After one of the characters dies (I'm doing my best to avoid a spoiler here), Downton must figure out how to feed a newborn baby without a mother.
Carson, the butler, discusses the matter with Mrs. Hughes, the head housekeeper.
Hughes: Mrs. Rose in the village has just had a baby. She's volunteered to nurse the child.
Carson: Good. And I suppose she'll stay here.
Hughes: Not for long. The doctor's sending up a pamphlet on feeding babies with something called "the percentage method." Boiled milk, water, honey, orange juice...that kind of thing.I suppose it's not surprising that some sort of baby formula existed back then. After all, women died frequently in childbirth. It's not as if all those babies were forced to simply die as well.
In this case, the Downton estate has the resources to hire a wet nurse — a woman who can feed the child from her own breasts. (You wonder how this would affect the milk supply for that woman's own child, but perhaps that's another discussion.)
I found an article on the history of infant formula that sheds some light on the matter:
The early years of the 20th century were notable for improvements in general sanitation, dairying practices and milk handling. Most infants were breast-fed, often with some formula feeding as well. Availability of the home icebox permitted safe storage of milk and infant formula, and by the 1920s, feeding of orange juice and cod liver oil greatly decreased the incidence of scurvy and rickets. Use of evaporated milk for formula preparation decreased bacterial contamination and curd tension of infant formulas.(I'm not entirely clear what "curd tension" is, but I think it contributed to the downfall of Saddam Hussein.)
The Wikipedia entry on infant formula addresses the "percentage method" directly (and, knowing the less-than-rigorous research methods used by "Downton Abbey" writers, it was probably the sole information they used to write the scene above):
...medical recommendations such as Thomas Morgan Rotch's "percentage method" (published in 1890) began to be distributed, and gained widespread popularity by 1907. These complex formulas recommended that parents mix cow's milk, water, cream, and sugar or honey in specific ratios to achieve the nutritional balance believed to approximate human milk reformulated in such a way as to accommodate the believed digestive capability of the infant.While the idea of mixing up formula at home may offend our modern sensibilities, human beings have probably spent most of their history feeding weird stuff to babies.
"Cod liver? Let's give it a shot!"