564 Scientific Intelligence. 



cultivated the region, and that the ancient people lived in a time 

 when the climate of this southwestern region was less arid than 

 at present and supported a denser population. The subject is 

 then tested by a study of southern Mexico. This is some 1,200 

 to 1,500 miles south and in a different climatic zone, on the 

 southern instead of the northern margin of the belt of trade- 

 winds. Here the study of lake strands and river terraces shows a 

 similar climatic history. 



Chapters XI to XIV inclusive develop a new and most import- 

 ant method of studying the climatic fluctuations of the past 2,000 

 to 3,000 years, that derived from the variable growth of trees. 

 The inception of this method is due to Professor Douglass, who 

 contributes the first chapter. The basis of the study rests upon 

 the principles that the thickness of the rings of growth measure 

 the food supply and this depends largely upon the amount of 

 moisture, especially where the quantity of moisture is limited and 

 the life struggle of the tree is against drought rather than against 

 competing vegetation. Cycles of years favorable for tree growth 

 can be detected and confirmed by the comparison of stumps. 

 Huntington has followed this method, using especially the stumps 

 of the giant sequoias of California and arrives at a general con- 

 firmation of the climatic curve as deduced from studies in Asia. 



The next section of the volume deals with the climatic changes 

 which have taken place in the northern part of the torrid zone, as 

 shown in Yucatan and Guatemala. It includes a chapter on " the 

 shift of the storm track " by Charles J. Kullmer. The conclusion 

 is that during a time of coolness and more abundant rain, the 

 trade-wind belts are compressed towards the equator. Conse- 

 quently regions like southern Yucatan and eastern Guatemala, 

 which now possess tropical forest, would at such a time have 

 had a drier and cooler climate and have been free from heavy 

 forest. As a result the region, as shown by the ruins, permitted 

 a high civilization where now it appears impossible. The argu- 

 ment is in accord with analogy given by the shifting toward the 

 equator exhibited by the trade-wind zone during the winter 

 season. 



Chapters XIX and XX take up the problem of explanation of 

 the ultimate causes of the changes determined from the previous 

 studies. It is concluded that the rapid oscillations, both of 

 smaller and larger amplitude, are most probably accounted for 

 by variations in solar radiation. Crust movements with related 

 changes in atmospheric composition seem, on the other hand, to 

 dominate those slower and longer changes which individualize 

 the geologic periods. Both seem to have been in operation 

 through all of geologic time. 



Part II of the volume is on the climates of geologic time, and 

 is by Charles Schuchert, Professor of Paleontology in Yale Uni- 

 versity. A review is given of the various lines of geologic evi- 

 dence, derived from tellites, aq leous sediments, and fossils. The 

 conclusions drawn as to the variations in climate from period to 





