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University of California Publications in Zoology [Vol. 12 



which join on the shoulders, and continue forward to cover the whole 

 back part of the head behind an abrupt line of demarcation joining 

 the ears. (See pi. 13, fig. 21.) 



The question suggests itself whether or not geographic variation 

 in proportion of white and black (as in Mephitis estor of the desert 

 as compared with M. occidentalis of the Pacific slope of California) is 

 correlated with the maximum condition of efficiency from the stand- 

 point of utility. Granted that the sole purpose of the skunk's con- 

 trasted markings is to offer a signal of warning, then the maximum 

 of efficiency will be reached when the proportion of black and white 

 is such as to bring the greatest degree of conspicuity amid the average 

 of the natural surroundings. 



The surroundings on the desert, even in the riparian thickets, are 

 far lighter-toned both night and day, than they are in the humid coast 

 region. It would seem, therefore, that to secure the greatest con- 

 spicuity the increased proportion of white must be provided for the 

 darker surroundings, and conversely the increased amount of black 

 must be presented in the lighter surroundings. This is the reverse of 

 the case in fact. Hence it looks as though the warning theory does 

 not gain substantial support from this direction. 



The problem of the significance of animal coloration promises 

 important results when attacked upon a basis of the facts to be 

 observed in geographic variation. 



LIST AND MEASUREMENTS IN MILLIMETERS OF MEPHITIS ESTOB FROM THE 

 COLORADO VALLEY 



Nature of 

 Mus. no. Sex material Locality 



10574 2 Skin and Calif, side, 5 mi. south 



skeleton of Needles 



10575 J Skin and Calif, side, Cheme- 



skeleton huevis Valley 



10576 $ Skin and Arizona side, foot of 



skull The Needles 



10577 £ Skull only Arizona side, Ehrenberg Mar. 26 

 12648 (f Skeleton Arizona side, Ehren- 



only berg 



10578 J 1 Skin and Calif, side, opposite 



skull Cibola 



10579 ? Skin and Calif, side, 20 mi. N. 



skull of Picacho 



10580 J Skin and Calif, side, near 



skull Pilot Knob 



