THE SERIES OF CLIMATIC CYCLES. 



341 



Table of dimatic cycles — Continued. 



Spatial differentiation of climates. — ^The principle of unif onnity leads to the 

 final conclusion that the world was not cooled throughout during glacial or 

 cooled periods, but that cold or drouth must have been pronounced in certain 

 areas, from which it shaded out more or less gradually in all directions. This 

 seems axiomatic from the present highly zonal distribution of climates on the 

 earth, as well as from our increasing knowledge of Pleistocene climates. As a 

 consequence, it appears inevitable that distinct climates must have appeared 

 in each glacial period, and that they must have shown a more or less zonal 

 grouping around centers of cold or aridity. The great topographic differences 

 arising out of variations in deformation must have been a constant cause of 

 climatic differentiation spatially, as deformation itself was temporarily. In 

 short, the elevation, sinking, or wearing down of different mountain ranges 

 and plateaus must have had in general the same differential effect upon 

 climate that is so characteristic of these land forms to-day. Associated with 

 deformation as a cause, according to Huntington (1914^: 578), is the grand 

 sun-spot cycle, which is thought to have produced the two great storm-belts, 

 subtropical and boreal, between which would he the relatively arid temperate 

 belt of decreased storminess. Hence it appears clear that every period of 

 glaciation or of pronounced cooling must have been a period of climatic 

 differentiation, resulting in more or less marked climatic zones, which were 

 themselves further differentiated by the distribution and forms of water and 

 land. In accordance with this assumption, climatic zones must have existed 

 around the poles whenever the oceanic circulation was restricted. It seems 

 probable that temperate, boreal, and polar zones existed continuously about 



