No. 646] THE DISTRIBUTION OF LIFE 431 



Sonoran and Tropical, some of which are known by 

 other terms in the eastern part of the country. Roughly, 

 the position of an isotherm, as well as the temperature 

 of a region at other times of the year, depends upon 

 latitude, altitude and distance from the sea. Hence it is 

 that the winter temperature of parts of Montana at a 

 considerable altitude and far from the sea reaches a 

 lower figure than has been recorded on the coasts of the 

 Arctic Ocean. The coldest temperature ever known upon 

 the face of the earth— minus 92 degrees F. in tlie in- 

 terior of Siberia— is mucli lower than has ever been 

 found by any of the "farthest north" expeditions. 



We may safely infer that the degree of win'ter cold, 

 below a certain point, is largely immaterial, for it makes 

 no difference to a tree whether the thermometer is ten 

 or sixty degrees below zero, nor to the lesser vegetation 

 and many rodents safely protected b\- a dev]) blanket of 

 snow. Even to the few species of birds wliicli linliiiually 

 spend the w^inter in high latitudes, wry low icinpcr.i- 

 tures are seldom disastrous, but rather is it (inc. when 

 numbers perish, to a failure of the t'nod -upnly during 

 sleet storms or long blizzards. XcitlHM- hii-ds ikw mam- 

 mals migrate so much because cold a- brcan-c lln-ir 

 usual foods are not to be r)l)taiiied in a(le(iuato amounts 

 during the winter. 



Certain forms of life may have to contend, in rela- 

 tively low latitudes and altitudes, with conditions wliicli 

 api)i-()Nimate those to l)e found nmcii farther north. AV. 

 T. Shaw hn>> but ju^t brought to our attention the fact 

 that in ca-.tern Washington, where Vp]n^r Sonoran con- 

 ditions are the rule, (-tiv.-dion and hihi'mation of the 

 Townsend Ground S.|iiin-J [('ifJhi^ h-n n^indi) are so 

 long continued that this animal enjoys hut four months 

 of activity during the yvwv. The sipiifi-rls emerge as 

 soon as the first growth -tajl- in the M.rinu-. hut ivtire 

 to their burrows for the loii^- sleep wiieii the aiM<l condi- 

 tions of early summer cause a desiccation of iheir food 



