600 THE POPULAR SCIENCE MONTHLY 
tions of mammals into the North American continent have taken place, 
and, so far as possible, the exact routes of migration. This problem is 
in a large part paleontological, requiring for its interpretation a satis- 
factory account of the paleontology of vertebrates, invertebrates and 
plants of North America and of Asia, with particular reference to the 
relations of adjacent areas. We must also have, associated with this 
information, a full statement of the crustal movements in these regions 
as interpreted by the stratigraphic geologists and the physiographers. 
Through the accumulated efforts of paleontologists in this country 
particularly, we have already a considerable mass of evidence bearing 
on the general relationships of the faunas of North America and Asia 
in comparatively recent geological time, but the detail of the problem 
is, as yet, scarcely indicated. Particularly for Pleistocene and Pliocene 
time our knowledge of the faunal succession is exceedingly meager, and 
we can scarcely expect to know anything satisfactorily until the Pleisto- 
cene mammalian paleontology of America has been worked out in 
detail. This work must be followed or accompanied by similar studies 
of the mammalian faunas of western and southern Asia. When this is 
completed we shall know the time of the various migratory movements, 
the nature of the faunas which migrated, the character of the land areas 
over which they have passed, and the climatic conditions which obtained 
along the routes of migration. The presumption is, that when this is 
done we shall have actual evidence of the time of man’s occupation of 
North America. 
Viewed in the large, and without regard to the detail which has 
just been indicated, it seems possible to present several reasonable 
conclusions with reference to the probable period of migration of man 
to America. It is shown by study of a map of linguistic stocks of the 
western hemisphere that the northern and southern continents taken 
together may be divided into between one hundred and two hundred 
provinces, based on the number of stocks represented. ‘These lan- 
guages vary greatly in their structure, and are not similar to the 
languages of other parts of the world. ‘There is every reason to 
believe that a large percentage of them have been developed by lin- 
guistic differentiation which occurred since man first occupied this con- 
tinent, and that measured in years the time required for this differentia- 
tion has been long. On the other hand, considering the American con- 
tinent as a whole, we find that the greatly differing physical environ- 
ments are not reflected to any extent in different physical types of people 
occupying this region. That the human family is not exempt from 
physical differentiation, such as is almost universally indicated in mam- 
mals which have for some time been distributed over large areas with 
varying environments, is clearly shown by the map of the old world. 
_ Jn that region the human race is known to have been spread over a wide 
