In addition to diurnal and seasonal rhythms which are well documented, there are other biological cycles with longer periodicities. It has long been known, of course, that plant and animal populations in the wild undergo tremendous quantitative changes.'8 For example, the records of the Hudson Bay Company provide fascinating material to document the statement that there have occurred large fluctuations in the numbers of fur animals as well as of the rodents on which they feed. 19 20 The analysis of historical records and of the findings in recent wild life surveys have led to the belief that many population changes are determined by climatic factors, and furthermore there has been a tendency to accept that some at least of these changes exhibit a cyclic character. While the evidence for a true periodicity is not always convincing, there seems to be little doubt that population fluctuations are often the expression of responses to changes in the physical environment. Well documented information bearing on this problem has come from the study of tree rings in the North American continent. Comparison of the thickness of tree rings has revealed that marked changes have occurred in the rate of plant growth during the past 2,000 years, probably as a result of variations in temperature and in atmospheric precipitation. There is reason to believe that these changes have also played an important part in the life of the Pueblo Indians-affecting the location of their settlements and the size of their population.2' It is worth mentioning here that, as repeatedly emphasized by Huntington, climatic changes have probably been influential also in determining the growth and decay of other civilizations all over the world.
Fairly accurate information derived from wild life surveys in our time has provided evidence that the climate conditions both the distribution and the abundance of several animal species-as illustrated by the history of rabbit populations in Australia.22 23 The European rabbit Oryctolagus cuniculus was introduced into Tasmania at the beginning of the 19th century and spread over much of the island. Very rapidly, color variants became established and they now exist with different frequencies in different areas--the black rabbits reaching a frequency of 20 per cent in places of highest rainfall. This particular example is of special interest because it illustrates that climatic factors can operate through genetic mechanisms. A related example is provided by the well-known fact that animals living in colder climates are usually larger than those of related species living in warmer climates. In most situations considered in the present report, however, mechanisms other than genetic must be invoked since the biological responses to climatic changes occur so rapidly that they can hardly be due to genetic alterations.
In certain cases, the explanation appears rather straightforward, for example with regard to plankton which changes continuously in abundance and composition from season to season and from year to year. In 1925 the warm equatorial counter-current off Columbia and Ecuador (El Nino) shifted its course so strongly to the South (as it does once every seven years) that the population of plankton, fish, and water birds normally found off the Peruvian coast fled or died, being replaced by warm water species.24 When the current from the Atlantic predominates on the English coast, it brings water rich in phosphate which favors one species of glassworm on which the herring feeds. In contrast, current from the Channel brings in water poor in phosphate, resulting in failure of the herring fishery. The "red tide" which swept immense numbers of dead fish into the Florida beaches in 1946 and again in 1952, as it does approximately once a decade, was caused by a microscopic flagellate which is always present in the waters off the Florida coast, but in numbers too small to be harmful; its population reaches toxic levels only when atmospheric circumstances bring about the local stagnation of low-salt brackish water in certain areas.
In contrast to these fairly simple situations, the biological findings remain unexplained in most cases. Over the past three decades Errington25 has analyzed the wild life surveys in Iowa with regard to populations of the ruffed grouse, the snowshoe hare, and especially the muskrats. The results of his analysis leave no doubt that the numbers of these animals have fluctuated enormously during the period under consideration. Yet there is no indication that heat, humidity, water levels, and other obvious variables can account for population changes, for disease states, or for patterns of behavior of the animals. While the findings are not explainable in terms of the meteorological data customarily recorded by the Weather Bureau, it is not impossible according to Errington that the biological patterns are related to the intensity of ultraviolet or other radiation-perhaps indirectly through some effect on the qualitative characteristics of the food available to the animals.
Decreases in the numbers of wild animals are commonly associated with a variety of disease states-both of metabolic and infectious character.25 26 It is therefore of importance to inquire into the evidence that climatological factors can actually affect resistance to disease. In fact, as already mentioned, there is a widespread belief among lay persons and physicians alike that certain types of weather disturbances are associated with particular illnesses.
In this country, Petersen and Mills7' 8 have long emphasized that the incidence and gravity of each type of pathological disorder can be correlated with either climate or weather. As illustration it will need suffice to mention here two types of weather which appear to be potentially harmful to man. One is the frontal or disturbed weather, the other the Fohn or Chinook with descending subtropical air in the whole troposphere, each type apparently bringing in its train a specific set of clinical and physiological events. Most familiar is the conviction expressed in many folklores that pains from scars and from arthritis sharpen during weather of the frontal type. Few are the persons indeed who do not believe that:
"A coming storm our shooting corns presage, Our aches will throb, our hollow tooth will rage."
There are many reports, on the other hand, that periods of Fohn in Switzerland and in Southern Germany are associated with increases in death rates, in automobile accidents, and in circulatory as well as mental disorders.3
In these special climatic situations, the pathological disorders seem to occur without any detectable change in any of the known geophysical surface elements. The patients may not even be aware of any bad weather in the usual sense. It would appear, therefore, that these weather disturbances operate through physical factors which are still obscure or even completely unrecognized. A few related observations made with microorganisms are worth mentioning at this time. Whereas the metabolism of bacteria and yeast seems to be attenuated during cyclonic periods, it is intensified during anticyclones. By recording automatically such activities as luminescence, motility, acid production, sporulation, etc., it was found that the changes occurred so rapidly (l/2-1 hour) that they could not be correlated with the usual daily weather curves.27