AND ITS INHABITANTS 187 



one hand, and Mediterranean rainfall on the other. Hence 

 it seems legitimate to infer that the main fluctuations in tree 

 growth, which are portrayed in Figure 38, represent climatic 

 pulsations of world-wide extent. 



Even yet we have not fully explained the nature of climatic 

 changes. They appear to have been not only pulsatory, but to 

 have produced different effects in different parts of the world. 

 It is an interesting coincidence that independently and on 

 wholly different lines of evidence the German glaciologist, 

 Penck, and the writer at almost the same time announced the 

 conclusion that climatic changes are the result of an alternate 

 equatorward and poleward shifting of the climatic zones. The 

 matter seems to be even more complicated than this, however, 

 for changes in the interior of continents seem to be different 

 from those on their borders or over the oceans. The net 

 result is that when such regions as the subtropical zone which 

 embraces California and the Mediterranean lands enjoy un- 

 usually abundant rainfall, the northern border of the zone of 

 equatorial rains becomes drier. The chief basis for this con- 

 clusion, so far as historic times are concerned, is the fact that 

 the Maya ruins in the dense forests of Central America seem 

 to indicate periods of relative dryness at the times when tree 

 growth in California indicates unusual moisture, and vice 

 versa. 



One more point needs emphasis if we would understand the 

 nature of these pulsatory shiftings of the climatic zones. 

 Apparently the most essential feature is a change in the tracks 

 of storms. At certain periods the total number of storms 

 appears to increase. Of even greater importance, however, is 

 the fact that at such times the storms show a tendency to follow 

 two belts instead of the one to which they are now largely 

 confined. The present belt of storms is well known. From 

 British Columbia it runs as a band hundreds of miles wide 

 centering along the international boundary line, but with a 



