This is not intended to be an extensive history of the Saint Bernard breed of dogs. There are many web sites with detailed histories which I have included links to below, and there are many books that cover the subject. As with all history, you will find variations and disagreements on some points, but learning the history of the breed will provide a good foundation for understanding the Saint Bernard and what breeders are striving to achieve.
The origins of the Saint Bernard are poorly documented and one of the issues subject to debate. Whether they are descended from the Roman Mollossus and the Tibetan Mastiff, or from a collective breed of "Kollektivrasse der groseen Alpenhunde" (large alpine dogs) may never be know for sure. What is known is that the initial breeding stock of the Saint Bernard came from came to the Hospice in the 1660's.
The Hospice of the Great St. Bernard, from which this breed eventually took its name, is located 8000 feet above sea level in the remote Mons Jovis Pass region of the Swiss Alps. It is thought that the first dogs brought to the hospice served primarily as watchdogs and companions to the monks of the Hospice during the long isolated winter periods. How these dogs developed from guardian and companion to search and rescue is unclear but it appears that they accompanied the monks who went on mountain patrols after snowstorms. The monks somehow learned to take advantage of the dog's natural skills and instincts and eventually were able to send dogs out in unaccompanied packs of two or three to seek lost or injured travelers. These packs were able to locate lost individuals, dig them out of snow if necessary and rouse them, and if they could not rouse them, lie down on top of them to keep the traveler warm while one of the dogs returned to the Hospice to rouse the monks. The first recorded life saving work by the dogs of Saint Bernard date back to 1774.
From 1816 to 1818, the Great Saint Bernard Pass experience many severe snowstorms and many dogs perished while doing rescue work. As a result of all of the losses the line of Saint Bernards from the Hospice came close to extinction. Records say the monks replenished their lines using dogs from nearby valleys. Beginning in 1830 three experimental breedings were done with Newfoundland dogs. The reasoning behind the outcross was the theory that the longer coat of the Newfoundland would help to protect the Saint Bernards in the harsh winter environment. It was good in theory but in practice the long coats gathered snow and ice that quickly incapacitated the dogs making them unsuitable for rescue work.
Longhaired puppies from future breedings were given to the people of the valley by the monks. These longhaired dogs were bred by their new owners and the longhaired line of Saint Bernards came to be the preferred Saint Bernard by many. Eventually the two lines were brought back together resulting in the Saint Bernard of today.
You can find some information on the Molussus at http://www.american-bulldog.com/molossus_myth.htm