Febeuaky lU, 1906.] 



SCIENCE. 



253 



as we have already seen, it occupies the 

 Transition zone, directly below the Boreal 

 zone inhabited by senex, and while the 

 ranges of the two are in direct contact for 

 many miles, both species remain true, show- 

 ing no tendency to intergrade. The his- 

 tory of merriami is by no means so simple, 

 and the road by which it reached its present 

 home by no means so direct.' It is not an 

 immediate offshoot from senex, for before 

 attaining its present status it passed 

 through another form, known as p^-icei. 

 The story, as I interpret it, is this : In the 

 northern part of California, south of the 

 Siskiyous and west of the northern Sierra, 

 the Boreal senex gave off a Transition zone 

 form which spread and became differen- 

 tiated in two directions: to the southward 

 along the west flank of the Sierra it de- 

 veloped long ears and the peculiarities of 

 coloration that distinguish quadrimacu- 

 laius; to the westward it developed the 

 long tail and other peculiarities that dis- 

 tinguish pricei. The latter pressed south- 

 ward through the coast ranges to Monterey 

 Bay, south of which it underwent another 

 change, assuming the characters by which 

 merriami differs from pricei, and continued 

 in the same direction to the Santa Barbara 

 Mountains, and then easterly to Mt. Pinoa, 

 where its range forked, the north branch 

 following the Tejon and Tehachapi Moun- 

 tains to the southern Sierra and thence 

 northward to a little beyond the Yosemite ; 

 the south branch pushing southeasterly over 

 the Sierra Liebre, Sierra Madre, San Ga- 

 briel and San Bernardino mountains, and 

 then south over San Jacinto and Palomar 

 to the San Pedro Martyr Mountains of 

 Lower California. In the Sierra region 

 merriami is restricted to the Upper Son- 

 oran zone, while its immediate ancestor, 

 pricei, belongs to the Transition zone, and 

 its remoter ancestor, senex, to the Boreal 

 zone. The change from Boreal senex to 

 Transition zone pricei and quadriynacidatus 



is merely a zone adaptation to an imme- 

 diately adjoining area; the change from 

 pricei to merriami is simpler and at the 

 same time more interesting, for the belt 

 occupied by pricei, M'hile mainly Transi- 

 tion, possesses the climatic peculiarity of 

 mild winters and relatively cool summers 

 and fulfils the temperature requirements 

 of both Transition and Upper Austral 

 zones, permitting an overlapping or ad- 

 mixture of the distinctive species of both. 

 In this belt merriamA became adapted to 

 Upper Austral conditions, so that in ex- 

 tending its range back to the Sierra it wa.s 

 natural that it should adhere to the Upper 

 Sonoran zone. 



The three Sierra members of the senex 

 group {senex, quadrimacxdatus and mer- 

 riami) therefore have reached their appro- 

 priate zones from opposite directions — 

 senex and its offshoot quadrimacxdatus 

 from the north by direct continuity of 

 range; merriami from the south (after 

 passing through another form) by the 

 roundabout way of the Coast ranges and 

 the Tejon and Tehachapi mountains.^ 



To sum up the story of the California 

 chipmunks from the standpoint of their 

 geographic origin : Of the nine species in- 

 habiting the middle Sierra region, six 

 {senex, amcenus, speciosus, merriami, pana- 

 mintinus and pictus) appear to have come 

 in from contiguous territory in their pres- 

 ent condition— as fvUly formed species — 

 although it is possible that one of them 

 {ammnus) originated here and extended its 

 range northward; and three {quadrimacu- 



* The chipmunks afford admirable illustrations 

 of a number of phases of variation and distribu- 

 tion besides those here discussed — as the occur- 

 rence of closely related forms end-to-end in the 

 same zone; the spreading out of a species in cer- 

 tain parts of its range to cover an adjacent zone 

 which elsewhere is avoided; the development of 

 protective and directive coloration anu markings; 

 the utility or non-utility of characters used in the 

 discrimination of species, and so on. 



