106 
This was about the sum of our knowledge 
of these sheep when Mr. Sheldon set out in 
1904 and 1905 to make a special study of the 
sheep question of the northern Rockies, and 
to trace out their geographical and physical 
relationships. Chapter XX. of this book gives 
a summary of the results of his two seasons’ 
work, and is illustrated by a map in colors 
showing the known distribution of the white 
and black sheep of Canada and Alaska, their 
areas of intergradation and the phases char- 
acteristic of special districts. Facing the map 
are half-tone figures of nine stages of color 
variation, with explanatory text. The subject 
is thus graphically and clearly illustrated by 
the distribution map, the facing explanatory 
text and shaded figures. The area embraced 
extends from about latitude 55° to latitude 
70°. In Alaska, from the Arctic coast south 
to latitude 60°, and in Yukon Territory and 
northeastward in the Mackenzie Mountains to 
about latitude 62° (generally speaking), the 
sheep are pure white, except in the Tanana 
Hills south of the Yukon River, where the 
white coat is varied with a few black hairs 
and slight indications of the color pattern of 
the fannini type; in British Columbia south 
of the Stikine River the sheep are uniformly 
black; but over an intervening region of from 
approximately six hundred and fifty miles 
north and south and about one hundred and 
fifty to two hundred miles east and west, 
“there is no area in which the color of the 
sheep is uniform.” 
Mr. Sheldon indicates on his map five areas 
(a, b, c, d, e) where the sheep are either pure 
white (a), or black (e), or are of intermediate 
or mixed shades (b, c, d); the b grade is 
nearly white, the d grade nearly black, c 
being the middle phase or the fannini type, 
which is intermediate geographically as well 
as in color. 
The facts of intergradation are thus forcibly 
and clearly presented—an intergradation con- 
tinuous and gradual from one extreme phase 
to the other through a vast expanse of country. 
The cause of this extensive and gradual merg- 
ing of these two widely diverse color types of 
sheep is not so easily demonstrable. Has it 
SCIENCE 
[N.S. Vou. XXXV. No. 890 
resulted from interbreeding or is it due to 
environment? Mr. Sheldon favors the former: 
hypothesis, but admits the possibility of its: 
having been “ produced by subtle and indeter- 
minate changes of environment to a much 
greater extent than the facts seem to me [him] 
to warrant.” 
The large size of these animals and the 
striking color differences between the extreme: 
phases that are thus shown to intergrade 
render this an impressive instance of inter- 
gradation, but parallel cases, though less stri- 
king, in other animals usually seem explain- 
able satisfactorily, and in many instances 
beyond question, on the hypothesis of the 
action of diverse conditions of environment. 
But whatever conclusion may finally be 
reached as to the cause, great credit is due 
Mr. Sheldon for his contribution of facts 
through a successful reconnoissance of the 
almost inaccessible haunts of the sheep in the 
Northern Rockies where lay the key to the 
problem—an undertaking few would have the 
hardihood to project or the endurance and 
persistence to accomplish. Besides the facts: 
of variation and range already outlined, his. 
contribution to the life-history of these ani- 
mals is of noteworthy importance, while the 
wide range of individual variation among 
members of the same herd, not only as regards 
coloration, but in respect to size, shape and 
curvature of the horns is noted in detail. He 
has also presented to the National Museum 
the large series of specimens of sheep ob- 
tained by him on his expeditions which go 
far to substantiate the facts of intergradation 
recorded and illustrated in his book, which 
may be read with equal interest by the nat- 
uralist, the big game hunter and the general 
reader. 
J. A. ALLEN 
Principia Mathematica. By Atrrep Nort 
WuitEHEAD, Sc.D., F.R.S., Fellow and Lec- 
turer of Trinity College, Cambridge, and 
BertraND Russet, M.A., F.R.S., Lecturer 
and late Fellow of Trinity College, Cam- 
bridge. Cambridge University Press. 1910. 
Vol. I., pp. xiii + 666. 
