Where does spit and polish originate?

In the Army, back when boots were made of leather, a highly polished, reflective surface could be obtained by applying Kiwi polish, then spitting (or using a damp rag) on the boot and polishing with a rag. This was a careful, time consuming enterprise, hence rarely engaged in unless before inspections or while in garrison. Spit and polished boots, crisp BDUs and a fresh regulation haircut made a strack soldier.