236 
the now-extinct Pleistocene mammals may have survived into com- 
paratively modern times, and that errors may creep in from other 
sources. In the light of our ])resent knowledge it is safest to con- 
clude that the western hemisphere was uniuhabitefl during the CJlacial 
period, although we must be willing to revise this opinion sliould new 
discoveries jiroduce overwhelming evidence to the contrary. 
Nevertlieless, the Indians are by no means recent immigrants 
into America. Early civilizations grow rather slowly, as we know 
from the histories of Egypt and Alesojiotamia. and a long preliminary 
peidod of agricultural development and experimentation in the dress- 
ing of stone, a gradual progress through hundreds of years, must have 
been necessary to produce the Mayan civilization that flourished in 
Central America at the beginning of the Christian era. Then the 
amazing number of languages^ and tlie diversity of customs and 
j)hysical types throughout the two continents are further evidence 
of long occupation, as long perhaps as twenty thousaiul years, or 
roughly from the final retreat of the ice-sheets to the present day. 
Even this length of time might seem too short, considering the slow- 
ness of such changes in other parts of the globe, were it not probable 
that many of these rlifferences developed in the Old World and cliar- 
acterized some of the tribes before they reached America. For we 
can hardly believe that they all entered in one body and at no time 
received any furtlier accessions from the outside; but rather that 
they came in many scattered bands separated by wide intervals of 
time, althougli they followed the same established route or routes. 
If the first l)ands arrived just at the close of the Pleistocene, they 
may have left those implements in the gravels at Trenton, New 
Jersey, whose position Wissler and Spier seem inclined to attribute 
to heavy floods produced by the melting ice-sheet.- Apart from this 
one instance, however, there seem to be no solid grounds for assign- 
ing quite as great an antiquity to any known aboriginal remains. 
Doubtless many of them date back several millenia, although we have 
no means of arriving at their true age because nowhere is there any 
1 We tlo not know bow mnny different linmiistir families there aie in Xortii and Siniiii America 
rombined. Powell, in ISfll, estimated fifty-eight in tlie region nortii of Mexico alone {Powell, J. W. : 
“Indian Linguistic Fnmihes’’; 7th Ann, Kept. Hur. .\m. Etlin., ipp. 1-142 (Washington, 1891)), init 
later investigations have brought that nundier down to about tliirty, with tlie probability of still 
lurther reduction in the future. 
^ Spier, Leslie: “The Trenton Argillite Culture”; Anth. Papers, Am. Mils. Xat. Hist., vol. xxii 
pp. 167-226 (New York, 1924). Wissler, Clark: “The Pre.sent Statu.s of the Antiriuity of Man in 
North .America”; Scientific Monthly, March, 1916, pp. 234-238. 
