104 University of Calif ornia Pitblications i)i Zoology [Vol. 12 



It may be carried close under the steep outside bank of an ox-bow 

 swing, only to be directly thrown back towards the opposite shore. 

 Stretches of rough water may be encountered where the object is 

 swamped at the crest of every wave. Or, along rocky parts of the 

 channel, swirls, large and small, arrest its passage. In the most violent 

 of these eddies a twenty-foot log was seen to up-end and sink from 

 sight, to reappear after its total submergence, a hundred yards down 

 stream. 



At times of falling water a great deal of drift lodges on mudbars 

 and projecting reefs of rock. It is imaginable that drift logs might 

 be reached by individuals, which freed again with subsequently rising 

 water, would carry their passengers until lodged under favoring cir- 

 cumstances on the opposite side of the river. In the account of 

 Xeotoma intermedia clesertorum, the only rodent of the Encelia asso- 

 ciation not checked by the river, it is suggested that in some such 

 way passage was secured from the California to the Arizona side of 

 the river. This wood rat now bids fair to occupy much appropriate 

 territory in southwestern Arizona not previously possessing an asso- 

 ciational homologue, that is a Neotoman representative. Neotoma may 

 be looked upon as a more hardy and ecologically less specialized rodent 

 than any of its assoeiational companions. It is certainly much the 

 largest, and is notoriously of aggressive disposition as a forager. 



Of the eight species of delimited rodents, not one individual of the 

 hundreds trapped was found on the "wrong" side of the river. As 

 far as they went, then, our efforts furnished no evidence that even 

 an occasional individual does get across. As already shown, there seems 

 to be nothing to attract the upland rodents to the water's edge, so 

 that possibility of securing safe transportation on a log or mass of 

 drift is doubly remote. Now, supposing that a single individual did 

 manage to reach the opposite shore, its species would not necessarily 

 be established there. In most cases (not, however, with Peromyscus 

 crinitus stepliensi and Perognathus formosus) there is already estab- 

 lished an assoeiational homologue, with which even a whole family of 

 the invaders would have to compete, with the chances at least as much 

 against success as favoring it. Hybridization might occur, granted 

 that no sexual antipathy arise, but, whatever the immediate results, it 

 is the impression of the writer that swamping would eventually be 

 likely to wipe out all trace of the invading species. This impression 

 is admittedl.y based upon fragmentary data which has not been sub- 

 jected to critical analysis. Whether or not ]\Iendelian behavior in 



