DR. EMERY'S book is remarkable in two ways. In the first place, it is a clear and concise statement of the parasitic hypothesis of the causation of new growths—a welcome innovation in a subject around which more dubious writing has been perpetrated than any other in medicine. In the second, it does not contain any original observations. After summarising and discarding the current definitions of “tumour,” the author sets up three postulates to which the required parasite should conform—viz. ultramicroscopic size, intracellular or intranuclear habitat, and production of a toxin capable of stimulating growth in the invaded cells. The remainder of the work is devoted to a rapid review of the more prominent features of tumours, showing how they fit in with these assumptions. Benign growths are those with few parasites in each cell, giving weak action of the toxin; in malignant growths the cells are heavily loaded, much toxin is produced, and growth is energetically stimulated. At once we come in contact with the subsidiary assumption that the cells of the body grow only when stimulated. It is at least arguable and probably true that, on the contrary, growth goes on so long as life lasts. The contrast to the form of growth presented by the limited reactions to known toxins is got over by assuming a nicely balanced symbiosis of host-cell and parasite, the parasite not getting out of bounds and killing the cell, and the cell not being sufficiently irritated to kill the parasite. Tumours: Their Nature and Causation. By Dr. W. D'Este Emery. Pp. xx + 146. (London: H. K. Lewis and Co., Ltd., 1918.) Price 5s. net.