My old farm book hunting earlier today also scored me the 1913 book "Sixty Lessons in Agriculture"...a tome on all facets of American farming written as lessons for middle school students.
The chapter on hogs included these fine specimens of the "lard" breeds, Poland-China, Duroc-Jersey and Chester White and a "bacon" breed, the Tamworth.
It is fascinating to me because of what these breeds look like and have been bred for 114 years later. While considered to be a massive, lard-producing hog in 1913, the Poland China, Chester White and Duroc are all part of what are considered "commercial" lines for modern breeding-- meaning they are meant to produce the ultra-lean, extremely muscular pigs.
Meanwhile, the Tamworth in 1913 was considered a "leaner" bacon-type hog. And indeed, from these photos that breed looks the leanest in build. But today, most would consider a Tamworth a fattier, "heritage" breed hog and its genetics are not often used as part of large commercial pig operations.
I think the moral of the story is that livestock have, and always will be, bred for an end use--be it lard, bacon, ultra-lean, or well marbled and the names we put on them are just that, names. The underlying genetics are whatever we as breeders are aiming for in our end game.
In the case of our farm, we are using a mix of what are considered "heritage" breeds--Gloucester Old Spot and Red Wattle with a more modern dose of Berkshire and a foreign dose of Meishan to create our "end game" of a pig that produces deep red, highly marbled, spectacular tasting meat with a nice but not too big lard cap and does it in a timely manner (250lbs by no later than 7 months) and doesn't need special housing or a special diet to get it done. So far, it is working well and maybe someone will grab a 2017 ag book 100 years from now and look back and wonder about how the "Dalarna Pig" came to be.