Vol.. 7, 1921 
PALEONTOLOGY: J. C. MERRIAM 
183 
expected from untried crosses, and has shown itself to be invaluable in 
everyday work. Detlefsen's own account is in agreement with this, for he 
says that his stock of white miniature has been used in class work and has 
always given the value to be expected from the published maps. When 
values that do not agree with the "normal" map are found, analysis has 
always shown some disturbing factor to be present; and in all cases where 
the point has been investigated it ha.s been found that maps based on data 
uniform with respect to this disturbing factor are entirely self-consistent. 
In view of these considerations it is clear that Detlefsen has misunder- 
stood the significance of the published maps. The conception that he 
has attacked is one that has not been held or urged by those who have con- 
structed chromosome maps. 
1 Detlefsen, J. A., These Proceedings, 6, 1920 (663-670). 
2 Sturtevant, A H., /. Exper. ZooL, 14, 1913 (43-59). 
3 Sturtevant, A. H., Zs. ind. Abst. Vererb. Lehre, 13, 1015 (234-287). 
4 Morgan, T. H., A. H. Sturtevant, H. J. Muller, and C. B. Bridges, New York, 1915, 
262 pp. 
5 Bridges, C. B., /. Exper. Zodl., 19, 1915 (1-21). 
6 Sturtevant, A. H., These Proceedings, 3, 1917 (555-558), and Carnegie Inst. Wash- 
ington Publ., No. 278, 1919 (305-341); Muller, H. J., These Proceedings, 3, 1917 
(619-626), and Genetics, 3, 1918 (422-499); etc. 
7 Morgan, T. H., The physical basis of heredity, Philadelphia, 1919, 305 pp. 
ORIGIN AND HISTORY OF THE BEAR FAMILY IN THE WEST- 
ERN HEMISPHERE, WITH PARTICULAR REFERENCE TO THE 
RELATION OF THIS QUESTION TO PROBLEMS OF GEOGRAPH- 
ICAL HISTORY 
By John C. M^rriam 
Carnegie Institution of Washington 
Read before the Academy, April 26, 1921 
Although bears of many varieties are widely distributed in America 
and have been present in large numbers for a very long period measured 
in terms of years, the history of this group ^shows that as we go back in 
the geological record no evidence of representatives of the bear type are 
present in formations of the third geological period preceding the present 
in America, while they are known in considerable numbers in the rocks 
of this age found in the Old World. There is, therefore, good reason to 
believe that the bear group is derived from the other side of the earth and 
that the ancestors of the present American bears migrated to this continent 
at a time geologically not far removed from the present. 
The bears of the world may be divided into two large groups, one some- 
