CHAPTER XIX. 



THE SOLAR HYPOTHESIS. 



In the investigation of any scientific problem the natural order of study is: (1) the 

 actual facts, past and present; (2) their causes; (3) their results; (4) the prediction of future 

 events. Thus far in this volume we have been endeavoring to ascertain the actual facts 

 as to the climatic events of the past two or three thousand years. Here and there we 

 have turned aside to a discussion of results as manifested in the topography of the earth, 

 its cover of vegetation, or its human inhabitants, but this has been merely to aid us in 

 ascertaining how far the climate has actually changed. If it be granted that the main 

 conclusions thus far set forth are correct, the way is open to the other three phases of the 

 subject — the study of causes and results, and the prediction of the future. Each of these 

 is so large as to demand a volume to itself. Hence in the following chapters I propose 

 merely to indicate briefly certain facts and relationships which appear to have a bearing 

 upon the cause of climatic changes. I shall discuss an hypothesis which presents a new 

 phase of the old hypotheses of the relation of the climate of the earth to the activity of 

 the sun on the one hand, and to deformation of the earth's crust on the other hand. I do 

 this with full appreciation of the fact that as yet the observational basis of the hypothesis 

 is small. I reahze that such an hypothesis is sure to be wrong in certain respects, and may 

 be entirely wrong. I do not present it with any expectation that it will at once be accepted, 

 or that it will supplant othec theories. It is offered merely as a first attempt to interpret 

 the theoretical bearing of the new facts set forth in this book. Whatever may be the 

 ultimate fate of the hypothesis, it may, perhaps, at least serve to stimulate the further 

 investigation of the phenomena here discussed and to promote the framing of other and 

 better hypotheses. 



Granting the validity of the conclusions set forth in previous chapters, the period from 

 the Pleistocene era to the present has been characterized by a series of climatic changes of 

 highly varying intensity. At one extreme come great changes, such as the glacial period, 

 lasting thousands or perhaps hundreds of thousands of years, and forming some of the 

 most noteworthy phenomena of geological history. At the other extreme come short 

 cUmatic cycles of various lengths such as 2.5, 11, 21, and 35 years. Few students question 

 the reality of either type of change, although there is much question as to whether they are 

 characterized by any permanent and regular periodicity or whether they occur merely at 

 irregular intervals. Hitherto these two types have commonly been regarded as distinct 

 phenomena, due in all probabiUty to diverse causes. The glacial changes have been 

 supposed to be a completed series of events, which might recur again, but whose causes are 

 not now operative. The minor cycles of the present time, on the other hand, have been 

 generally looked upon either as more or less accidental phenomena due to fortuitous 

 combinations of atmospheric influences, or else as the result of causes which may or may 

 not be connected with the glacial period, but which at least are separated from that period 

 by a pronounced and unbridged gap. This gap appears now to be bridged, and in this fact 

 lies the most important contribution of this volume to our knowledge of the laws of nature. 

 Between the two extreme tj^es of climatic change typified by glacial periods and 11 -year 

 cycles, there seem to be two others of intermediate magnitude. First, we have the change 

 by which the climate of the world in general appears now to be different from what it was 

 2,000 to 3,000 years ago. This change is perhaps of the same degree as the changes of 



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