The torch must pass from one man to the next. It is the way of this world.
Those that once patrolled the game as the primary predators of the jungle have to bow out to father time.
The hour is upon Seán O'Brien.
The British and Irish Lions flanker's legacy is secured as a generational Leinster and Ireland talent.
The Carlow man made rugby a more attainable option for young men in the south-east for more than a decade as Leinster's tentacles began to reach beyond Dublin and the schools system.
Fellow Lions Test animal Tadhg Furlong was one of those with a dream in his head.
The Wexford man has worked alongside O'Brien, the man who made the hard road easier to travel.
The contribution of O'Brien stretches beyond the overall transformation of Leinster from underachievers to perennial finalists, even champions.
He has led the way to expanding the attraction of Leinster to young athletes 'down the country.'
For example, the Ireland U20 squad for the World Cup includes a tight-head prop from Tullow in Charlie Ward and a back-five forward from Wexford, Brian Deeny.
Better again, these two teenagers will be available again at that level next season, indicating above-average international talent.
There is what O'Brien has done for those around him and those who dare one day to follow him.
Sadly, he has lost his battle with a hip injury in what would have been his last appearance for Leinster, allowing the admirable Josh van der Flier to start at number seven.
There are few better to speak of the Tullow Tank's influence on the organisation than Furlong.
"It's so much experience, isn't it? Twelve years in Leinster now, and everything he's contributed.
"On the pitch, you've seen what he's like," he said.
"In the dressing room, team meetings and leadership, I think you can't look past the impact Seánie has had on the growth of the game within Leinster."
The Blues will need everyone at their best to get the better of Glasgow at Celtic Park this evening.
The Scots will aim to give Stuart Hogg the perfect 'adieu' with an unchanged '15'.
However, it is a chance for Leinster to move on from the heartbreak of the Champions Cup final.
"It's pretty much that, isn't it?" said Furlong.
"Look, we're an ambitious group and obviously losing a European final is pretty devastating. If you think back to it, it's still disappointing.
"It's a chance to brighten up our holidays.
"I suppose it's not just for the group of players who are taking the pitch either, it's the wider group and everybody who has contributed in the Guinness Pro14 this season."
To leave the season behind on back-to-back defeats in the finals would be a hard blow to shake.
The devastation would be bound to last well into the summer.
Coach Cullen has recalled captain Jonathan Sexton, Rob Kearney and Scott Fardy for Ross Byrne, Dave Kearney and the injured Devin Toner in what is their best available side.
"I don't think you'll ever forget losing a final," added Furlong.
"I remember we lost a Pro12 final to Connacht and Leo (Cullen) said he lost his first European final with Leicester against Wasps and the memory of that day still stays with him.
"It's something that you learn from it, and you live with it. But, yeah, if we can find a way to win at the weekend, which is obviously not going to be any sort of easy task, it definitely boosts the feeling in the camp about the year we had."
Or else, they will have to stew on what might have been.
Glasgow: S Hogg; T Seymour, K Steyn, S Johnson, DTH van der Merwe; A Hastings, A Price; J Bhatti, F Brown, Z Fagerson, S Cummings, J Gray, R Harley, C Gibbins (capt), M Fagerson. Replacements: G Stewart, O Kebble, S Halnukonuka, R Wilson, T Gordon; G Horne, P Horne, H Jones.
Leinster: J Larmour, G Ringrose, R Henshaw, J Lowe; J Sexton, L McGrath; C Healy, S Cronin, T Furlong, J Ryan, S Fardy, R Ruddock (capt), J van der Flier, J Conan. Replacements: B Byrne, E Byrne, A Porter, R Molony, M Deegan; N McCarthy, R Byrne, R O'Loughlin.
PRO14 LEAGUE FINAL
GLASGOW v LEINSTER
Live TG4 & Eirsport 1
(Today, KO 6.30pm)