What claim did Anna Seward make about Erasmus Darwin’s preface to The Botanic Garden?

In her Memoirs of the Life of Dr. Darwin, Anna Seward claimed on three separate occasions that the preface to Erasmus Darwin’s The Botanic Garden was plagiarized from her own poem, "Verses Written in Dr. Darwin’s Botanic Garden" (1788).1

Furthermore, she asserted that Darwin had previously published that same poem under her name in The Gentleman’s Magazine in 1783, though she maintained this was done without her permission.1