Just using this as a scratch pad; please ignore.
American Handy-Book of the American Brewing, Malting and Auxiliary Trades/Pure Yeast Culture
By pure yeast is meant a yeast derived from a single cell by methods excluding the possibility of infection.
If yeast is mixed with the wort in the brewery and allowed to ferment in open tubs, it is evident that bacteria, wild yeasts and mycoderma that may be present in the air are apt to fall into the fluid, where they have opportunities to develop and subsequently settle, in part, in the yeast sediment.
The old way.
In former years, no special pitching yeast was employed, the wort being simply left to spontaneous fermentation. Frequently the fermenting tubs were not cleaned out, and the new wort was pumped on the old sediment. This operation being repeated a number of times, the sediment would acquire a certain characteristic composition. If the beer turned out satisfactory, the yeast was not thrown away when the tubs were cleaned, but was transferred to other vessels. In that way the barm gradually grew up. What this barm really was, no one knew.
Investigation of yeast.
Inquiry into the nature of this body grew active with the improvement of the microscope, which about the beginning of the nineteenth century became an important factor in scientific research. Being applied to yeast, this mass was found to be composed of countless numbers of very small cells, each of which constitutes a living being very prolific in multiplication and endowed with the peculiar power of inciting fermentation.
Pasteur's pure yeast.
The full import of these investigations was not realized until Pasteur startled the scientific world with his classical experiments which demonstrated conclusively that the yeast mass is mixed