This, the third Isaiah Bowman Memorial Lecture, is the first that has been devoted to what is primarily a physical aspect of geography, although the subject is not without its human implications. In the true Bowman tradition, Dr. Ahlmann’s qualifications to analyse and assess the rapidly growing mass of physical data (much of which were inspired by his own researches) are unrivalled. The glaciologists will welcome equally this admirably concise review of one aspect of their science and the “fascinating presentation of its bio-geographical significance.”

The idea of a simple, direct relationship between glacier variation and climatic fluctuation may appear plausible, and indeed in much past discussion such a relationship has often been assumed. Development of our knowledge has, however, demonstrated that the relationships between glaciers and climate are highly complicated and still far from clear.

In an early section, Ahlmann comments with parental modesty on the importance of the glaciological results of the Norwegian—British—Swedish Expedition to Queen Maud Land, as illustrated by the preliminary results produced by Schytt and Robin. In a succeeding section dealing with the factors influencing glaciation he emphasizes the changing relative importance of the individual climatic elements through the ablation season in an individual glacier and their regional variation as the controllers of the “state of health” of glaciers.

Recent studies have paid increased attention to the significance of the survival of ice masses from one climatic regimen into another, both in modern high polar glaciers and in the late Pleistocene ice sheets of lower latitudes. Delayed response to increased accumulation and survival of movements beyond the period of application of the causative stress are probably normal features of glaciers. Glaciers may have an inherent periodicity independent of climatic factors. On the basis of such complications, it is probably true to say that departure from a condition of being in equilibrium with environment is the general rule among glaciers.

A discussion of recent systematic studies by Wallén on the Kársa Glacier and Schytt and Woxnerud on the Stor Glacier in Swedish Lapland leads to a general appraisal of the recent glacier recession in Scandinavia, Iceland, Spitsbergen and Greenland. Typical of the author’s penetrating approach to glacier fluctuation is his comment on the remarkable 60-mile recession in Glacier Bay, Alaska, since the eighteenth century. He would put the stress on the great advance which took place in the eighteenth century and its cause, as the major problem, and suggests that the retreat should not be taken as evidence of the importance of the present climatic fluctuation.

A brief summary of some recent data on the present climatic fluctuation, with its complicated reversals of seasonal trends in the past decade, is followed by a consideration of the causes of the recent glacier recession and the present climatic fluctuation. The author critically reviews current theories of increased zonal circulation as a major factor, on the whole favourably, and notes the growing tendency to accept the “solar variation” origin for both short- and long-range fluctuations. He stresses the significant fact that of all the endless series of climatic fluctuations since the beginning of history, “the present one is the first that we can measure, investigate and, possibly, explain.”

Turning to the field of bio-geography attention is drawn to the great economic significance of the present climatic fluctuation in the northern sea routes and fishery fields and, on land, in its influence on forestry, agriculture and hunting. This is especially the case in the marginal countries, as is shown, for example, by a recent symposium on this subject published by the Finnish Geographical Society.

Finally, the author visualizes the recent changes, in the broader setting, in the fluctuations of the past few thousand years and looks forward to a bright future for glaciological research.

Many of Dr. Ahlmann’s friends and admirers will recall the inspiring foreword he wrote to the first issue of the Journal of Glaciology and rejoice that, in spite of heavy prior claims on his time, he is able to continue actively in the field where, as elsewhere, his influence is pre-eminent.