288 
GEOLOGY: W. E. EKBLAW 
The line derived from specimen ED is of special interest, since within it 
appeared the greatest diversities that were found during the entire investiga- 
tion. The progeny of ED had a mean spine number of 9.91 and a mean diam- 
eter of 23.51 units (fig. 1, E). At the same time the mean spine number of the 
parent line (E) was 10.99 and the mean diameter was 27.05 units, giving a 
difference in mean spine number of 1.08 and in mean diameter of 3.54 units. 
Furthermore the differences persisted for many generations and until the line 
was discontinued. Specimen ED therefore fulfilled the conditions usually 
required of a mutation, i.e., it was a sudden large variation that was inherited. 
From line ED there were derived three branches, EDA, EDB and EDC, that 
quickly exceeded in diameter and spine number any other branches in the 
entire family 58. The largest specimen appeared in branches EDB. It had 
20 spines and a diameter of 40 units (fig. 1, F). These branches, however, 
soon died out for some unknown reason, although they were cultivated as 
carefully as possible. 
The general conclusion reached is that within a large family of Arcella den- 
tata produced by vegetative reproduction from a single specimen, there are 
many heritably » diverse branches. These diversities are due both to very 
slight variations and to sudden large variations ('mutations'). The formation 
of such hereditarily diverse branches appears to be a true case of evolution 
that has been observed in the laboratory and that occurs in a similar way in 
nature. 
1 Middleton, A. R. J. Exp. Zool., 19, 1915. (451-503.) 
2 Jennings, H. S. Genetics, 1, 1916, (407-534.) 
THE IMPORTANCE OF NIVATION AS AN EROSIVE FACTOR, AND 
OF SOIL FLOW AS A TRANSPORTING AGENCY, IN 
NORTHERN GREENLAND 
By W. Elmer Ekblaw 
Crocker Land Expedition, American Museum of Natural History, and University 
of Illinois 
Communicated by J. M. Clarke, July 12, 1918 
Nivation and solifluctidfi, two closely related and important physiographic 
processes of Arctic lands, are perhaps nowhere better illustrated than in 
those coastal areas of northern Greenland not covered by the permanent 
ice-cap. The climate and the topography are favorable to the high develop- • 
ment of these processes; the rather heavy snowfall that melts gradually 
during the short summer promotes the work of nivation; and the high relief, 
with numerous small plateaus and generally steep slopes, affords opportunity 
for the action of solifluction. The presence of an ' ice-table' everywhere, not 
deep below the surface, is an added favorable condition. As a consequence 
