In https://philosophynow.org/issues/48/John_Herschel, author Toni Carey started out by talking about William Herschel, who died in 1822, before Darwin’s book came out. The author went on to talk about his son, “John Frederick William Herschel (1792-1871), who inherited all the Herschel talents, and their modesty as well.” It was to this son, apparently, that Darwin wrote. Sorry for my confusion of father and son.