Febkuaky 16, 1906.] 



SCIENCE. 



2b I 



zone. Above this still is the Hudsonian 

 zone, and it also has its distinctive chip- 

 munk — a small but highly colored species 

 {alpinus) —which fills the zone completely 

 and climbs up a short distance on the alpine 

 slopes above timber line. Crossing the sum- 

 mit and descending the steep east side of 

 the Sierra, the Canadian zone is found to 

 be inhabited by the same three species that 

 occupy this zone on the west slope. Below 

 this, in the nut pine belt of the Transition 

 zone, is a brilliantly colored species (pana- 

 mintinus) very different from any we have 

 seen. Still lower, in the Upper Sonoran 

 sage brush of Owens Valley, is a small gray 

 species (pictus) not related to any of the 

 others. Owens "Valley is a long narrow 

 and deep valley between the Sierra and the 

 AVhite Mountains. Crossing this valley 

 and ascending the west slope of the White 

 Moimtains we reenter the nut pine belt and 

 find the same chipmunk (panamintinus) 

 Jhat we found in the same belt on the other 

 side of the valley (on the east slope of the 

 Sierra). Continuing the ascent we enter 

 the Canadian zone, which covers the greater 

 part of the summit of the White Moun- 

 tains, and in it find a widely different chip- 

 munk (inyoensis) , which proves to be an- 

 other member of the beautiful speciosiis 

 group— a group we have already found 

 represented in the same zone on both east 

 and west slopes of the Sierra. 



Thus in a distance of a hundred miles, 

 from the west base of the Sierra to the 

 summit of the White Mountains, are in- 

 cluded the ranges of no less than nine spe- 

 cies of chipmunks. This is made possible 

 by the height and steepness of the moun- 

 tains, the abrupt changes in altitude with 

 consequent differences in temperature com- 

 pressing the life zones into narrow parallel 

 belts; whereas in level regions, as well 

 known, these same belts are spread out 

 broadly over the land. 



The Sierra chipmunks furnish striking 



illustrations of the occurrence of species 

 without isolation, and some of them of the 

 evolution of species without isolation, for 

 not only are there no visible barriers be- 

 tween the ranges of adjoining species, but 

 the species themselves actually overlap 

 along the zone borders, individual animals 

 belonging to the zone above and the zone 

 below occurring together on the same 

 ground. But while there are no barriers 

 between the species, each belongs to and is 

 characteristic of a definite climatic life 

 zone ; and the fact that the life zones over- 

 lap slightly along the edges explains the 

 slight geographic overlapping of the spe- 

 cies themselves. The reason the species do 

 not intergrade along the lines of contact 

 doubtless is that they are not closely enough 

 related— their differentiation into fully de- 

 veloped species (with the probable excep- 

 tion of speciosus and inyoensis) having 

 taken place long ago. 



The question now arises as to the origin 

 of the nine species. Most of these, if 

 studied independently of their relations in 

 other parts of the country, resemble one 

 another sufficiently to justify the inference 

 that they have been derived from one 

 another. 



Far from holding this view, however, my 

 belief is that some of them came from 

 closely related forms in remote geographic 

 areas, others from antecedent forms now 

 extinct, and not more than three or four 

 from species still inhabiting the region. 

 The case is of a class often encountered by 

 the systematist and student of distribution, 

 where, without a comprehensive knowledge 

 of the relationships and geographic distri- 

 bution of the group as a whole and of its 

 component species and subspecies, there is 

 little hope of arriving at correct conclu- 

 sions. Let us look closely at, the facts, for 

 they have an important bearing on more 

 than one problem in evolution and distri- 

 bution. 



