I find a word of preface is expected for this volume, and it comes welcomely to me here to thank the University of Edinburgh for the invitation to give these lectures. The invitation indeed overcame some hesitation on my part which readers of these pages, I can think, may find intelligible.

As to acknowledgements, Sir S. R. Christophers, F.R.S., a friend of many years, has been so kind as to read through for me in proof an excursion into the parasitology of malaria made in my final chapter. To my friend Mr J. Reid Moir, F.R.S., I am indebted for a like good office in regard to the text's occasional references to ways and works of prehistoric man, and other kindred themes.

At numerous places, especially in its opening chapters, my text turns to the writings of a sixteenth-century physician, Jean Fernel, a figure probably now little known except to the historian of Medicine. It is hoped, however, to issue about him in the near future a more documented account, biographical and bibliographical.

Lastly I would acknowledge with gratitude the care and skill given to the issue of this volume by the University Press, and I would convey my thanks to Dr Robert Chambers for his readiness to allow me the reproduction of two noteworthy microphotographs from his laboratory.