No. 603] THE ROLE OF ISOLATION 177 



pairs of groups, within which a comparison of average 

 ear-length is possible. In twelve cases the mean figure is 

 greater for the Samoa mice, in two cases it is greater for 

 the Eureka mice, while in one case the two figures do not 

 differ appreciably. The probabilities against such a pre- 

 ponderance being due to chance are of course high. The 

 mean difference in ear-length between the two lots, com- 

 puted according to a method described by me in an earlier 

 paper,^ is 0.87 mm. Those who have made careful meas- 

 urements of mice will regard such a difference in the 

 length of this appendage as far from trivial. 



Let me now say something as to the environmental con- 

 ditions under which these two races of ruhidiis live. 

 Those which I have designated as the "Eureka" or "red- 

 wood" race were trapped by me during two different 

 years, within a distance of two miles from the southern 

 limits of the city of Eureka, California. The region is 

 one covered in large part by redwood forest, most of 

 which is of second growth, although there are some small 

 areas that have never been logged. The predominant tree 

 is the redwood {Sequoia sempervirens) , but several other 

 conifers are common, the most abundant of these being 

 the Sitka spruce {Picea sifchensis), Douglas fir {Pseu- 

 dotsuga taxifolia), and lowland fir [Ahies grandis). The 

 red alder (Alnus rubra), cascara {Rhamnus purshiana), 

 waxberry (Myrica calif ornica), red elderberry (Sam- 

 hucus racemosa), and a willow (SaUx hooheriana) appear 

 to be the chief non-coniferous trees of this district.^ The 

 *'wild lilac" {Ceanothus thgrsiflorus) is likewise com- 

 mon in some of the more open areas, often reaching the 

 proportions of a small tree. 



Except in recently cleared tracts, the region is one of 

 dense underbrush, the shrubbery and vines forming, in 

 fact, a veritable jungle which is frequently hard to pene- 



T Journal of Experimental Zoology, Vol. 18, April, 1915, particularly, pp. 



