JANUARY 19, 1912] 
fully the following explanation given in the 
accompanying text: 
“Let us regard the cat as made up of a fore 
part and a hind part, whose moments of 
inertia Z,, I, are equal when the legs are fully 
extended at right angles to the body. The 
photographs given in Plate IJ. show that it 
first contracts its fore legs (thereby making 
T, less than J,) and then turns its fore part 
round. This latter action necessitates the 
hind part being turned in the opposite direc- 
tion (since the total angular momentum about 
the axis is zero) but to a less extent, since J, 
is greater than J,, The animal then contracts 
its hind legs, extends its forelegs, and gives 
its hind part a turn. This necessitates the 
fore part being turned in the reverse direction 
but, again, to a less extent, since J, is now 
greater than J,. It will thus be seen that by 
continued action of this kind the cat can turn 
itself through any required angle, though at 
no time has it any angular momentum about 
mis) C Bhai? ? 
The explanation offered by Professor Hay- 
ford, although a possible one, accordingly does 
not agree with the actual performance of a 
eat, as observed by photography. 
J. R. Brenton 
UNIVERSITY OF FLORIDA, 
December 18, 1911 
SCIENTIFIC BOOKS 
The Wilderness of the Upper Yukon: A 
Hunter's Explorations for Wild Sheep in 
Sub-Arctic Mountains. By CHARLES SHEL- 
pon. New York, Charles Scribner’s Sons. 
1911. S8vo. Pp. xxi+ 354; 4 colored and 
46 half-tone plates; 4 maps, one in colors. 
The distribution and relationships of the 
mountain sheep of Canada and Alaska present 
one of the most interesting and puzzling 
problems in North American mammalogy. 
For the purpose of obtaining more definite 
information on this subject Mr. Sheldon, a 
hunter-naturalist of well-known qualifications 
for such a task, spent the seasons of 1904 and 
1905 in the Northern Rockies, exploring the 
Ogilvie, the Selwyn and Plateau mountains 
and the Watson River country in 1904, and 
SCIENCE 
105 
the Pelly, Rose and Glenlyon mountains in 
1905. As a narrative of exploration in prac- 
tically new fields, the book is an important 
contribution to our knowledge of the physical 
conditions and natural history of the region 
traversed, aside from its bearing upon the 
special quest for which these journeys were 
undertaken. Its excellent literary form, its 
abundant and admirable illustrations and the 
author’s enthusiasm and sympathy with his 
surroundings, add a value and a charm to his 
pages unusual in books of hunting adventure. 
Maps are given of the districts traversed, 
excellent. half-tones illustrate scenic features 
and there are four colored plates from draw- 
ings by Carl Rungius of sheep and other big 
game. 
The sheep of northern Canada and Alaska 
are quite different from the well-known big- 
horn of the Rocky Mountains of southern 
Canada, the United States and northern Mex- 
ico. The first northern form to become scien- 
tifically known was the Ovis dalli described by 
E. W. Nelson in 1884 from specimens col- 
lected in the upper Yukon region of Alaska. 
This sheep is pure white at all seasons except 
for adventitious staining from soil or vegeta- 
tion; it is smaller and has less massive horns 
than the various forms of the Rocky Mountain 
bighorn. 
In 1897 a black form was described as Ovis 
stonei from specimens obtained in the Che- 
onee Mountains south of the Stikine River in 
northern British Columbia. Although the 
Alaska form is pure white, and the other so 
dark colored as to be known as the black sheep, 
the structural differences that characterize 
them are slight and inconstant. 
A few years later (in 1901) a sheep inter- 
mediate in coloration between the white and 
black sheep was described as Ovis fannini, 
based on specimens collected near Dawson 
City. As the sheep of this general region 
became better known it was found that the 
sheep of the fannini type were very unstable 
in respect to coloration and were apparently 
intergrades between the white form of Alaska, 
the Yukon and Northwest territories and the 
black form of northern British Columbia. 
