things (books 4 - 6)   

To keep true measure. And yet they no whit
Do get a larger fruit of gladsomeness
Than got the woodland aborigines
In olden times. For what we have at hand-
If theretofore naught sweeter we have known-
That chiefly pleases and seems best of all;
But then some later, likely better, find
Destroys its worth and changes our desires
Regarding good of yesterday.
And thus
Began the loathing of the acorn; thus
Abandoned were those beds with grasses strewn
And with the leaves beladen. Thus, again,
Fell into new contempt the pelts of beasts-
Erstwhile a robe of honour, which, I guess,
Aroused in those days envy so malign
That the first wearer went to woeful death
By ambuscades- and yet that hairy prize,
Rent into rags by greedy foemen there
And splashed by blood, was ruined utterly
Beyond all use or vantage. Thus of old
'Twas pelts, and of to-day 'tis purple and gold
That cark men's lives with cares and weary with war.
Wherefore, methinks, resides the greater blame
With us vain men today: for cold would rack,
Without their pelts, the naked sons of earth;
But us it nothing hurts to do without
The purple vestment, broidered with gold
And with imposing figures, if we still
Make shift with some mean garment of the Plebs.
So man in vain futilities toils on
Forever and wastes in idle cares his years-
Because, of very truth, he hath not learnt
What the true end of getting is, nor yet
At all how far true pleasure may increase.
And 'tis desire for better and for more
Hath carried by degrees mortality
Out onward to the deep, and roused up
From the far bottom mighty waves of war.
But sun and moon, those watchmen of the world,
With their own lanterns traversing around
The mighty, the revolving vault, have taught
Unto mankind that seasons of the years
Return again, and that the Thing takes place
After a fixed plan and order fixed.
Already would they pass their life, hedged round
By the strong towers; and cultivate an earth
All portioned out and boundaried; already,
Would the sea flower and sail-winged ships;
Already men had, under treaty pacts,
Confederates and allies, when poets began
To hand heroic actions down in verse;
Nor long ere this had letters been devised-
Hence is our age unable to look back
On what has gone before, except where reason
Shows us a footprint.
Sailings on the seas,
Tillings of fields, walls, laws, and arms, and roads,
Dress and the like, all prizes, all delights
Of finer life, poems, pictures, chiselled shapes
Of polished sculptures- all these arts were learned
By practice and the mind's experience,
As men walked forward step by eager step.
Thus time draws forward each and everything