My dissertation asks why, in the years between the end of the Great War in 1918 and the East/West division of 1949, German philosophers, biologists, and anthropologists came into dialogue in an effort to ascertain exactly what sorts of entities animals are, what sort of animal the human is, and what sorts of political community, moral obligation, and epistemological values ought to follow from man's eccentric position in the natural world. I aim to chart how the shifting taxonomy of knowledge in this period (in particular, changing understandings of the relationship between the natural sciences and the humanistic disciplines) opened new horizons of possibility for thought regarding "the animal" and rendered the human-animal distinction integral to debates in German intellectual life concerning the fate of Western culture. Among the intellectual developments central to this study are: the birth of philosophical anthropology as an independent sub-discipline in the 1920s; the emergence and philosophical reception of theoretical biology, which seemed to offer a sophisticated alternative to the mechanistic reductionism of contemporary physiology and the troubling contingency of Darwinian evolutionary outcomes; and the founding of the discipline of ethology during the Third Reich. The participants in these intellectual movements saw themselves as contributing to a thoroughgoing interrogation of the foundations of Western metaphysics in response to the acute sense of cultural crisis widespread among German intellectuals during the Wilhelmine period but given new life by the failure of the war effort of 1914-18. Most broadly, then, my dissertation aims to discern the political, ethical, and indeed metaphysical implications of these ostensibly scientific and epistemological discussions, asking how and why attempts to delimit the human-animal relation came to have such normative purchase.