Geology. 161 



mont valley. In the eastern Alps, of the fifty-five glaciers studied, 

 the Yernagt of the Oetzthal is the only one of importance which 

 lias lengthened. This glacier has grown remarkably during the 

 past fifty years, and in 1901 advanced fifty meters. The retreat 

 of glaciers in the Italian Alps continues, and several small ones 

 have disappeared. The glaciers of the French Alps show an 

 increased rate of retreat in 1901, and those of the Chambeyron 

 massif are destined soon to disappear. The Marinet group, in 

 spite of successive advances and retreats, is shown by Kilian to 

 have suffered gradual shortening since the Pleistocene. The Alps 

 at the present time offer unequalled opportunity for studying 

 recent glacial work, Speaking generally, the glaciers of other 

 parts of the world were likewise decreasing during 1901. 



4. The Cause of the Glacial Period ; by H. L. True. 174 pp., 

 illustrated. Cincinnati : The Robert Clarke Co. — Dr. True has 

 prepared an interesting summary and discussion of the current 

 theories regarding the cause of the ice age and presents one of 

 his own, viz : that there was a sudden toppling ot the earth due 

 to overweighting of certain parts. 



5. looth Characters and Revision of North American Species 

 of the Genus Equus ; by J. W. Gidley. Bull. American Mu- 

 seum of Nat. Hist., vol. xiv, pp. 91-141, New York. — Among the 

 many examples of evolution based on fossil material, that of the 

 horse is one of the best known and, to most minds, striking and 

 interesting. From the historical point of view it is also of deep 

 interest as having been the first example in which a complete 

 demonstration of an evolutionary process was ever made. Insep- 

 arably linked with the working out of this problem are the names 

 of Huxley, Marsh, and Kowalewsky. And the types perhaps 

 historically most important are those of Marsh in the great col- 

 lections bequeathed by him to the Museum of Yale University. 

 But it is to be noted, however, that these earlier demonstrations 

 were confined to the more salient facts. The methods of more 

 thorough and systematic collection developed within recent years 

 have rendered it possible to work out the finer details of this 

 evolutionary process, and now promise to place us in possession 

 of the actual specific descent of the several lines of horses. In 

 the contribution before us, Mr. Gidley has determined with 

 exactness the various extinct species of the genus Equus, a most 

 difficult and necessary preliminary to the larger problems of 

 descent. He has been able to do this by investigating in a far 

 more thorough manner than has ever hitherto been attempted, 

 the complicated process of individual tooth change. His further 

 investigations of these problems, based on the splendid collections 

 he has made for the American Museum in the past few years, will 

 be awaited with much interest. w. 



6. The Evolution of the Horse ; by W. D. Matthew. 

 Supplement to the American Museum Journal, vol. iii, No. 1, 

 Jan. 1903. — A popular exposition of the evolution of the horse, 

 which is evidently based upon and called forth by. these later 

 discoveries and researches of Mr. Gidley, as reviewed above, w. 



