534 GEOLOGY. 



each has its own appropriate grouping in plant society, and neither 

 has a grouping that seems to have any relation to the homes of the 

 aborigines. 



Aside from the spreading due to the outward growth of the limbs of 

 the parent>-tree and the slight aid of winds, the distribution of these 

 trees seems to be chiefly dependent on squirrels, which have the habit 

 of carrying the nuts short distances and burying them for future use;. 

 Now if 15 years be taken as the average time at which a seedling under 

 native conditions comes into bearing, and if a squirrel is always pres- 

 ent to carry the first-borne nuts an average distance of 75 feet for 

 burial, and always in the right direction, and always neglects to recover 

 them, and they always grow and escape destruction, the average rate 

 of migration would be five feet per year, or a mile in 1000 years. At 

 least four species of the family are found 300 miles back from the former 

 ice-limit, and the migration must have been greater than this to the 

 amount that these trees were driven beyond the ice-border by the 

 severity of the glacial climate. An appreciable portion of the dis- 

 tance was against a rising slope where the drainage was antagonistic, 

 and it needs to be observed that streams, swamps, wet meadows, and 

 other features were barriers to assistance by squirrels. Where the 

 drainage favored, the dispersal might obviously be much accelerated. 

 But if only the adverse slopes be considered, the time-estimate is larger 

 than those derived from the erosion of falls and other physical methods. 

 In the present state of knowledge, it is for each to judge for himself 

 whether the uncertainties of the biological method of estimate are 

 greater or less than are those of the physical, and what is the purport 

 of their combined testimony. 



The Dynasty of Man. 



Human dispersal. — As yet there is little geological evidence rela- 

 tive to the place of man's origin, or to the earliest stages of his develop- 

 ment. Various considerations connected with his physical nature 

 and his distribution seem to point to the warm zone of the eastern 

 hemisphere, preferably southern Asia, as the place of his appearance. 

 There are some grounds for the inference that the earliest develop- 

 ments of those qualities that especially gave -him dominance were 

 associated with the open tracts of the sub-tropical zone, where rela- 



