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THE AMERICAN NATURALIST [Vol. XLVIII 



pterus is an arboreal type which is nocturnal, and has de- 

 veloped characters which enable it more easily to travel 

 from tree to tree. Arctomys is the most highly modified 

 form, and has departed most widely in its habits; it is 

 entirely terrestrial, seeks shelter in artificial burrows, 

 eats grass, and hibernates. 



Were we to study the characters separating these gen- 

 era, we should find that they are all characters which 

 enable the animal best to occupy the ecologic niche it fills. 

 If now we select any one of these genera and examine its 

 species, we perceive that .the differences we find are not 

 such as could clearly be related to differences in mode of 

 life or habits, but rather such differences as are induced 

 by the circumstances mentioned above, such differences 

 being size, color, length of feet and tail, texture of fur, 

 etc.— i. e., extrinsic variations. 



An interesting example of both extrinsic and intrinsic 

 modifications in an incipient stage may be found in the 

 song-sparrows of western United States. Let us compare 

 the form of the humid northwest coast belt, Melospiza 

 melodia morphna, with the form of the arid Arizona des- 

 erts, M. m. fallax. The differences to be observed in color 

 and size are very noticeable, and would undoubtedly lead 

 to their separation into two distinct species were it not for 

 the complete chain of intermediate forms. But even if the 

 chain of intermediate forms were not complete, and after 

 a period of segregation the numerous intergrading sub- 

 species became broken up into a few well-marked species, 

 nevertheless, unless a change in mode of life of the bird 

 were involved, however far the extremes of color and size 

 might tend, they could not be given generic distinction 

 because of the intermediate forms, inhabiting semi-arid or 

 semi-humid regions, which would be almost certain to 

 exist. It hapens, however, that Melospiza melodia mor- 

 phna, and M. m. fallax, do differ considerably in mode of 

 life, the former being a beach comber, the latter a nomad 

 of the desert. It would be expected, therefore, that if 

 these two subspecies were isolated, the modifications re- 



