sentences:  Commonly, longer novellas are referred to as novels; Robert Louis Stevenson's "Strange Case of Dr Jekyll and Mr Hyde" (1886) and Joseph Conrad's "Heart of Darkness" (1899) are sometimes called novels, as are many science fiction works such as H. G. Wells' "The War of the Worlds" (1897) and Philip Francis Nowlan's "Armageddon 2419 A.D." (1928).Less often, longer works are referred to as novellas.The subjectivity of the parameters of the novella genre is indicative of its shifting and diverse nature as an art form.