The boundaries of Colonialism, like those of many literary eras, are difficult to draw. The history of Colonialism as a policy or practice goes back for centuries, and arguably the story of Colonialism is not over yet. Thus literature of several ages reflects concerns about Colonialism in depictions of encounters with native peoples and foreign landscapes and in vague allusions to distant plantations. As colonial activity gained momentum in the late nineteenth century, so the reflection of that activity —as a celebration of European might or as fears of what lay in the wilderness—grew in intensity. Thus rough boundaries for the literary movement of Colonialism would begin in 1875, when historians date the start of a "New Imperialism," through the waning empires of World War I and up to the beginning of World War II, around 1939, although the years after World War I reflect primarily nostalgia for an era that was rapidly coming to a close. Colonialism is primarily a feature of British literature, given that the British dominated the imperial age; even colonial writers of other nationalities often wrote in English or from an English setting. The literature of Colonialism is characterized by a strong sense of ambiguity: uncertainty about the morality of imperialism, about the nature of humanity, and about the continuing viability of European civilization. Perhaps the essential colonial critique is Joseph Conrad's Heart of Darkness, though such works as Olive Schreiner's Story of an African Farm and E. M. Forster's A Passage to India similarly explore the paradoxes of Colonialism. Colonial literature is also full of high adventure, romance, and excitement, as depicted in Rudyard Kipling's spy thriller Kim or the adventure tales of H. Rider Haggard. Isak Dinesen's memoirs, including Out of Africa, similarly romanticize the wildness of the colonial landscape and the heroism of adventurous colonizers.