252 The American Geologist. October, looi. 
ropean ice-sheet, appear to have occurred some 5,000 to 10,000 
years ago. According to my studies as a glaciaHst, it seems to 
me that FHnders Petrie has given as satisfactory estimates as 
can be made with our present knowledge, in his recent sugges- 
tions assigning 100,000 years as the probable duration since 
Paleolithic man appeared in the Somme valley, and 10,000 
years since Neolithic man came into western Europe. Eolithic 
man, known by his very rude stone implements in stream de- 
posits which are preserved on high plateaus in southern Eng- 
land, belonged doubtless to a time considerably earlier than 
100,000 years ago; so that we may perhaps allot twice that 
period for the existence of man and the development of his 
principal races. 
All mankind, however, constitute only a single species. A 
requisite condition of distinctness separating species is the 
inability to produce hybrid offspring, or, when such offspring 
is possible to nearly allied species, it is incapable of reproduc- 
tion. Judged by this essential part of the definition of every 
zoologic or botanic species, the whole human family is specific- 
ally a zoologic unit. All its races and varieties are freely fer- 
tile with each other. However different in color, stature, 
features, mental attainment, or all other qualities which dis-' 
tinguish races and nations, they blend together. Such inter- 
mingling, mostly of the black and yellow races, has apparently 
been the origin of the Malay, Australian, and Polynesian peo- 
ples. In America, on the contrary, no considerable foreign ini- 
termixture seems to have occurred since the very early time 
when this continent was first peopled. The immigration came 
probably from northeastern Asia, across Bering strait, and 
perhaps in part also, at nearly the same time, from Europe, 
over a land connection by the Faeroe islands, Iceland, and 
Greenland. 
An objection to such migrations of primitive man during 
the Glacial period may be based on the ice-covered condition 
of North America at that time, wholly enveloped by an ice- 
sheet upon its northern half, northward from the Ohio, Mis- 
souri, and Columbia rivers, excepting the greater part of 
Alaska. If the preglacial and early Glacial altitude of the con- 
tinent had been the same as now, this objection would be valid, 
and we should be obliged to refer these ancient migrations 
