254 



SCIENCE. 



[N. S. Vol. XXIII. No. 581. 



latus, inyoensis and alpinus) appear to 

 have been developed within the region in 

 the areas they now inhabit. So far as 

 origin is concerned, therefore, we have to 

 do with only the last three. Of these, one 

 — inyoensis— was clearly derived from the 

 speciosus stock by the gradual accentuation 

 of minute variations; another, quadri- 

 maculatus, appears to have originated from 

 senex in the same way, as already ex- 

 plained, leaving only one, alpinus, whose 

 history is by no means obvious. Since 

 alpinus has no near relatives, there is little 

 in the way of a clue to its ancestry. As 

 already suggested, it may be the remnant 

 of an Arctic- Alpine group, of which it and 

 oreocetes of the high mountains of western 

 Montana are the sole survivors. In any 

 case its origin must be sought far back in 

 the past. 



I have dwelt thus at length on the Cali- 

 fornia chipmunks for three reasons: (1) 

 The problems they present to the student 

 of variation and evolution are fairly repre- 

 sentative of problems presented by other 

 groups; (2) the compact distribution of 

 the species in close-lying parallel belts in 

 conformity with the life zones has the ad- 

 vantage of bringing them into near rela- 

 tions, so that the facts of variation and 

 behavior are easily discerned; (3) the study 

 of the group emphasizes the necessity of a 

 knowledge of the geographic distribution 

 of species in order that their interrelations 

 and probable origins may be understood. 



The term geographic distribution must 

 not be taken to mean merely the area a 

 species occupies, to be shown by a color 

 patch on the map, but includes a compre- 

 hensive knowledge of the geographic en- 

 vironment, taking into account the climate 

 and the aspects of nature with which each 

 species is associated and by which it is 

 profoundly impressed. Moritz Wagner, in 

 a ptiragraph recently quoted by Doctor 

 Jordan, said: It is 'the study of all the 



important phenomena embraced in the 

 geography of animals and plants, which is 

 the surest guide to the study of the real 

 phases in the process of the formation of 

 species.' To the systematist and student 

 of evolution this knowledge is so funda- 

 mental that it is hard to see how correct 

 conclusions can be reached without it. In 

 studying problems in nature the geo- 

 graphic point of view is the natural method 

 of approach; it is a method so full of sug- 

 gestions and explanations that we can ill 

 afford to do without it. And how can it 

 be otherwise, for do we not all admit that 

 organisms are profoundly affected and 

 modified by their environment ? The utter 

 hopelessness of attempting to work out the 

 variations, affinities and probable origin of 

 a group of related species of animals or 

 plants without giving heed to the geo- 

 graphic distribution of the various forms 

 has just been illustrated by the case of the 

 Sierra chipmunks. If to a knowledge of 

 present distribution can be added a few 

 facts from the paleontological history of 

 the group a flood of light is at once thrown 

 on the problem. 



INTERGEADATION AND REGIONAL INFLUENCES. 



In studying geographic variation in the 

 various groups of terrestrial animals and 

 plants one soon learns that among closely 

 related species and subspecies some forms 

 intergrade while others do not; and that 

 among those that do intergrade, the change 

 from one to another may take place ab- 

 ruptly in a narrow belt, or gradually, by 

 imperceptible steps, over a wide area. 

 These two kinds of intergradation do not 

 usually occur in the same area, for the rea- 

 son that each is associated with a definite 

 set of geographic and physiographic condi- 

 tions of which climate plays the most im- 

 portant part. As a general rule it may be 

 asserted that where the geographic change 

 from one faunal area to another is sradual. 



