Board Thread:Warhammer 40k General Discussion/@comment-24332685-20140106171610/@comment-3055683-20140107084918

They talk about this very subject in the novel "The Emperor's Gift" by Aaron Dembski-Bowden. Basically, following the Chapter's creation, all the subsequent Astartes of the newly formed Grey Knights are descended not from any of the original 8 Astartes founders or the original 20 Space Marine Legions of the First Founding, but from a unique gene-seed engineered from the genome of the Emperor of Mankind Himself -- what is known as "the Emperor's Gift." One of the quotes from the book states:

"One unbreakable shield against the coming darkness, One last blade forged in defiance of fate, Let them be my legacy to the galaxy I conquered, And my final gift to the species I failed."

- Inscription upon the Arcus Daemonica, attributed to the Emperor of Mankind