So… the latin on the board at Lonesome Dove reads:
Uva uvam vivendo varia fit
I know the correct phrase is from Juvenal, and is Uva uvam videndo varia fit: “a grape, seeing another grape, changes color”
and I’m pretty sure that “uva uvam vivendo varia fit” is just nonsense. But suppose you wanted to change the phrase so that it read:
“a grape, living with another grape, changes color”? – That, after all, is what happend with Gus and Cal, and is probably what McMurty had in mind.
Very rarely is the ablative of accompaniment used without “cum” (Allen & Greenough, §413). It would, I suspect, be more correct to say “Uva cum uvâ vivendo varia fit”. Rarissimè sine “cum” ablativus comitandi significatione scribitur. Commodiùs dicetur (‘ut credo, dicetur’ futuri temporis activo modo latiné, praesentis subjunctivo anglicé), suspicor: “Uva cum uvâ vivendo varia fit”.
Note also this phrase means more “A grape living by means of a grape [i.e., ‘upon a grape’ or ‘eating a grape’] ripens.” Setiùs, ambiguum dictum tuum est, nota.
Note that you can say classically “Uva in diem vivendo varia fit” and “Uva in horam vivendo varia fit” for “a grape by living hand-to-mouth [depending just on God] ripens” or “a grape by living in the moment ripens”. But those are idiomatic phrases (well, I suppose that’s stretching a point to say ‘idiomatic’ unless amongst grapes, but ‘in diem vivere’ and ‘in horam vivere’ are idiomatic!), and if you said "uva in uvam vivendo" hoping for the sense of “a grape in living solely for another grape ripens” or “in the pocket (!) of another grape”, as it were, you really should say “uva ovae [dativo] vivendo varia fit”.
Ego quoquè, sensu quem requiris et casu accusativo, “ovam” accomodare non possum, nec idiomatibus quidem.
POST SCRIPTUM Hunc jocum nota, etiam. Note the joke in Lonesome Dove: “We rent rigs…We don’t rent pigs. Ova ovam [sic] vivendo varia fit.” Id est, “We don’t rent pigs because, when you live with a pig you become a pig.” [Horses and cattle are OK, of course, and we’re talking rigs!]
but ‘in diem vivere’ and ‘in horam vivere’ are idiomatic!), and if you said "uva in uvam vivendo" hoping for the sense of “a grape in living solely for another grape ripens” or “in the pocket (!) of another grape”, as it were, you really should say “uva ovae [dativo] vivendo varia fit”.