No. 603] THE ROLE OF ISOLATION 181 



mainly of the same age or older, and likewise reared 

 from birth at La Jolla. Not a single individual of the 

 latter stock was as pale as either of the two females of 

 Samoa parentage. The male of the Samoa race was, 

 however, of about the average shade of the redwood 

 descendants. As stated above, some of the wild parents, 

 trapped on the peninsula, were likewise as dark as many 

 of the redwood series. 



No certain conclusions can, of course, be based upon 

 these three individuals. But the condition of the two 

 females certainly lends support to the belief that the 

 peculiar coat color of the Samoa race, however it was ac- 

 quired, has become fixed germinally. 



Reference to the map shows that the northern penin- 

 sula of Humboldt Bay is largely isolated, so far as land- 

 living rodents are concerned. In addition to the ocean 

 and the bay, a marshy tract extends from the latter to 

 the Mad River, which, in turn, interposes a further barrier 

 on the north, and nearly converts the peninsula into an 

 island. Beyond the mouth of Mad River, this same type 

 of sand-dune formation extends uninterruptedly to the 

 mouth of Little River, about six miles to the north, where 

 it ends abruptly and the shore line becomes precipitous. 



Now this northward extension of the sand-dune region 

 is not isolated by any physical barrier from the redwood 

 forest, which here comes near to the coast. It occurred 

 to me, therefore, to attempt the collection of Peromyscus 

 from a point somewhere within this region. The locality 

 chosen was close to the ocean, about two miles south of 

 Little River and four to five miles north of Mad River. 

 Here the conditions were found to be closely similar to 

 those on the exposed side of the northern peninsula of 

 Humboldt Bay. The dunes were on the whole lower, how- 

 ever, and some minor differences were noted in the flora. 

 The belt of shifting sand here ranges from five or six 

 hundred feet to perhaps a fourth of a mile, giving place 

 on the landward side to a narrow meadow or marshy 

 area, succeeded by a high, steep, wooded ridge. 



About ninety traps were set on two consecutive nights. 



