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THE NATIONAL GEOGRAPHIC MAGAZINE 



by the environment to which it has been 

 subjected during the ages. The larger 

 effects evidenced by profound modifica- 

 tions in the anatomy are the result of 

 continued pressure extending far back in 

 time. The far more numerous, modern, 

 and superficial changes known to natu- 

 ralists as geographic variations are every- 

 where in evidence. 



By the collection of great series of 

 specimens in North America and else- 

 where in the world it has been proved 

 that it is common for a single species of 

 mammal to occupy a great area, includ- 

 ing such diverse climatic conditions as 

 humid forested districts near the sea- 

 level, sections of arid desert plains in 

 the interior, and high rugged mountain 

 slopes. In each area of differing condi- 

 tions it is ordinarily found that represen- 

 tatives of a species, under certain con- 

 ditions, vary from those in other areas 

 mainly in shades of color and in propor- 

 tions. 



GEOGRAPHY AND COLOR 



In arid areas the colors are usually dis- 

 tinctly paler and grayer, in the humid 

 districts they are darker and browner. 

 Other conditions also effect these changes 

 among members of the same species, as 

 is shown in some of the most arid and 

 desert plains of the southwestern United 

 States, where mammals living among 

 dark-colored lava beds are darker than 

 those found, sometimes within a few 

 rods, on paler adjoining soil. Complete 

 isolation under the same climatic and 

 other conditions sometimes produces 

 marked changes, as is well illustrated by 

 the difference between the Abert and 

 Kaibab squirrels on the two sides of the 

 Grand Canyon in Arizona (see page 448) . 



The different forms of a species oc- 

 cupying areas under varying conditions 

 are commonly termed geographic races. 

 They grade imperceptibly into one an- 

 other along the border between their 

 ranges, step by step with the gradations 

 of the climatic and other conditions which 

 have produced their differences. 



ANIMAL CHEMISTS CHANGE STARCH INTO 

 WATER 

 One of the most striking modifications 

 of mammalian economy by environment 



is that shown in many small mammals ot" 

 our southwestern desert region and ad- 

 jacent parts of Mexico, in which such 

 species as the kangaroo-rats, pocket-mice, 

 prairie-dogs, and others are able to exisi 

 under the most arid conditions without 

 drinking. The liquid necessary for sup- 

 plying their bodily needs is obtained 

 through chemical action in their digestive 

 tracts, whereby some of the starchy parts 

 of their food are changed into water: 



Over considerable areas in the water- 

 less deserts on the peninsula of Lower 

 California periods of from three to five 

 years sometimes pass without a drop of 

 rain falling. In these areas the small 

 desert mammals named above, as well as 

 wood-rats, white-footed mice, cotton- 

 tails, and jack-rabbits, are numerous and 

 successfully pass these dry periods with- 

 out inconvenience. The absolute inde- 

 pendence of water of these animals has 

 been demonstrated in southern California 

 in the case of pocket-mice kept for 

 months in captivity in a box and fed 

 solely upon thoroughly dried seeds with- 

 out their showing the slightest sign of 

 discomfort. 



Our small mammals may be roughly 

 classified by their food habits into three 

 main groups : Rodents, or gnawing ani- 

 mals ; carnivores, or flesh eaters, and in- 

 sectivores, or insect eaters. 



GNAWERS MOST NUMEROUS OE MAMMALS 



The rodents vastly outnumber all other 

 mammals and are typified by the squir- 

 rels, rats, and mice ; their food is mainly 

 vegetable matter, but many of them eat 

 insects and meat whenever available. 

 The carnivores, including such species as 

 the weasel, mink, and marten, are mainly 

 flesh eaters, preying largely upon rodents, 

 but they also eat insects and fruits of 

 many kinds. The insectivores include 

 the moles and shrews, which, with all the 

 bats found within our limits, are almost 

 exclusively eaters of worms and insects. 



While rodents primarily feed on vege- 

 table matter, it is surprising to note the 

 large number of species among them 

 which commonly feed on insects and have 

 strong carnivorous propensities. This is 

 not so much the case with such larger 

 rodents as the beaver, porcupine, and 

 woodchuck, but most of the smaller kinds, 



